Under the Microscope: Asked to name a dangerous insecticide, most people would say DDT. It is equally likely that most people would be unaware that DDT has saved millions of lives because of its ability to kill the mosquito that transmits malaria.
UN efforts now underway to eliminate worldwide use of DDT may unjustly penalise poor tropical countries where DDT may be the only viable weapon to combat endemic malaria. Andrew Spielman and Michael d'Antonio have written a comprehensive book called Mosquito: A Natural History of Our Most Persistent and Deadliest Foe (Faber and Faber, 2001).
Malaria affects 40 per cent of the world's population and is endemic in about 100 countries. After dysentery and tuberculosis, malaria is the third deadliest world disease. Between 300 and 500 million people are infected by malaria every year, of whom over one million die, mostly children under five in Asia and Africa.
The word malaria comes from the Italian mal aria because the Romans believed that malaria was caused by bad air. We now know that mosquitoes of the genus Anopheles transmit malaria. The malaria is transmitted through the bite of the female who injects a parasite into the human victim's blood.
Malaria has been a scourge since ancient times. Alexander the Great died from malaria in 323 BC. In 1897, about 40 per cent of the 178,000-strong British army in India was stricken with malarial fever. These diseases not only ravaged tropical areas in Third World countries but also caused epidemics in the southern US. From the late 1800s to the mid-1900s successful anti-malarial campaigns were carried out in the US and elsewhere. Mosquito breeding grounds were targeted and it was shown that areas could be rendered free of mosquito-borne infection.
The most famous malarial fighter was the American Dr Fred Soper. In 1938, an outbreak of malaria in Brazil killed 20,000 people. The government called in Dr Soper, whose team of 40,000 workers fumigated houses with a natural pesticide and sprayed a mixture of diesel oil and copper acetoarsenite on pools of water. In 22 months he eradicated the mosquito from an area of 18,000 square miles.
The Swiss company, J.R. Geigy, started to manufacture dichloro-diphenyl-trichloroethane (DDT) in the late 1930s. It was soon shown to be the best anti-malarial insecticide and it was used extensively in the second World War to protect allied troops in Asia. Dr Soper controlled the malarial mosquito in Sardinia with DDT and he declared that it would be possible to clear the mosquito from the entire world.
The use of DDT against malaria peaked in the 1950s and the 1960s. Spraying houses with DDT reduced Sri Lanka's malaria from 2.8 million cases and 7,300 deaths to 17 cases and zero deaths. However, in 1962 Rachel Carson published her book, Silent Spring, in which she challenged the widespread assumption that DDT is safe. Carson pointed to the phenomenon of food-chain concentration. This means that, in watery environments, the smallest organisms concentrate tiny amounts of chemical pesticides in their bodies, passing them up the food chain to the largest predators. In this way DDT could eventually end up in mother's milk. Public opinion turned against DDT, and the US declared the use of DDT illegal in 1972. But Rachel Carson made no mention of the fact that the malaria fighters had saved 10 million lives through the use of DDT.
In the July 2000, Vol. 6, No. 7 edition of Nature Medicine, Amir Attaran and others argue that the benefits of DDT in saving lives from malaria in tropical countries are worth the risks. It is true that DDT has caused toxic effects in the non-human environment. Bio-accumulated DDT can thin eggshells and cause reproductive failure in birds of prey, but this resulted from huge agricultural use of DDT.
Dusting a 100-hectare cotton field calls for over 100,000 kilograms of DDT in four weeks. DDT spraying for malaria control is far less intensive. The interior surfaces of houses at risk are sprayed once or twice a year, leaving a residue of DDT on the walls. Half a kilogram treats a large house. Guyana's entire high-risk population for malaria can be protected with the DDT that might otherwise be used to protect a 0.4 square kilometre field of cotton in a season.
Evidence from animal studies indicates that DDT can cause neurological and immune problems and some epidemiological studies have linked human cancers and hormone-disrupting effects, such as reduced lactation in women. But there is much room for doubt. Most reports correlating DDT exposure with elevated risk of cancer or reduced lactation have not been replicated despite repeated attempts.
DDT is an affordable and effective anti- malarial agent for poor tropical countries, Attaran argues. The national malaria-control budget for India, using DDT, is sufficient to cover only 65 per cent of high-risk persons. Switching to Malathion, the next-cheapest alternative, reduces that coverage to 21 per cent, leaving 71 million more people unprotected.
Poor, heavily indebted countries must make poverty reduction their top priority and the stimulation of foreign investment will require the near elimination of malaria from economically productive zones. Any insistence to do so without DDT could impoverish countries no less than past colonial exploitation. William Reville is associate professor of biochemistry and director of microscopy at UCC