The Excellent Powder–Saving Millions of Lives With DDT–Videos

Posted on August 8, 2012. Filed under: Agriculture, American History, Babies, Biology, Blogroll, Business, Chemistry, College, Communications, Diasters, Economics, Education, Energy, Federal Government, Food, government, Health Care, High School, history, Inflation, Investments, Law, liberty, Life, Links, media, People, Philosophy, Politics, Programming, Psychology, Quotations, Rants, Raves, Religion, Resources, Science, Strategy, Video, Wealth, Wisdom | Tags: DDT, Malaria, Saving Lives |

John Stossel – DDT

Demonizing DDT: Challenging The Scare Campaign That Has Cost Millions of Lives

“In The Excellent Powder: DDT’s Political and Scientific History, Richard Tren and Donald Roberts argue that the infamous insecticide is the world’s greatest public-health success stories, saving millions of lives by preventing insect-borne disease. Unfortunately for those in areas still infested with mosquitoes and other flying bugs, DDT is also the world’s most-misunderstood substance, the target of a decades-long scientifically ignorant and ideologically motivated campaign that has vastly limited its use and applications.

From Rachel Carson in the 1960s to contemporary critics, DDT has been the object of what Roberts, a professor of tropical public health at the Uniformed Services University of the Health Sciences, calls “scare campaigns” that link DDT to “theoretical harms to wildlife and human life that simply don’t exist.”

Dubbed “the excellent powder” by Winston Churchill for its life-saving qualities, DDT has the potential to transform the developing world from a malarial hell into something else again. Yet as Tren, the winner of the 2009 Julian L. Simon Award, warns, under current international conventions, global DDT production is scheduled to be halted in 2017, thereby consigning much of the world to less-effective and more-expensive alternatives that will consign millions of poor people to living hell.

Reason.tv’s Nick Gillespie sat down with Tren and Roberts, who are part of Africa Fighting Malaria, to talk about how DDT got such a bad rap and what can be done to set the record straight.”

15-108 Science Matters:DDT & Modern Environmental Movement II

Malaria

“…Malaria is a mosquito-borne infectious disease of humans and other animals caused by protists (a type of microorganism) of the genus Plasmodium. The protists first infect the liver, then act as parasites within red blood cells, causing symptoms that typically include fever and headache, in severe cases progressing to coma or death. The disease is widespread in tropical and subtropical regions in a broad band around the equator, including much of Sub-Saharan Africa, Asia, and the Americas.

Five species of Plasmodium can infect and be transmitted by humans. Severe malaria is largely caused by P. falciparum while the disease caused by P. vivax, P. ovale, and P. malariae is generally a milder form that is rarely fatal. The zoonotic species P. knowlesi, prevalent in Southeast Asia, causes malaria in macaques but can also cause severe infections in humans. Malaria is prevalent in tropical regions because the significant amounts of rainfall, consistently high temperatures and high humidity, along with stagnant waters in which mosquito larvae readily mature, provide them with the environment they need for continuous breeding. Disease transmission can be reduced by preventing mosquito bites by distribution of mosquito nets and insect repellents, or with mosquito-control measures such as spraying insecticides and draining standing water.

The World Health Organization has estimated that in 2010, there were 216 million documented cases of malaria. Around 655,000 people died from the disease, many of whom were children under the age of five.^[1] The actual number of deaths may be significantly higher, as precise statistics are unavailable in many rural areas, and many cases are undocumented. P. falciparum — responsible for the most severe form of malaria — causes the vast majority of deaths associated with the disease. Malaria is commonly associated with poverty, and can indeed be a cause of poverty and a major hindrance to economic development.

Despite a clear need, no vaccine offering a high level of protection currently exists. Efforts to develop one are ongoing. Several medications are available to prevent malaria in travelers to malaria-endemic countries (prophylaxis). A variety of antimalarial medications are available. Severe malaria is treated with intravenous or intramuscular quinine or, since the mid-2000s, the artemisinin derivative artesunate, which is superior to quinine in both children and adults and is given in combination with a second anti-malarial such as mefloquine. Resistance has developed to several antimalarial drugs, most notably chloroquine and artemisinin.

Signs and symptoms

Main symptoms of malaria^[2]

Typical fever patterns of malaria

The signs and symptoms of malaria typically begin 8–25 days following infection.^[3] However, symptoms may occur later in those who have taken antimalarial medications as prevention.^[4] The presentation may include fever, shivering, arthralgia (joint pain), vomiting, hemolytic anemia, jaundice, hemoglobinuria, retinal damage,^[5] and convulsions. Approximately 30% of people however will no longer have a fever upon presenting to a health care facility.^[4]

The classic symptom of malaria is cyclical occurrence of sudden coldness followed by rigor and then fever and sweating lasting about two hours or more, occurring every two days in P. vivax and P. ovale infections, and every three days for P. malariae. P. falciparum infection can cause recurrent fever every 36–48 hours or a less pronounced and almost continuous fever.^[6] For reasons that are poorly understood, but that may be related to high intracranial pressure, children with malaria frequently exhibit abnormal posturing, a sign indicating severe brain damage.^[7] Cerebral malaria is associated with retinal whitening, which may be a useful clinical sign in distinguishing malaria from other causes of fever.^[8]

Severe malaria is usually caused by P. falciparum, and typically arises 6–14 days after infection.^[9] Non-falciparum species have however been found to be the cause of ~14% of cases of severe malaria in some groups.^[4] Consequences of severe malaria include coma and death if untreated—young children and pregnant women are especially vulnerable. Splenomegaly (enlarged spleen), severe headache, cerebral ischemia, hepatomegaly (enlarged liver), hypoglycemia, and hemoglobinuria with renal failure may occur. Renal failure is a feature of blackwater fever, where hemoglobin from lysed red blood cells leaks into the urine.^[9]

Cause

A Plasmodium sporozoite traverses the cytoplasm of a mosquito midgut epithelial cell in this false-colour electron micrograph.

Malaria parasites are from the genus Plasmodium (phylum Apicomplexa). In humans, malaria is caused by P. falciparum, P. malariae, P. ovale, P. vivax and P. knowlesi.^[10][11] Among those infected, P. falciparum is the most common species identified (~75%) followed by P. vivax (~20%).^[4] P. falciparum accounts for the majority of deaths.^[12] P. vivax proportionally is more common outside of Africa.^[13] There have been documented human infections with several species of Plasmodium from higher apes; however, with the exception of P. knowlesi—a zoonotic species that causes malaria in macaques^[11]—these are mostly of limited public health importance.^[14]

Life cycle

The definitive hosts for malaria parasites are female mosquitoes of the Anopheles genus, which act as transmission vectors to humans and other vertebrates, the secondary hosts. Young mosquitoes first ingest the malaria parasite by feeding on an infected vertebrate carrier and the infected Anopheles mosquitoes eventually carry Plasmodium sporozoites in their salivary glands. A mosquito becomes infected when it takes a blood meal from an infected vertebrate. Once ingested, the parasite gametocytes taken up in the blood will further differentiate into male or female gametes and then fuse in the mosquito’s gut. This produces an ookinete that penetrates the gut lining and produces an oocyst in the gut wall. When the oocyst ruptures, it releases sporozoites that migrate through the mosquito’s body to the salivary glands, where they are then ready to infect a new human host. The sporozoites are injected into the skin, alongside saliva, when the mosquito takes a subsequent blood meal. This type of transmission is occasionally referred to as anterior station transfer.^[15]

Only female mosquitoes feed on blood; male mosquitoes feed on plant nectar, and thus do not transmit the disease. The females of the Anopheles genus of mosquito prefer to feed at night. They usually start searching for a meal at dusk, and will continue throughout the night until taking a meal.^[16] Malaria parasites can also be transmitted by blood transfusions, although this is rare.^[17]

Recurrent malaria

Malaria recurs after treatment for three reasons. Recrudescence occurs when parasites are not cleared by treatment, whereas reinfection indicates complete clearance with new infection established from a separate infective mosquito bite; both can occur with any malaria parasite species. Relapse is specific to P. vivax and P. ovale and involves re-emergence of blood-stage parasites from latent parasites (hypnozoites) in the liver.^[4] Describing a case of malaria as cured by observing the disappearance of parasites from the bloodstream can, therefore, be deceptive. The longest incubation period reported for a P. vivax infection is 30 years.^[9] Approximately one in five of P. vivax malaria cases in temperate areas involve overwintering by hypnozoites, with relapses beginning the year after the mosquito bite.^[18]

Pathogenesis

The life cycle of malaria parasites. A mosquito causes infection by taking a blood meal. First, sporozoites enter the bloodstream, and migrate to the liver. They infect liver cells, where they multiply into merozoites, rupture the liver cells, and return to the bloodstream. Then, the merozoites infect red blood cells, where they develop into ring forms, trophozoites and schizonts that in turn produce further merozoites. Sexual forms are also produced, which, if taken up by a mosquito, will infect the insect and continue the life cycle.

Malaria infection develops via two phases: one that involves the liver or hepatic system (exoerythrocytic), and one which involves red blood cells, or erythrocytes (erythrocytic). When an infected mosquito pierces a person’s skin to take a blood meal, sporozoites in the mosquito’s saliva enter the bloodstream and migrate to the liver where they infect hepatocytes, multiplying asexually and asymptomatically for a period of 8–30 days.^[19] After a potential dormant period in the liver, these organisms differentiate to yield thousands of merozoites, which, following rupture of their host cells, escape into the blood and infect red blood cells to begin the erythrocytic stage of the life cycle.^[19] The parasite escapes from the liver undetected by wrapping itself in the cell membrane of the infected host liver cell.^[20]

Within the red blood cells, the parasites multiply further, again asexually, periodically breaking out of their hosts to invade fresh red blood cells. Several such amplification cycles occur. Thus, classical descriptions of waves of fever arise from simultaneous waves of merozoites escaping and infecting red blood cells.^[19]

Some P. vivax sporozoites do not immediately develop into exoerythrocytic-phase merozoites, but instead produce hypnozoites that remain dormant for periods ranging from several months (6–12 months is typical) to as long as three years. After a period of dormancy, they reactivate and produce merozoites. Hypnozoites are responsible for long incubation and late relapses in P. vivax infections, although their existence in P. ovale is uncertain.^[21]

The parasite is relatively protected from attack by the body’s immune system because for most of its human life cycle it resides within the liver and blood cells and is relatively invisible to immune surveillance. However, circulating infected blood cells are destroyed in the spleen. To avoid this fate, the P. falciparum parasite displays adhesive proteins on the surface of the infected blood cells, causing the blood cells to stick to the walls of small blood vessels, thereby sequestering the parasite from passage through the general circulation and the spleen.^[22] The blockage of the microvasculature causes symptoms such as in placental and cerebral malaria. In cerebral malaria the sequestrated red blood cells can breach the blood–brain barrier possibly leading to coma.^[23]

Micrograph of a placenta from a stillbirth due to maternal malaria. H&E stain. Red blood cells are anuclear; blue/black staining in bright red structures (red blood cells) indicate foreign nuclei from the parasites

Although the red blood cell surface adhesive proteins (called PfEMP1, for P. falciparum erythrocyte membrane protein 1) are exposed to the immune system, they do not serve as good immune targets, because of their extreme diversity; there are at least 60 variations of the protein within a single parasite and even more variants within whole parasite populations.^[22] The parasite switches between a broad repertoire of PfEMP1 surface proteins, thus staying one step ahead of the pursuing immune system.^[24]

Some merozoites turn into male and female gametocytes. If a mosquito pierces the skin of an infected person, it potentially picks up gametocytes within the blood. Fertilization and sexual recombination of the parasite occurs in the mosquito’s gut. New sporozoites develop and travel to the mosquito’s salivary gland, completing the cycle. Pregnant women are especially attractive to the mosquitoes, and malaria in pregnant women is an important cause of stillbirths, infant mortality and low birth weight,^[25] particularly in P. falciparum infection, but also in other species infection, such as P. vivax.^[26]

Genetic resistance

Due to the high levels of mortality and morbidity caused by malaria—especially the P. falciparum species—it is thought to have placed the greatest selective pressure on the human genome in recent history. Several diseases may provide some resistance to it including sickle cell disease, thalassaemias, glucose-6-phosphate dehydrogenase deficiency as well as the presence of Duffy antigens on the subject’s red blood cells.^[27][28]

The impact of sickle cell anemia on malaria immunity is of particular interest. Sickle cell anemia causes a defect to the hemoglobin molecule in the blood. Instead of retaining the biconcave shape of a normal red blood cell, the modified hemoglobin S molecule causes the cell to sickle or distort into a curved shape. Due to the sickle shape, the molecule is not as effective in taking or releasing oxygen, and therefore malaria parasites cannot complete their life cycle in the cell. Individuals who are homozygous for sickle cell anemia seldom survive this defect, while those who are heterozygous experience immunity to the disease. Although the potential risk of death for those with the homozygous condition seems to be unfavorable to population survival, the trait is preserved because of the benefits provided by the heterozygous form.^[29]

Malarial hepatopathy

Hepatic dysfunction as a result of malaria is rare and is usually a result of a coexisting liver condition such as viral hepatitis and chronic liver disease.^[30] Hepatitis, which is characterized by inflammation of the liver, is not actually present in what is called malarial hepatitis; the term as used here invokes the reduced liver function associated with severe malaria.^[30] While traditionally considered a rare occurrence, malarial hepatopathy has seen an increase in malaria endemic areas, particularly in Southeast Asia and India.^[30] Liver compromise in people with malaria correlates with a greater likelihood of complications and death.^[30]

Diagnosis

Malaria is typically diagnosed by the microscopic examination of blood using blood films or using antigen-based rapid diagnostic tests.^[31][32] Rapid diagnostic tests that detect P. vivax are not as effective as those targeting P. falciparum.^[33] They also are unable to tell how many parasites are present.^[4] Areas that cannot afford laboratory diagnostic tests often use only a history of subjective fever as the indication to treat for malaria.^[34] Polymerase chain reaction based tests have been developed, though these are not widely implemented in malaria-endemic regions as of 2012, due to their complexity.^[4]

Classification

Malaria is divided into severe and uncomplicated by the World Health Organization (WHO).^[4] Severe malaria is diagnosed when any of the following criteria are present, otherwise it is considered uncomplicated.^[35]

• Decreased consciousness
• Significant weakness such that the person is unable to walk
• Inability to feed
• Two or more convulsions
• Low blood pressure (less than 70 mmHg in adults or 50 mmHg in children)
• Breathing problems
• Circulatory shock
• Kidney failure or hemoglobin in the urine
• Bleeding problems, or hemoglobin less than 5 g/dl
• Pulmonary edema
• Low blood glucose (less than 2.2 mmol/l / 40 mg/dl)
• Acidosis or lactate levels of greater than 5 mmol/l
• A parasite level in the blood of greater than 2%

Prevention

Anopheles albimanus mosquito feeding on a human arm. This mosquito is a vector of malaria and mosquito control is an effective way of reducing the incidence of malaria.

Methods used to prevent malaria include medications, mosquito eradication and the prevention of bites. The presence of malaria in an area requires a combination of high human population density, high mosquito population density and high rates of transmission from humans to mosquitoes and from mosquitoes to humans. If any of these is lowered sufficiently, the parasite will eventually disappear from that area, as happened in North America, Europe and much of the Middle East. However, unless the parasite is eliminated from the whole world, it could become re-established if conditions revert to a combination that favours the parasite’s reproduction.^[36] Many countries are seeing an increasing number of imported malaria cases owing to extensive travel and migration.

Many researchers argue that prevention of malaria may be more cost-effective than treatment of the disease in the long run, but the capital costs required are out of reach of many of the world’s poorest people. There is a wide disparity in the costs of control (i.e. maintenance of low endemicity) and elimination programs between countries. For example, in China—whose government in 2010 announced a strategy to pursue malaria elimination in the Chinese provinces—the required investment is a small proportion of public expenditure on health. In contrast, a similar program in Tanzania would cost an estimated one-fifth of the public health budget.^[37]

Vector control

Man spraying kerosene oil to protect against mosquitoes carrying malaria, Panama Canal Zone 1912

Efforts to eradicate malaria by eliminating mosquitoes have been successful in some areas. Malaria was once common in the United States and southern Europe, but vector control programs, in conjunction with the monitoring and treatment of infected humans, eliminated it from those regions. In some areas, the draining of wetland breeding grounds and better sanitation were adequate. Malaria was eliminated from most parts of the USA in the early 20th century by such methods, and the use of the pesticide DDT and other means eliminated it from the remaining pockets in the South by 1951.^[38] (see National Malaria Eradication Program)

Before DDT, malaria was successfully eradicated or controlled in tropical areas like Brazil and Egypt by removing or poisoning the breeding grounds of the mosquitoes or the aquatic habitats of the larva stages, for example by applying the highly toxic arsenic compound Paris Green to places with standing water. This method has seen little application in Africa for more than half a century.^[39]

A more targeted and ecologically friendly vector control strategy involves genetic manipulation of malaria mosquitoes. Advances in genetic engineering technologies make it possible to introduce foreign DNA into the mosquito genome and either decrease the lifespan of the mosquito, or make it more resistant to the malaria parasite.^[40] Sterile insect technique is a genetic control method whereby large numbers of sterile males mosquitoes are reared and released. Mating with wild females reduces the wild population in the subsequent generation; repeated releases eventually eradicate the target population. Progress towards transgenic, or genetically modified, insects suggests that wild mosquito populations could be made malaria resistant. Successful replacement of current populations with a new genetically modified population relies upon a drive mechanism, such as transposable elements to allow for non-Mendelian inheritance of the gene of interest. Although this approach has been used successfully to eradicate some parasitic diseases of veterinary importance, technological problems have hindered its effective deployment with malaria vector species.^[40]

Indoor residual spraying

Indoor residual spraying (IRS) is the practice of spraying insecticides on the interior walls of homes in malaria-affected areas. After feeding, many mosquito species rest on a nearby surface while digesting the bloodmeal, so if the walls of dwellings have been coated with insecticides, the resting mosquitoes will be killed before they can bite another victim and transfer the malaria parasite.^[41]

The first pesticide used for IRS was DDT.^[38] Although it was initially used exclusively to combat malaria, its use quickly spread to agriculture. In time, pest control, rather than disease control, came to dominate DDT use, and this large-scale agricultural use led to the evolution of resistant mosquitoes in many regions. The DDT resistance shown by Anopheles mosquitoes can be compared to antibiotic resistance shown by bacteria. The overuse of antibacterial soaps and antibiotics led to antibiotic resistance in bacteria, similar to how overspraying of DDT on crops led to DDT resistance in Anopheles mosquitoes. During the 1960s, awareness of the negative consequences of its indiscriminate use increased, ultimately leading to bans on agricultural applications of DDT in many countries in the 1970s.^[42]

The World Health Organization currently advises the use of 12 insecticides in IRS operations, including DDT as well as alternative insecticides (such as the pyrethroids permethrin and deltamethrin).^[43] This public health use of small amounts of DDT is permitted under the Stockholm Convention on Persistent Organic Pollutants (POPs), which prohibits the agricultural use of DDT.^[42] However, because of its legacy, many developed countries previously discouraged DDT use even in small quantities.^[44]

One problem with all forms of IRS is insecticide resistance via evolution. Mosquitoes that are affected by IRS tend to rest and live indoors, and due to the irritation caused by spraying, their descendants tend to rest and live outdoors, meaning that they are not as affected—if affected at all—by the IRS, which greatly reduces its effectiveness as a defense mechanism.^[45]

Mosquito nets

Mosquito nets create a protective barrier against malaria-carrying mosquitoes that bite at night.

Mosquito nets help keep mosquitoes away from people and significantly reduce infection rates and transmission of malaria. The nets are not a perfect barrier and they are often treated with an insecticide designed to kill the mosquito before it has time to search for a way past the net. Insecticide-treated nets (ITNs) are estimated to be twice as effective as untreated nets and offer greater than 70% protection compared with no net.^[40] Although ITNs are proven to be very effective against malaria, only about 13% of households in sub-Saharan countries own them.^[46] Since the Anopheles mosquitoes feed at night, the preferred method is to hang a large “bed net” above the center of a bed to drape over it completely.^[47]

Other methods

Community participation and health education strategies promoting awareness of malaria and the importance of control measures have been successfully used to reduce the incidence of malaria in some areas of the developing world.^[48] Recognizing the disease in the early stages can stop the disease from becoming fatal. Education can also inform people to cover over areas of stagnant, still water, such as water tanks that are ideal breeding grounds for the parasite and mosquito, thus cutting down the risk of the transmission between people. This is generally used in urban areas where there are large centers of population in a confined space and transmission would be most likely in these areas.^[49]

Other interventions for the control of malaria include mass drug administrations^[33] and intermittent preventive therapy.^[50]

Medications

Several drugs, most of which are used for treatment of malaria, can be taken to prevent contracting the disease during travel to endemic areas. Chloroquine may be used where the parasite is still sensitive.^[51] However, due to resistance one of three medications—mefloquine (Lariam), doxycycline (available generically), or the combination of atovaquone and proguanil hydrochloride (Malarone)—is frequently needed.^[51] Doxycycline and the atovaquone and proguanil combination are the best tolerated; mefloquine is associated with higher rates of neurological and psychiatric symptoms.^[51]

The prophylactic effect does not begin immediately upon starting the drugs, so people temporarily visiting malaria-endemic areas usually begin taking the drugs one to two weeks before arriving and should continue taking them for four weeks after leaving (with the exception of atovaquone proguanil that only needs to be started two days prior and continued for seven days afterwards). Generally, these drugs are taken daily or weekly, at a lower dose than is used for treatment of a person who contracts the disease. Use of prophylactic drugs is seldom practical for full-time residents of malaria-endemic areas, and their use is usually restricted to short-term visitors and travelers to malarial regions. This is due to the cost of purchasing the drugs, negative adverse effects from long-term use, and because some effective anti-malarial drugs are difficult to obtain outside of wealthy nations.^[52] The use of prophylactic drugs where malaria-bearing mosquitoes are present may encourage the development of partial immunity.^[53]

Treatment

The treatment of malaria depends on the severity of the disease; whether people can take oral drugs or must be admitted depends on the assessment and the experience of the clinician.

Uncomplicated malaria

Uncomplicated malaria may be treated with oral medications. The most effective strategy for P. falciparum infection is the use of artemisinins in combination with other antimalarials (known as artemisinin-combination therapy).^[54] This is done to reduce the risk of resistance against artemisinin.^[54] These additional antimalarials include amodiaquine, lumefantrine, mefloquine or sulfadoxine/pyrimethamine.^[35] Another recommended combination is dihydroartemisinin and piperaquine.^[35] In the 2000s (decade), malaria with partial resistance to artemisins emerged in Southeast Asia.^[55][56]

Severe malaria

Severe malaria requires the parenteral administration of antimalarial drugs. Until the mid-2000s the most used treatment for severe malaria was quinine, but artesunate has been shown to be superior to quinine in both children^[57] and adults.^[58][59] Treatment of severe malaria also involves supportive measures that are optimally performed in a critical care unit, including management of high fevers (hyperpyrexia) and the subsequent seizures that may result from it, and monitoring for respiratory depression, hypoglycemia, and hypokalemia.^[60] Infection with P. vivax, P. ovale or P. malariae is usually treated on an outpatient basis (while a person is at home). Treatment of P. vivax requires both treatment of blood stages (with chloroquine or ACT) as well as clearance of liver forms with primaquine.^[61]

Prognosis

Disability-adjusted life yearfor malaria per 100,000 inhabitants in 2004.

no data    <10    10–100    100–500    500–1000    1000–1500    1500–2000

2000–2500    2500–2750    2750–3000    3000–3250    3250–3500    ≥3500

Severe malaria can progress extremely rapidly and cause death within hours or days.^[9] In the most severe cases of the disease, fatality rates can reach 20%, even with intensive care and treatment.^[4] Over the longer term, developmental impairments have been documented in children who have suffered episodes of severe malaria.^[62] It causes widespread anemia during a period of rapid brain development and also direct brain damage. This neurologic damage results from cerebral malaria to which children are more vulnerable.^[62] When properly treated, people with malaria can usually expect a complete recovery.^[63]

Epidemiology

Map showing the distribution of malaria in the world.^[64]  : ♦ Elevated occurrence of chloroquine- or multi-resistant malaria : ♦ Occurrence of chloroquine-resistant malaria : ♦ No plasmodium falciparum or chloroquine-resistance : ♦ No malaria

Based on documented cases, the WHO estimates that there were 216 million cases of malaria in 2010 resulting in 655,000 deaths.^[1] An estimate in The Lancet, based on a systematic analysis of all available mortality data combined with empirical methods for estimating causes of death, places the number of deaths in 2010 at 1.24 million.^[65][66] The majority of cases occur in children under five years old;^[67] pregnant women are also especially vulnerable. Despite efforts to reduce transmission and increase treatment, there has been little change in which areas are at risk of this disease since 1992.^[68] Indeed, if the prevalence of malaria stays on its present upwards course, the death rate could double in the next twenty years.^[69] Precise statistics are unknown because many cases occur in rural areas where people do not have access to hospitals or the means to afford health care. As a consequence, the majority of cases are undocumented.^[69]

Although coinfection with HIV and malaria does increase mortality, this is less of a problem than with HIV/tuberculosis coinfection, due to the two diseases usually attacking different age ranges, with malaria being most common in the young and active tuberculosis most common in the old.^[70] Although HIV/malaria coinfection produces less severe symptoms than the interaction between HIV and TB, HIV and malaria do contribute to each other’s spread. This effect comes from malaria increasing viral load and HIV infection increasing a person’s susceptibility to malaria infection.^[71]

Malaria is presently endemic in a broad band around the equator, in areas of the Americas, many parts of Asia, and much of Africa; however, it is in sub-Saharan Africa where 85–90% of malaria fatalities occur.^[72] The geographic distribution of malaria within large regions is complex, and malaria-afflicted and malaria-free areas are often found close to each other.^[73] Malaria is prevalent in tropical regions because of the significant amounts of rainfall, consistent high temperatures and high humidity, along with stagnant waters in which mosquito larvae readily mature, providing them with the environment they need for continuous breeding.^[74] In drier areas, outbreaks of malaria have been predicted with reasonable accuracy by mapping rainfall.^[75] Malaria is more common in rural areas than in cities; this is in contrast to dengue fever where urban areas present the greater risk.^[76] For example, several cities in Vietnam, Laos and Cambodia are essentially malaria-free, but the disease is present in many rural regions.^[77] By contrast, malaria in Africa is present in both rural and urban areas, though the risk is lower in the larger cities.^[78] The Wellcome Trust, UK, has funded the Malaria Atlas Project to map global endemic levels of malaria, providing a more contemporary and robust means with which to assess current and future malaria disease burden.^[79] This effort led to the publication of a map of P. falciparum endemicity in 2010.^[80] As of 2010, countries with the highest death rate per 100,000 population are Cote d’Ivoire with (86.15), Angola (56.93) and Burkina Faso (50.66) – all in Africa.^[81]

History

Malaria has infected humans for over 50,000 years, and Plasmodium may have been a human pathogen for the entire history of the species.^[82] Close relatives of the human malaria parasites remain common in chimpanzees. Some new evidence suggests that the most virulent strain of human malaria may have originated in gorillas.^[83]

References to the unique periodic fevers of malaria are found throughout recorded history, beginning in 2700 BC in China.^[84] Malaria may have contributed to the decline of the Roman Empire,^[85] and was so pervasive in Rome that it was known as the “Roman fever”.^[86] Several regions in ancient Rome were considered at-risk for the disease because of the favorable conditions present for malaria vectors. This included areas such as southern Italy, the island of Sardinia, the Pontine Marshes, the lower regions of coastal Etruria and the city of Rome along the Tiber River. The presence of stagnant water in these places was preferred by mosquitoes for breeding grounds. Irrigated gardens, swamp-like grounds, runoff from agriculture, and drainage problems from road construction led to the increase of standing water.^[87]

The term malaria originates from Medieval Italian: mala aria — “bad air”; the disease was formerly called ague or marsh fever due to its association with swamps and marshland.^[88] Malaria was once common in most of Europe and North America,^[89] where it is no longer endemic,^[90] though imported cases do occur.^[91]

British doctor Ronald Ross received the Nobel Prize for Physiology or Medicine in 1902 for his work on malaria.

Malaria was the most important health hazard encountered by U.S. troops in the South Pacific during World War II, where about 500,000 men were infected.^[92] According to Joseph Patrick Byrne, “Sixty thousand American soldiers died of malaria during the African and South Pacific campaigns.”^[93] Scientific studies on malaria made their first significant advance in 1880, when a French army doctor working in the military hospital of Constantine in Algeria named Charles Louis Alphonse Laveran observed parasites for the first time, inside the red blood cells of people suffering from malaria. He therefore proposed that malaria is caused by this organism, the first time a protist was identified as causing disease.^[94] For this and later discoveries, he was awarded the 1907 Nobel Prize for Physiology or Medicine. The malarial parasite was called Plasmodium by the Italian scientists Ettore Marchiafava and Angelo Celli.^[95] A year later, Carlos Finlay, a Cuban doctor treating people with yellow fever in Havana, provided strong evidence that mosquitoes were transmitting disease to and from humans.^[96] This work followed earlier suggestions by Josiah C. Nott,^[97] and work by Sir Patrick Manson, the “father of tropical medicine”, on the transmission of filariasis.^[98]

In April 1894, a Scottish physician Sir Ronald Ross visited Sir Patrick Manson at his house on Queen Anne Street, London. This visit was the start of four years of collaboration and fervent research that culminated in 1898 when Ross, who was working in the Presidency General Hospital in Calcutta, proved the complete life-cycle of the malaria parasite in mosquitoes. He thus proved that the mosquito was the vector for malaria in humans by showing that certain mosquito species transmit malaria to birds. He isolated malaria parasites from the salivary glands of mosquitoes that had fed on infected birds.^[99] For this work, Ross received the 1902 Nobel Prize in Medicine. After resigning from the Indian Medical Service, Ross worked at the newly established Liverpool School of Tropical Medicine and directed malaria-control efforts in Egypt, Panama, Greece and Mauritius.^[100] The findings of Finlay and Ross were later confirmed by a medical board headed by Walter Reed in 1900. Its recommendations were implemented by William C. Gorgas in the health measures undertaken during construction of the Panama Canal. This public-health work saved the lives of thousands of workers and helped develop the methods used in future public-health campaigns against the disease.^[101]

The first effective treatment for malaria came from the bark of cinchona tree, which contains quinine. This tree grows on the slopes of the Andes, mainly in Peru. The indigenous peoples of Peru made a tincture of cinchona to control malaria. The Jesuits noted the efficacy of the practice and introduced the treatment to Europe during the 1640s, where it was rapidly accepted.^[102] It was not until 1820 that the active ingredient, quinine, was extracted from the bark, isolated and named by the French chemists Pierre Joseph Pelletier and Joseph Bienaimé Caventou.^[103][104] Quinine become the predominant malarial medication until the 1920s, when other medications began to be developed. In the 1940s, chloroquine replaced quinine as the treatment of both uncomplicated and severe falciparum malaria until resistance supervened, first in Southeast Asia and South America in the 1950s and then globally in the 1980s.^[59] Artemisinins, discovered by Chinese scientists in the 1970s, are now the recommended treatment for falciparum malaria, administered in combination with other antimalarials as well as in severe disease.^[105]

Society and culture

Malaria is not just a disease commonly associated with poverty but also a cause of poverty and a major hindrance to economic development.^[106] Tropical regions are affected most; however, malaria’s furthest extent reaches into some temperate zones with extreme seasonal changes. The disease has been associated with major negative economic effects on regions where it is widespread. During the late 19th and early 20th centuries, it was a major factor in the slow economic development of the American southern states.^[107] A comparison of average per capita GDP in 1995, adjusted for parity of purchasing power, between countries with malaria and countries without malaria gives a fivefold difference ($1,526 USD versus $8,268 USD). In countries where malaria is common, average per capita GDP has risen (between 1965 and 1990) only 0.4% per year, compared to 2.4% per year in other countries.^[108]

Poverty is both a cause and effect of malaria, since the poor do not have the financial capacities to prevent or treat the disease. In its entirety, the economic impact of malaria has been estimated to cost Africa $12 billion USD every year. The economic impact includes costs of health care, working days lost due to sickness, days lost in education, decreased productivity due to brain damage from cerebral malaria, and loss of investment and tourism.^[67] In some countries with a heavy malaria burden, the disease may account for as much as 40% of public health expenditure, 30–50% of admissions to hospital, and up to 50% of outpatient visits.^[109] The slow demographic transition in Africa may be partly attributed to malaria. Total fertility rates were best explained by child mortality, as measured indirectly by infant mortality, in a 2007 study.^[110]

A study on the effect of malaria on IQ in a sample of Mexicans found that exposure during the birth year to malaria eradication was associated with increases in IQ. It also increased the probability of employment in a skilled occupation. The author suggests that this may be one explanation for the Flynn effect and that this may be an important explanation for the link between national malaria burden and economic development.^[111] The cognitive abilities and school performance are impaired in sub-groups of people (with either cerebral malaria or uncomplicated malaria) when compared with healthy controls. Studies comparing cognitive functions before and after treatment for acute malarial illness continued to show significantly impaired school performance and cognitive abilities even after recovery. Malaria prophylaxis was shown to improve cognitive function and school performance in clinical trials when compared to placebo groups.^[62] April 25 is World Malaria Day.^[81]

Counterfeit and substandard drugs

Sophisticated counterfeits have been found in several Asian countries such as Cambodia,^[112] China,^[113] Indonesia, Laos, Thailand, and Vietnam, and are an important cause of avoidable death in those countries.^[114] The WHO said that studies indicate that up to 40% of artesunate based malaria medications are counterfeit, especially in the Greater Mekong region and have established a rapid alert system to enable information about counterfeit drugs to be rapidly reported to the relevant authorities in participating countries.^[115] There is no reliable way for doctors or lay people to detect counterfeit drugs without help from a laboratory. Companies are attempting to combat the persistence of counterfeit drugs by using new technology to provide security from source to distribution.^[116]

Another clinical and public health concern is the proliferation of substandard antimalarial medicines resulting from inappropriate concentration of ingredients, contamination with other drugs or toxic impurities, poor quality ingredients, poor stability and inadequate packaging.^[117] A 2012 study demonstrated that roughly one-third of antimalarial medications in Southeast Asia and Sub-Saharan Africa failed chemical analysis, packaging analysis, or were falsified.^[118]

War

Throughout history, the contraction of malaria (via natural outbreaks as well as via infliction of the disease as a biological warfare agent) has played a prominent role in the fortunes of government rulers, nation-states, military personnel, and military actions. “Malaria Site: History of Malaria During Wars” addresses the devastating impact of malaria in numerous well-known conflicts, beginning in June 323 B.C. That site’s authors note: “Many great warriors succumbed to malaria after returning from the warfront and advance of armies into continents was prevented by malaria. In many conflicts, more troops were killed by malaria than in combat.”^[119] The Centers for Disease Control (“CDC”) traces the history of malaria and its impacts farther back, to 2700 BCE.^[120]

In 1910, Nobel Prize in Medicine-winner Ronald Ross (himself a malaria survivor), published a book titled The Prevention of Malaria that included a chapter titled “The Prevention of Malaria in War.” The chapter’s author, Colonel C. H. Melville, Professor of Hygiene at Royal Army Medical College in London, addressed the prominent role that malaria has historically played during wars and advised: “A specially selected medical officer should be placed in charge of these operations with executive and disciplinary powers [...].”

Significant financial investments have been made to procure existing and create new anti-malarial agents. During World War I and World War II, the supplies of the natural anti-malaria drugs, cinchona bark and quinine, proved to be inadequate to supply military personnel and substantial funding was funneled into research and development of other drugs and vaccines. American military organizations conducting such research initiatives include the Navy Medical Research Center, Walter Reed Army Institute of Research, and the U.S. Army Medical Research Institute of Infectious Diseases of the US Armed Forces.^[119]

Additionally, initiatives have been founded such as Malaria Control in War Areas (MCWA), established in 1942, and its successor, the Communicable Disease Center (now known as the Centers for Disease Control) established in 1946. According to the CDC, MCWA “was established to control malaria around military training bases in the southern United States and its territories, where malaria was still problematic” and, during these activities, to “train state and local health department officials in malaria control techniques and strategies.” The CDC’s Malaria Division continued that mission, successfully reducing malaria in the United States, after which the organization expanded its focus to include “prevention, surveillance, and technical support both domestically and internationally.”^[120]

Eradication efforts

Several notable attempts are being made to eliminate the parasite from sections of the world, or to eradicate it worldwide. In 2006, the organization Malaria No More set a public goal of eliminating malaria from Africa by 2015, and the organization plans to dissolve if that goal is accomplished.^[121] Several malaria vaccines are in clinical trials, which are intended to provide protection for children in endemic areas and reduce the speed of transmission of the disease. As of 2012, The Global Fund to Fight AIDS, Tuberculosis and Malaria has distributed 230 million insecticide-treated nets intended to stop mosquito-born transmission of malaria.^[122] According to director Inder Singh, the U.S.-based Clinton Foundation has significantly reduced the cost of drugs to treat malaria, and is working to further reduce the spread of the disease.^[123] Other efforts, such as the Malaria Atlas Project focus on analyzing climate and weather information required to accurately predict the spread of malaria based on the availability of habitat of malaria-carrying parasites.^[79]

Malaria has been successfully eradicated in certain areas. The Republic of Mauritius, a tropical island located in the western Indian Ocean, considered ecological connections to malaria transmission when constructing their current plan for malaria control. To prevent mosquitoes from breeding in aquatic areas, DDT is used in moderate amounts. Additionally, larvae-eating fish are placed in water sources to remove the malaria vectors before they become a threat to the human population. Obstructions are also removed from these sources to maintain water flow and reduce stagnant water. Similarly, marsh or swamp-like environments are drained and filled to diminish mosquito breeding grounds. These actions have produced positive results. The program has cut infection and death rates tremendously, and is cost effective, only requiring $1USD per head each year. This success is a clear indication that responses to adverse environmental conditions can decrease rates of disease.^[124]

Research

With the onset of drug-resistant Plasmodium parasites, new strategies are required to combat the widespread disease. One such approach lies in the introduction of synthetic pyridoxal-amino acid adducts, which are channeled into the parasite. Thus, trapped upon phosphorylation by plasmodial PdxK (pyridoxine/pyridoxal kinase), the proliferation of Plasmodium parasites is effectively hindered by a novel compound, PT3, a cyclic pyridoxyl-tryptophan methyl ester, without harming human cells.^[125]

Malaria parasites contain apicoplasts, an organelle usually found in plants, complete with their own functioning genomes. These apicoplasts are thought to have originated through the endosymbiosis of algae and play a crucial role in various aspects of parasite metabolism, for example in fatty acid biosynthesis.^[126] As of 2003, 466 proteins have been found to be produced by apicoplasts^[127] and these are now being investigated as possible targets for novel anti-malarial drugs.^[126]

Malaria vaccines have been an elusive goal of research. The first promising studies demonstrating the potential for a malaria vaccine were performed in 1967 by immunizing mice with live, radiation-attenuated sporozoites, which provided significant protection to the mice upon subsequent injection with normal, viable sporozoites.^[128] Since the 1970s, there has been a considerable effort to develop similar vaccination strategies within humans. It was determined that an individual can be protected from a P. falciparum infection if they receive over 1,000 bites from infected yet irradiated mosquitoes.^[129]

Immunization

Immunity (or, more accurately, tolerance) does occur naturally, but only in response to repeated infection with multiple strains of malaria.^[130] A completely effective vaccine is not yet available for malaria, although several vaccines are under development.^[131] SPf66 was tested extensively in endemic areas in the 1990s, but clinical trials showed it to be insufficiently effective.^[132] Other vaccine candidates, targeting the blood-stage of the parasite’s life cycle, have also been insufficient on their own.^[133] Several potential vaccines targeting the pre-erythrocytic stage are being developed, with RTS,S showing the most promising results so far.^[129]

http://en.wikipedia.org/wiki/Malaria

DDT

‘…DDT (dichlorodiphenyltrichloroethane) is an organochlorine insecticide which is a white, crystalline solid, tasteless, and almost odorless. Technical DDT has been formulated in almost every conceivable form including solutions in xylene or petroleum distillates, emulsifiable concentrates, water-wettable powders, granules, aerosols, smoke candles, and charges for vaporisers and lotions.^[2]

First synthesized in 1874, DDT’s insecticidal properties were not discovered until 1939, and it was used with great success in the second half of World War II to control malaria and typhus among civilians and troops. The Swiss chemist Paul Hermann Müller was awarded the Nobel Prize in Physiology or Medicine in 1948 “for his discovery of the high efficiency of DDT as a contact poison against several arthropods.”^[3] After the war, DDT was made available for use as an agricultural insecticide, and soon its production and use skyrocketed.^[4]

In 1962, Silent Spring by American biologist Rachel Carson was published. The book catalogued the environmental impacts of the indiscriminate spraying of DDT in the US and questioned the logic of releasing large amounts of chemicals into the environment without fully understanding their effects on ecology or human health. The book suggested that DDT and other pesticides may cause cancer and that their agricultural use was a threat to wildlife, particularly birds. Its publication was one of the signature events in the birth of the environmental movement, and resulted in a large public outcry that eventually led to DDT being banned in the US in 1972.^[5] DDT was subsequently banned for agricultural use worldwide under the Stockholm Convention, but its limited use in disease vector control continues to this day and remains controversial.^[6][7]

Along with the passage of the Endangered Species Act, the US ban on DDT is cited by scientists as a major factor in the comeback of the bald eagle, the national bird of the United States, from near-extinction in the contiguous US.^[8]

Properties and chemistry

DDT is similar in structure to the insecticide methoxychlor and the acaricide dicofol. It is a highly hydrophobic, nearly insoluble in water but has a good solubility in most organic solvents, fats, and oils. DDT does not occur naturally, but is produced by the reaction of chloral (CCl₃CHO) with chlorobenzene (C₆H₅Cl) in the presence of sulfuric acid, which acts as a catalyst. Trade names that DDT has been marketed under include Anofex (Geigy Chemical Corp.), Cezarex, Chlorophenothane, Clofenotane, Dicophane, Dinocide, Gesarol (Syngenta Crop.), Guesapon, Guesarol, Gyron (Ciba-Geigy Corp. – now Novartis), Ixodex, Neocid (Reckitt & Colman, Ltd), Neocidol (Ciba-Geigy Corp. – now Novartis), and Zerdane.^[4]

Isomers and related compounds

o,p’ -DDT, a minor component in commercial DDT.

Commercial DDT is a mixture of several closely–related compounds. The major component (77%) is the p,p’ isomer which is pictured at the top of this article. The o,p’ isomer (pictured to the right) is also present in significant amounts (15%). Dichlorodiphenyldichloroethylene (DDE) and dichlorodiphenyldichloroethane (DDD) make up the balance. DDE and DDD are also the major metabolites and breakdown products in the environment.^[4] The term “total DDT” is often used to refer to the sum of all DDT related compounds (p,p’-DDT, o,p’-DDT, DDE, and DDD) in a sample.

Production and use statistics

From 1950 to 1980, DDT was extensively used in agriculture—more than 40,000 tonnes were used each year worldwide^[9]—and it has been estimated that a total of 1.8 million tonnes have been produced globally since the 1940s.^[1] In the U.S., where it was manufactured by Ciba,^[10] Montrose Chemical Company, Pennwalt^[11] and Velsicol Chemical Corporation,^[12] production peaked in 1963 at 82,000 tonnes per year.^[4] More than 600,000 tonnes (1.35 billion lbs) were applied in the U.S. before the 1972 ban. Usage peaked in 1959 at about 36,000 tonnes.^[13]

In 2009, 3314 tonnes were produced for the control of malaria and visceral leishmaniasis. India is the only country still manufacturing DDT, with China having ceased production in 2007.^[14] India is the largest consumer.^[15]

Mechanism of insecticide action

In insects it opens sodium ion channels in neurons, causing them to fire spontaneously, which leads to spasms and eventual death. Insects with certain mutations in their sodium channel gene are resistant to DDT and other similar insecticides. DDT resistance is also conferred by up-regulation of genes expressing cytochrome P450 in some insect species.^[16]

In humans, however, it may affect health through genotoxicity or endocrine disruption. See Effects on human health.

History

Commercial product containing 5% DDT

Commercial product (Powder box, 50 g) containing 10% DDT ; Néocide. CibaGeigy DDT ; “Destroys parasites such as fleas, lice, ants, bedbugs, cockroaches, flies, etc.. Néocide Sprinkle caches of vermin and the places where there are insects and their places of passage. Leave the powder in place as long as possible. ” “Destroy the parasites of man and his dwelling”. “Death is not instantaneous, it follows inevitably sooner or later. ” “French manufacturing” ; “harmless to humans and warm-blooded animals” “sure and lasting effect. Odorless.

First synthesized in 1874 by Othmar Zeidler,^[4] DDT’s insecticidal properties were not discovered until 1939 by the Swiss scientist Paul Hermann Müller, who was awarded the 1948 Nobel Prize in Physiology and Medicine for his efforts.^[3]

Use in the 1940s and 1950s

DDT is the best-known of several chlorine-containing pesticides used in the 1940s and 1950s. With pyrethrum in short supply, DDT was used extensively during World War II by the Allies to control the insect vectors of typhus — nearly eliminating the disease in many parts of Europe. In the South Pacific, it was sprayed aerially for malaria and dengue fever control with spectacular effects. While DDT’s chemical and insecticidal properties were important factors in these victories, advances in application equipment coupled with a high degree of organization and sufficient manpower were also crucial to the success of these programs.^[17] In 1945, it was made available to farmers as an agricultural insecticide,^[4] and it played a minor role in the final elimination of malaria in Europe and North America.^[6] By the time DDT was introduced in the U.S., the disease had already been brought under control by a variety of other means.^[18] One CDC physician involved in the United States’ DDT spraying campaign said of the effort that “we kicked a dying dog.”^[19]

In 1955, the World Health Organization commenced a program to eradicate malaria worldwide, relying largely on DDT. The program was initially highly successful, eliminating the disease in “Taiwan, much of the Caribbean, the Balkans, parts of northern Africa, the northern region of Australia, and a large swath of the South Pacific”^[20] and dramatically reducing mortality in Sri Lanka and India.^[21] However widespread agricultural use led to resistant insect populations. In many areas, early victories partially or completely reversed, and in some cases rates of transmission even increased.^[22] The program was successful in eliminating malaria only in areas with “high socio-economic status, well-organized healthcare systems, and relatively less intensive or seasonal malaria transmission”.^[23]

DDT was less effective in tropical regions due to the continuous life cycle of mosquitoes and poor infrastructure. It was not applied at all in sub-Saharan Africa due to these perceived difficulties. Mortality rates in that area never declined to the same dramatic extent, and now constitute the bulk of malarial deaths worldwide, especially following the disease’s resurgence as a result of resistance to drug treatments and the spread of the deadly malarial variant caused by Plasmodium falciparum. The goal of eradication was abandoned in 1969, and attention was focused on controlling and treating the disease. Spraying programs (especially using DDT) were curtailed due to concerns over safety and environmental effects, as well as problems in administrative, managerial and financial implementation, but mostly because mosquitoes were developing resistance to DDT.^[22] Efforts shifted from spraying to the use of bednets impregnated with insecticides and other interventions.^[23][24]

Silent Spring and the U.S. ban

As early as the 1940s, scientists in the U.S. had begun expressing concern over possible hazards associated with DDT, and in the 1950s the government began tightening some of the regulations governing its use.^[13] However, these early events received little attention, and it was not until 1957, when the New York Times reported an unsuccessful struggle to restrict DDT use in Nassau County, New York, that the issue came to the attention of the popular naturalist-author, Rachel Carson. William Shawn, editor of The New Yorker, urged her to write a piece on the subject, which developed into her famous book Silent Spring, published in 1962. The book argued that pesticides, including DDT, were poisoning both wildlife and the environment and were also endangering human health.^[5]

Silent Spring was a best seller, and public reaction to it launched the modern environmental movement in the United States. The year after it appeared, President Kennedy ordered his Science Advisory Committee to investigate Carson’s claims. The report the committee issued “add[ed] up to a fairly thorough-going vindication of Rachel Carson’s Silent Spring thesis,” in the words of the journal Science,^[25] and recommended a phaseout of “persistent toxic pesticides”.^[26] DDT became a prime target of the growing anti-chemical and anti-pesticide movements, and in 1967 a group of scientists and lawyers founded the Environmental Defense Fund (EDF) with the specific goal of winning a ban on DDT. Victor Yannacone, Charles Wurster, Art Cooley and others associated with inception of EDF had all witnessed bird kills or declines in bird populations and suspected that DDT was the cause. In their campaign against the chemical, EDF petitioned the government for a ban and filed a series of lawsuits.^[27] Around this time, toxicologist David Peakall was measuring DDE levels in the eggs of peregrine falcons and California condors and finding that increased levels corresponded with thinner shells.

In response to an EDF suit, the U.S. District Court of Appeals in 1971 ordered the EPA to begin the de-registration procedure for DDT. After an initial six-month review process, William Ruckelshaus, the Agency’s first Administrator rejected an immediate suspension of DDT’s registration, citing studies from the EPA’s internal staff stating that DDT was not an imminent danger to human health and wildlife.^[13] However, the findings of these staff members were criticized, as they were performed mostly by economic entomologists inherited from the United States Department of Agriculture, whom many environmentalists felt were biased towards agribusiness and tended to minimize concerns about human health and wildlife. The decision not to ban thus created public controversy.^[17]

The EPA then held seven months of hearings in 1971–1972, with scientists giving evidence both for and against the use of DDT. In the summer of 1972, Ruckelshaus announced the cancellation of most uses of DDT—an exemption allowed for public health uses under some conditions.^[13] Immediately after the cancellation was announced, both EDF and the DDT manufacturers filed suit against the EPA, with the industry seeking to overturn the ban, and EDF seeking a comprehensive ban. The cases were consolidated, and in 1973 the U.S. Court of Appeals for the District of Columbia ruled that the EPA had acted properly in banning DDT.^[13]

The U.S. DDT ban took place amidst a growing public mistrust of industry, with the Surgeon General issuing a report on smoking in 1964, the Cuyahoga River catching fire in 1969, the fiasco surrounding the use of diethylstilbestrol (DES), and the well-publicized decline in the bald eagle population.^[26]

Some uses of DDT continued under the public health exemption. For example, in June 1979, the California Department of Health Services was permitted to use DDT to suppress flea vectors of bubonic plague.^[28] DDT also continued to be produced in the US for foreign markets until as late as 1985, when over 300 tons were exported.^[1]

Restrictions on usage

In the 1970s and 1980s, agricultural use was banned in most developed countries, beginning with Hungary in 1968^[29] then in Norway and Sweden in 1970, Germany and the United States in 1972, but not in the United Kingdom until 1984. Vector control use has not been banned, but it has been largely replaced by less persistent alternative insecticides.

The Stockholm Convention, which took effect in 2004, outlawed several persistent organic pollutants, and restricted DDT use to vector control. The Convention has been ratified by more than 170 countries and is endorsed by most environmental groups. Recognizing that total elimination in many malaria-prone countries is currently unfeasible because there are few affordable or effective alternatives, public health use is exempt from the ban pending acceptable alternatives. Malaria Foundation International states, “The outcome of the treaty is arguably better than the status quo going into the negotiations…For the first time, there is now an insecticide which is restricted to vector control only, meaning that the selection of resistant mosquitoes will be slower than before.”^[30]

Despite the worldwide ban, agricultural use continues in India^[31] North Korea, and possibly elsewhere.^[15]

Today, about 3-4,000 tonnes each year are produced for vector control.^[14] DDT is applied to the inside walls of homes to kill or repel mosquitoes. This intervention, called indoor residual spraying (IRS), greatly reduces environmental damage. It also reduces the incidence of DDT resistance.^[32] For comparison, treating 40 hectares (99 acres) of cotton during a typical U.S. growing season requires the same amount of chemical as roughly 1,700 homes.^[33]

Environmental impact

Degradation of DDT to form DDE (by elimination of HCl, left) and DDD (by reductive dechlorination, right)

DDT is a persistent organic pollutant that is readily adsorbed to soils and sediments, which can act both as sinks and as long-term sources of exposure contributing to terrestrial organisms ^[2]. Depending on conditions, its soil half life can range from 22 days to 30 years. Routes of loss and degradation include runoff, volatilization, photolysis and aerobic and anaerobic biodegradation. Due to hydrophobic properties, in aquatic ecosystems DDT and its metabolites are absorbed by aquatic organisms and adsorbed on suspended particles, leaving little DDT dissolved in the water itself. Its breakdown products and metabolites, DDE and DDD, are also highly persistent and have similar chemical and physical properties.^[1] DDT and its breakdown products are transported from warmer regions of the world to the Arctic by the phenomenon of global distillation, where they then accumulate in the region’s food web.^[34]

Because of its lipophilic properties, DDT has a high potential to bioaccumulate, especially in predatory birds.^[35] DDT, DDE, and DDD magnify through the food chain, with apex predators such as raptor birds concentrating more chemicals than other animals in the same environment. They are very lipophilic and are stored mainly in body fat. DDT and DDE are very resistant to metabolism; in humans, their half-lives are 6 and up to 10 years, respectively. In the United States, these chemicals were detected in almost all human blood samples tested by the Centers for Disease Control in 2005, though their levels have sharply declined since most uses were banned in the US.^[36] Estimated dietary intake has also declined,^[36] although FDA food tests commonly detect it.^[37]

Marine macroalgae (seaweed) help reduce soil toxicity by up to 80% within six weeks.^[38]

Effects on wildlife and eggshell thinning

DDT is toxic to a wide range of living organisms, including marine animals such as crayfish, daphnids, sea shrimp and many species of fish. It is less toxic to mammals, but may be moderately toxic to some amphibian species, especially in the larval stage. DDT, through its metabolite DDE, caused eggshell thinning and resulted in severe population declines in multiple North American and European bird of prey species.^[39] Eggshell thinning lowers the reproductive rate of certain bird species by causing egg breakage and embryo deaths. DDE related eggshell thinning is considered a major reason for the decline of the bald eagle,^[8] brown pelican,^[40] peregrine falcon, and osprey.^[1] However, different groups of birds vary greatly in their sensitivity to these chemicals. ^[2] Birds of prey, waterfowl, and song birds are more susceptible to eggshell thinning than chickens and related species, and DDE appears to be more potent than DDT.^[1] Even in 2010, more than forty years after the U.S. ban, California condors which feed on sea lions at Big Sur which in turn feed in the Palos Verdes Shelf area of the Montrose Chemical Superfund site seemed to be having continued thin-shell problems. Scientists with the Ventana Wildlife Society and others are intensifying studies and remediations of the condors’ problems.^[41]

The biological thinning mechanism is not entirely known, but there is strong evidence that p,p’-DDE inhibits calcium ATPase in the membrane of the shell gland and reduces the transport of calcium carbonate from blood into the eggshell gland. This results in a dose-dependent thickness reduction.^{[1][42][43][44]} There is also evidence that o,p’-DDT disrupts female reproductive tract development, impairing eggshell quality later.^[45] Multiple mechanisms may be at work, or different mechanisms may operate in different species.^[1] Some studies show that although DDE levels have fallen dramatically, eggshell thickness remains 10–12 percent thinner than before DDT was first used.^[46]

Effects on human health

Potential mechanisms of action on humans are genotoxicity and endocrine disruption. DDT may be directly genotoxic,^[47] but may also induce enzymes to produce other genotoxic intermediates and DNA adducts.^[47] It is an endocrine disruptor; The DDT metabolite DDE acts as an antiandrogen (but not as an estrogen). p,p’-DDT, DDT’s main component, has little or no androgenic or estrogenic activity.^[47] Minor component o,p’-DDT has weak estrogenic activity.

Acute toxicity

DDT is classified as “moderately toxic” by the United States National Toxicology Program (NTP)^[48] and “moderately hazardous” by the World Health Organization (WHO), based on the rat oral LD₅₀ of 113 mg/kg.^[49] DDT has on rare occasions been administered orally as a treatment for barbiturate poisoning.^[50]

Chronic toxicity

Diabetes

DDT and DDE have been linked to diabetes. A number of studies from the US, Canada, and Sweden have found that the prevalence of the disease in a population increases with serum DDT or DDE levels.^{[51][52][53][54][55][56]}

Developmental toxicity

DDT and DDE, like other organochlorines, have been shown to have xenoestrogenic activity, meaning they are chemically similar enough to estrogens to trigger hormonal responses in animals. This endocrine disrupting activity has been observed in mice and rat toxicological studies, and available epidemiological evidence indicates that these effects may be occurring in humans as a result of DDT exposure. The US Environmental Protection Agency states that DDT exposure damages the reproductive system and reduces reproductive success. These effects may cause developmental and reproductive toxicity:

• A review article in The Lancet states, “research has shown that exposure to DDT at amounts that would be needed in malaria control might cause preterm birth and early weaning … toxicological evidence shows endocrine-disrupting properties; human data also indicate possible disruption in semen quality, menstruation, gestational length, and duration of lactation.”^[24]
• Human epidemiological studies suggest that exposure is a risk factor for premature birth and low birth weight, and may harm a mother’s ability to breast feed.^[57] Some 21st-century researchers argue that these effects may increase infant deaths, offsetting any anti-malarial benefits.^[58] A 2008 study, however, failed to confirm the association between exposure and difficulty breastfeeding.^[59]
• Several recent studies demonstrate a link between in utero exposure to DDT or DDE and developmental neurotoxicity in humans. For example, a 2006 University of California, Berkeley study suggests that children exposed while in the womb have a greater chance of development problems,^[60] and other studies have found that even low levels of DDT or DDE in umbilical cord serum at birth are associated with decreased attention at infancy^[61] and decreased cognitive skills at 4 years of age.^[62] Similarly, Mexican researchers have linked first trimester DDE exposure to retarded psychomotor development.^[63]
• Other studies document decreases in semen quality among men with high exposures (generally from IRS).^[64][65][66]
• Studies generally find that high blood DDT or DDE levels do not increase time to pregnancy (TTP.)^[67] There is some evidence that the daughters of highly exposed women may have more difficulty getting pregnant (i.e. increased TTP).^[68]
• DDT is associated with early pregnancy loss, a type of miscarriage. A prospective cohort study of Chinese textile workers found “a positive, monotonic, exposure-response association between preconception serum total DDT and the risk of subsequent early pregnancy losses.”^[69] The median serum DDE level of study group was lower than that typically observed in women living in homes sprayed with DDT.^[70]
• A Japanese study of congenital hypothyroidism concluded that in utero DDT exposure may affect thyroid hormone levels and “play an important role in the incidence and/or causation of cretinism.”^[71] Other studies have also found the DDT or DDE interfere with proper thyroid function.^[72][73]

Other

Occupational exposure in agriculture and malaria control has been linked to neurological problems (i.e. Parkinsons)^[74] and asthma.^[75]

Carcinogenicity

DDT is suspected to cause cancer. The NTP classifies it as “reasonably anticipated to be a carcinogen,” the International Agency for Research on Cancer classifies it as a “possible” human carcinogen, and the EPA classifies DDT, DDE, and DDD as class B2 “probable” carcinogens. These evaluations are based mainly on the results of animal studies.^[1][24]

There is evidence from epidemiological studies (i.e. studies in human populations) that indicates that DDT causes cancers of the liver,^[24][36] pancreas^[24][36] and breast.^[36] There is mixed evidence that it contributes to leukemia,^[36] lymphoma^[36][76] and testicular cancer.^[24][36][77] Other epidemiological studies suggest that DDT/DDE does not cause multiple myeloma,^[24] or cancers of the prostate,^[24] endometrium,^[24][36] rectum,^[24][36] lung,^[36] bladder,^[36] or stomach.^[36]