By Richard Black BBC Science correspondent

African cattle: Millions fall prey to parasites every year

Researchers in Kenya say they have identified a number of genes which make cattle resistant to the disease, and believe they can now develop breeds which are both resistant and productive.

They report their findings in the US journal Proceedings of the National Academy of Sciences.

Sleeping sickness in cattle costs Africa billions of dollars each year in lost agricultural productivity.

It is caused by parasites called trypanosomes, which are spread by the tsetse-fly.

There are breeds which have some resistance to the trypanosomes - and that resistance must be down to their genes.

Scientists at the International Livestock Research Institute in Nairobi have now identified resistance genes in two different breeds: the N'Dama and the Boran.

Dr John Gibson says his team aims to create new breeds which are more resistant.

"This provides a really exciting possibility that through a cross-breeding programme, we could combine these factors and obtain a breed which will get infected, but they will recover relatively rapidly, and they will continue to grow and reproduce and produce milk," he said.

It is an intriguing concept, combining traditional cross-breeding with modern genetic science - and letting the genetics rather than the appearance guide which breeds you combine and which offspring you then work with.

It has been done in Africa before, combining various strains of rice to create the hardy Nerica variety.

Dr Gibson's team aims to develop cattle which are not only trypanosome-resistant, but which look strong and give good yields of milk - traits which farmers not surprisingly also find attractive.