SINGLE shots of a genetically engineered vaccine have successfully protected livestock in Kenya against two lethal viral diseases, rinderpest and capripox, which kill millions of animals each year. The British and Kenyan researchers who developed the vaccine hope that one day it will help eradicate the diseases entirely.
Cattle with rinderpest salivate profusely, and they die as a result of the dehydrating effects of severe diarrhoea. Capripox virus infection leads to 鈥渓umpy skin disease鈥�. Unsightly lumps appear on the animals鈥� hides, making them unsaleable. They eventually die from the effects of lumps that develop internally. The two viruses cause similar conditions in sheep and goats.
Because rinderpest and capripox flourish in the same areas, a vaccine that prevented both would be especially valuable. Existing capripox vaccines leave painful sores in 20 per cent of animals, while rinderpest vaccines are unstable and can lose their effectiveness in the hot climates where they are in greatest demand.
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Donald Black, head of the capripox virus group at the Institute for Animal Health in Pirbright, Surrey, says that results with the double-barrelled vaccine in Kenya have been very promising. 鈥淭he majority of the animals we injected were solidly protected,鈥� he says. Some showed mild symptoms when injected with the viruses up to a year after vaccination, but all recovered fully. All the unvaccinated animals died.
Black began preparations to test the vaccine in Kenya three years ago, in collaboration with the Kenya Agricultural Research Institute in Muguga. The vaccine consists of a harmless form of the capripox virus, which protects against the form that causes lumpy skin disease. To make the harmless virus effective against rinderpest as well, Black inserted rinderpest genes into it. These genes manufacture the rinderpest fusion and haemagglutinin proteins, which have been found to trigger the strongest immune responses in animals.
After proving that the vaccine worked in British cattle, Black began testing it in Kenya, supported by Britain鈥檚 Overseas Development Administration (Technology, 23 October 1993). He is currently evaluating it in 90 animals 鈥� 30 zebu cattle, which are native to Kenya, 30 sheep and 30 goats. 鈥淲e bought all the animals on local markets, so they are just what ordinary Kenyan farmers would have,鈥� says Black.
Already the vaccine has been shown to protect animals against both viruses one month, six months and 12 months after vaccination. Black and his colleagues are now tinkering with the vaccine in a bid to make it even more effective. They hope to insert a different molecular 鈥渟witch鈥�, or promoter, into the vaccine to activate the rinderpest genes. The one in the existing vaccine is a 鈥渓ate promoter鈥�, so called because it switches on the genes late in the life cycle of the virus. Replacing it with an 鈥渆arly promoter鈥� may trigger a stronger immune response.
The capripox research group at Pirbright is due to be closed down in April, but Black hopes the ODA will continue to fund the trials. He has submitted his results for publication in Epidemiology & Infection.
