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Study finds gene clues to African cattle disease

SCI & TECH
By | May 19th 2011

By PETER ORENGO

Scientists studying the tsetse fly-borne disease "sleeping sickness" and a devastating version found in cattle say they have found two genes that may rescue the livelihoods of millions of farmers in a tsetse fly-plagued regions Sub-Saharan Africa.

Research results from the International Livestock Research Institute (ILRI) in Africa and the University of Liverpool, were published in the Proceedings of the National Academy of Sciences (PNAS).

The researchers drew on the fact that while the humped cattle breeds characteristic of much of Africa are susceptible to disease-causing trypanosome parasites, a humpless West African breed, called the N’Dama, is not seriously affected by the disease.

In a vast tsetse belt across Africa, stretching from Senegal on the west coast to Tanzania on the east coast, and from Chad in the north to Zimbabwe in the south, the disease each year renders millions of cattle too weak to plow land or to haul loads, and too sickly to give milk or to breed, before finally killing off most of those infected.

This means that in much of Africa, where tractors and commercial fertilisers are scarce and prohibitively expensive, cattle are largely unavailable for tilling and fertilising croplands or for producing milk and meat for families.

The tsetse fly and the disease it transmits are thus responsible for millions of farmers having to till their croplands by hand rather than by animal-drawn plow.

Responses

The scientists used genetic approaches to distinguish differences between the trypano-tolerant N’Dama from West Africa, and trypano-susceptible Kenyan Boran cattle.

The scientists first identified the broad regions of their genomes controlling their different responses to infection with trypanosome parasites, but this was insufficient to identify the specific genes controlling resistance to the disease. So the scientists began adding layers of information obtained from other approaches. They sequenced genes from these regions to look for differences in those sequences between the two breeds.

Having been domesticated in Africa some 8,000 or more years ago, this most ancient of African breeds has had time to evolve resistance to the parasites.

This makes the N’Dama a valued animal in Africa’s endemic regions. But the N’Dama tends to be smaller, to produce less milk, and to be less docile than their bigger, humped cousins.

"The two genes discovered in this research could provide a way for cattle breeders to identify the animals that are best at resisting disease when infected with trypanosome parasites, which are transmitted to animals and people by the bite of infected tsetse flies," said author Steve Kemp, a geneticist on joint appointment with the Nairobi-based ILRI and University of Liverpool.

Sh400 billion

Although best known for causing human sleeping sickness, the trypanosome parasite’s most devastating blow to human welfare comes in an animal form, with sick, unproductive cattle costing mixed crop-livestock farmers and livestock herders huge losses and opportunities. The annual economic impact has been estimated at around Sh400 billion.

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