Sponging cattle with insecticide halves malaria incidence in study

Malaria is endemic in much of southern Asia, and current methods for controlling the disease are limited by cost considerations. But a study published 9 June in The Lancet shows, for what is believed to be the first time, that sponging livestock with insecticide might offer a far cheaper but just as effective a method, which may also have added benefits.

One method health workers use to control malaria in southern Asia is insecticide spraying inside homes. This method is expensive, since it uses large amounts of insecticide. In 1999, a research team led by Dr Mark Rowland of the London School of Hygiene and Tropical Medicine, in the UK, showed that treating livestock with insecticides could kill mosquitoes. Because the two species of mosquitoes that spread malaria in southern Asia — Anopheles stephensi and A. culicifacies — feed preferentially on domestic animals, Rowland's group theorized that treating livestock should reduce the spread of malaria in this region.

The researchers tested their theory in six Afghan refugee settlement villages in north- west Pakistan. They randomly assigned the villages to two groups. Livestock in the villages assigned to the first group were treated with insecticide during the 1995 and 1997 malaria seasons but not in 1996, while livestock in the second group of villages received the insecticide treatment during the 1996 season but not in 1995 or 1997. In this crossover design, each village acted as its own control. Researchers supervised the villagers as they sponged the livestock with the insecticide deltamethrin. At the end of the study, the researchers compared malaria incidence in each village during livestock treatment seasons with incidence in seasons without livestock treatment.

The results were dramatic. Treating livestock with insecticide produced a 56% fall in the incidence of falciparum malaria, the most deadly form of the disease, and a 31% fall in the incidence of vivax malaria, a disabling but rarely fatal form. The livestock insecticide method was thus as effective as standard indoor spraying, but the cost was 80% less — US$ 0.34 per person protected vs US$ 0.07 for the animal sponging method.

"The livestock method should be good for epidemic control since it can be done more quickly than indoor spraying," Rowland commented to the Bulletin. Though he says the method could stem malaria in much of southern Asia, he is quick to point out that it will only work in regions where mosquitoes prefer feeding on livestock. That's not the case in sub-Saharan Africa, for example, where malaria is spread primarily by A. gambiae mosquito species which don't home in on livestock but prefer to feed indoors on humans. Further testing is needed to find out if the method will work against livestock- loving mosquitoes in Africa, China and South America, he says.

Local people must be committed to the programme, since all the domestic animals in a village must be treated and the insecticide must be applied every six weeks. That wasn't a problem in the test villages, where people were eager to continue with the regimen when they noticed their animals thriving. Indeed, the researchers found significant weight gains among treated cattle and in some villages there were increased milk yields, though these gains weren't statistically significant.

Deltamethrin, the insecticide used in the study, is one of the safest insecticides known, says Rowland. But epidemiologist and malaria expert Dr Syed Jamil Hasan Kazmi at the University of Karachi in Pakistan says that deltamethrin does pose a danger to humans if ingested orally. He says further studies should investigate whether the milk and meat of treated animals contain pesticide residues. Dr Bernard Nahlen of the WHO-based Roll Back Malaria Initiative adds: "Monitoring of insecticide resistance and potential changes in mosquito feeding behaviour will also be important in this type of malaria control activity which relies on the use of insecticides".

Kazmi praises the new study, and believes its findings could greatly benefit rural areas of southern Asia. "However, its effectiveness in urban areas is still questionable. Urban areas like Mumbai, Karachi, New Delhi, Decca, etc. are the growing hotspots of malaria and there are no cattle available for the lethal lady [female mosquito] so she has to rely on human beings," says Kazmi. Still, plenty of regions in southern Asia do host live- stock and in these areas Rowland's method could greatly reduce the cost of controlling malaria.

Christie Aschwanden,
Nederland, Colorado, USA

World Health Organization Genebra - Genebra - Switzerland