British scientists have come closer to devising a genetic test to tell which cattle have the form of sleeping sickness that kills people. The researchers say such a test would have substantial impact in controlling the disease in east Africa.

Sleeping sickness is not Africa's biggest killer, but its toll of 100,000 lives each year is still substantial.

The tse-tse fly transmits the illness, a parasitic infection that affects the nervous system. The strain that afflicts east and central Africa is an acute form that causes quick death if untreated.

The World Health Organization says the disease is becoming more common after nearly being eliminated in the early 1960s. That is an unwelcome by-product of development. A Scottish veterinarian who has studied the problem in Uganda, Sue Welburn, says as people acquire more livestock, a reservoir for the illness, they suffer more infections. "We hadn't expected to find so many animals infected with these parasites," he says. "But there hasn't really been any control for the last 10 years. I worked there 10 years ago and I hadn't expected things to be still, even though we have the disease, I hadn't expected it to be such a large reservoir of disease in these domestic livestock."

Now Dr. Welburn and colleagues at the University of Edinburgh have taken steps that could lead to a genetic test to identify which cattle carry the infection and could threaten people.

The problem is that livestock also carry a form of sleeping sickness that does not infect humans, but is indistinguishable under the microscope from the infectious kind. Dr. Welburn says a test that could make the distinction would help public health officials control the disease at the source, the cattle. This is crucial because the arsenic compound used to treat human sleeping sickness patients has severe side effects, killing up to 5 percent of those who take it. "So any test that we can have so we can determine which regions actually do have human infective parasites in their animals, then we can start to think about a management campaign to treat animals within a certain radius, for example, of a sleeping sickness case being reported," he says.

The test Dr. Welburn has in mind would draw blood from cattle to look for the presence of a gene in the parasite that lets the organism survive in people.

In a recent study in the journal "Lancet," Dr. Welburn and her University of Edinburgh colleagues confirmed that a parasite gene called SRA is the one that does this. When they analyzed sleeping sickness parasites in blood from 29 infected Ugandans, the SRA gene was present. It was also present in all eight cattle (parasite) samples that survived in human blood in laboratory dishes. The gene was not present in any of the 33 cattle parasite samples that human blood and its immune system molecules killed.

Dr. Welburn now is pursuing the next step. "It would be perfect if we can develop a diagnostic," he says. "I hope that we will be able now to move forward and develop some sort of card test that can be used at the district level or at the market level as a tool, really, to assist more appropriate methods of control."

Such a simple, portable test is necessary for poor regions because, as World Health Organization sleeping sickness control expert Jean Jannin says, the current genetic analysis techniques require sophisticated laboratories unavailable where the disease is endemic. "We cannot use this technique now for a large diagnosis of human population. So it's very interesting for research now and for getting more information about the circulation of the parasite, but it's not yet possible technique for direct diagnosis of population," she says.

Dr. Welburn in Edinburgh is working on the problem with support from the British government and donor organizations. But she says she still must find a drug company willing to invest in such a test for Africans.