The organochlorine compound DDT (Dichloro-Diphenyl-Trichloroethane) was first synthesized in 1874, but its insecticidal properties were not discovered until 1939 by the Swiss scientist Paul Muller, who was awarded the 1948 Nobel Prize in Physiology and Medicine for his efforts. DDT kills by opening sodium ion channels in insect neurons, causing the neuron to fire spontaneously. This leads to spasms and eventual death. Insects with mutations in their sodium channel gene or with up-regulation of genes expressing cytochrome P450 may become resistant to DDT and similar insecticides.
In the early years of World War II DDT was used with great effect to combat mosquitoes spreading malaria, typhus, and other insect-borne human diseases among both military and civilian populations. After the war, DDT was made available as an agricultural insecticide, and its production and use skyrocketed.
In 1955 the World Health Organization began a program to eradicate malaria worldwide, relying largely on DDT. Though this effort was initially highly successful (reducing mortality rates from 192 per 100,000 to a low of 7 per 100,000), resistance soon emerged in many insect populations as a consequence of the widespread agricultural use of DDT. In the 1960s, the environmental impacts of indiscriminate spraying of DDT became known. As a persistent organic pollutant, DDT accumulated in the food chain and had severe effects on fish, amphibians, birds, and rather less well known impacts on mammals, including humans. DDT can still be found in the fat reserves of polar bears, penguins, and possibly you, thousands of miles away from where it was ever sprayed. In 1987 the US EPA classified DDT as a probable human carcinogen. DDT is also known to be an endocrine disruptor and to cause developmental problems in infants.
In the 1970s and 1980s, agricultural use of DDT was banned in most developed countries, in 1970 in Scandinavia, 1972 in the USA, but not until 1984 in the UK. The Stockholm Convention which came into effect in 2004 outlawed several persistent organic pollutants, and restricted the use of DDT to the control of insect vectors of human diseases. After these bans, the populations of many severely threatened species, such as the American bald eagle, rebounded.
In September 2006, the World Health Organization announced that DDT will be used as one of the three main tools against malaria, and recommended indoor spraying in epidemic areas and places with high malaria transmission. USAID now funds the use of DDT overseas. DDT sprayed inside a home provides protection from mosquitoes for up to six months. New studies show that despite mosquito resistance to DDT, it also acts as a powerful insect repellent.
Malaria afflicts between 300 million and 500 million people each year. The World Health Organization estimates that around 1 million people die of malaria and malaria-related illness every year, with 90% of these deaths in Africa, mostly in children under the age of five. To put that in perspective, that is equivalent to the death toll of around ten of the nuclear bombs dropped on Hiroshima during World War II. Malaria also weakens the economies of poor countries. People who become infected cannot work or die. Infected children can suffer brain damage. The World Bank estimates that malaria costs Africa more than US$100 billion annually and this cost is growing by 1.3 per cent each year. In 2004, when Uganda publicly contemplated reintroducing DDT to fight malaria, the European Union made threats that the country’s US$32 billion agriculture exports could be at risk if tough new measures were not taken to ensure DDT residues did not find their way into food crops.
As a result of the WHO program, the number of African countries spraying DDT inside houses has exploded. Eritrea, Madagascar, Ethiopia, Swaziland, Senegal, Ghana, Angola, South Africa, Mauritius, Mozambique, Zimbabwe, Namibia, Zambia and Burkina Faso are all using the chemical. Uganda, where more than 100,000 people died from malaria in 2006, began spraying it this year in a pilot project, and Tanzania and Malawi may follow. But Rwanda, Burundi and Kenya (a major producer of pyrethrum, the main alternative to DDT) are so far refusing to adopt the use of the chemical. In 1995, South Africa stopped spraying DDT to control malaria, citing international pressures, but as soon as the ban started, the incidence of malaria rose.
DDT is cheap. Safer pyrethrum-based insecticides are 20 times more costly, often too expensive for developing countries. The price of controlling malaria in Africa has been estimated at US$1 billion per year, but foreign aid targeting the disease has never topped US$200 million.
So my question to you is this: imagine you are the president of the world, but with a limited budget. What would you do?
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This article reviews recent research on Rift Valley fever virus (RVFV) infection, encompassing four main areas: epidemiology and outbreak prediction, viral pathogenesis, human diagnostics and therapeutics, and vaccine and therapeutic candidates. RVFV continues to extend its range in Africa and the Middle East. Better definition of RVFV-related clinical syndromes and human risk factors for severe disease, combined with early-warning systems based on remote-sensing, simplified rapid diagnostics, and tele-epidemiology, hold promise for earlier deployment of effective outbreak control measures. Advances in understanding of viral replication pathways and host cell-related pathogenesis suggest means for antiviral therapeutics and for more effective vaccination strategies based on genetically engineered virus strains or subunit vaccines. RVFV is a significant health and economic burden in many areas of Africa, and remains a serious threat to other parts of the world. Development of more effective methods for RVFV outbreak prevention and control remains a global health priority.
