Artificial cows effectively combat tsetse flies

Marilynn Larkin Contributing Editor The Lancet

A tsetse fly. Tsetse flies that infect cattle with nagana (animal trypanosomosis) have been "all but eradicated from Zimbabwe's farming areas" with the use of "artificial cows," says David Hall of the University of Greenwich, UK. Results of a recently completed project by Hall and co-workers documenting the demise of the tsetse flies were reported last month at the 26th meeting of the International Scientific Council for Trypanosomiasis Research and Control (Ouagadougou, Burkina Faso; Oct 1-5), which had as a special theme the launch of the Pan African Tsetse Eradication Campaign. "In Africa, cattle provide meat, milk, labour, and they're part of the culture, so the last thing you want to do is kill them," says Hall. "Wherever cattle are, people are. So if an area is infested with tsetse flies, then it's not really suitable for habitation because people won't go in without their animals." Artificial cows are devices impregnated with insecticides that attract tsetse flies by using kairomones-a blend of chemicals emitted by one species and detected by another. They don't look like cows because the flies "don't have very good eyesight, and in fact, they like a blue color best. So what they're seeing is a silhouette that smells right," explains Hall. "The cows are made of cotton cloth suspended on a metal frame. The cloth is a blue rectangle (1.8 x 1 m) of cloth with a central black stripe about 1 m wide. The black stripe is impregnated with insecticide - a deltamethrin suspension concentrate. Tsetse are attracted to the horizontal blue-black shape but land preferentially on the black region. So we can confine the insecticide to this region" explains Steve Torr. According to Torr, "Tsetse use host kairomones to locate their hosts by a process termed 'odour-mediated upwind anemotaxis'. In short, the tsetse detects the smell and responds by flying upwind. This response leads tsetse to the vicinity of the host. The final close-range location of the host is primarily a visual response to the shape and colour of the host - or target." The cows were introduced into Zimbabwe in the 1980s, when thousands of cattle were infected with nagana, and some 60 000 of the devices are now in use. Over time, nagana cases have plummeted-as has the use of aerial and ground pesticide spraying-and have remained at near-zero levels for the past 5 years. The cows have also been used effectively in Zambia, Kenya, and Ethiopia, says Hall, although it is more difficult to track data in these countries. The device has been most successful when deployed as a government operation, he notes. "The question is, is that sustainable? We've been looking at whether individual farmers or villages or groups of people can use it themselves. The device is relatively simple, you don't need many of them, and so in principle at least, people should be able to have them. But there's a bit of a problem because the tsetse fly is not like a crop pest, where a farmer can treat and protect his field and never mind to a certain extent what his neighbour is doing. Tsetse is a strong fly and it moves around a lot, so it's no good to have one farmer using the device by himself." Tsetse are still present in Zimbabwe but they are largely confined to the country's national parks and safari areas by Marilynn Larkin Contributing Editor The Lancet   MLEditor@aol.com LINK TO TLID: http://infection.thelancet.com/

 

November 2001