A vital vaccine

The decoding of genetic information from one of Africa's most destructive parasites should speed the development of a cost-effective vaccine for East Coast fever - a major constraint to cattle production in sub-Saharan Africa. Scientists from the United States (TIGR) and Kenya (ILRI) have combined forces in order to identify the proteins that enable the parasite, Theileria parva, responsible for causing the disease, to invade the white blood cells in cattle. In the long-term, the research may also have implications for human health by assisting scientists to develop a vaccine against malaria and to improve understanding of cancers.

credit: FAO

The vector for the single-celled protozoan is the Brown Ear Tick (Rhipicephalus appendiculatus). Once inside the animal, the Theileria parasites invade the white blood cells of the lymphatic system, where they multiply and interfere with the animal's immunity. It then not only becomes vulnerable to secondary infections but is affected by the uncontrollable division of the white blood cells, which clog vital organs and ultimately results in death between 2-4 weeks after initial infection. The parasite is estimated to cost farmers in Africa more than US$170 million each year in direct losses and more than 24 million cattle are at risk of contracting the disease.

In Zambia, more than 200,000 animals have been wiped out as a result of the disease during the past six years. The disease is more prevalent in the southern part of the country where a vaccine, which has proved effective in other parts of the country, has failed to work. Since the vaccine was introduced in 1987, the mortality rate in new-born animals has been reduced from 40% to less than 5% in the Eastern Province. But, according to Bruce Makanda, Principal Veterinary Officer for the Ministry of Agriculture, the vaccine is not effective on all strains of the parasite and control of the disease in the southern part of the Zambia is proving more difficult.

Use of a vaccine has also been used as part of a multidonor Programme for Integrated Tick and Tickborne Disease Control in Eastern, Central and Southern Africa. The trivalent vaccine, a mixture of three strains that helps to provide the broadest possible immunity against the parasite, is not flawless as it only provides immunity in 80-85% of animals which are immunized. However, for subsistence farmers unable to afford expensive acaracides to prevent tick infestation and antibiotics to treat the fever, the relatively small cost of vaccination is often preferable. Re-stocking with improved indigenous breeds in Uganda, for instance, is also being encouraged as they tend to develop an immunity against East Coast fever in comparison to exotic cattle, which prove to be particularly vulnerable to the disease.

However, many farmers do not have access to a vaccine and those that can afford acaricides remain dependent on regular use of sprays or dips to kill ticks in an attempt to prevent their cattle being affected by the disease. Whilst these chemicals can be damaging to the farmers health and the environment, overuse results in tick resistance (see Undramatic loss?). But it is not the tick itself that is the problem, so effective control of East Coast fever depends on the development of a therapy which treats the parasite and not the vector. As a small number of animals infected with East Coast fever recover naturally, it is possible that the development of a genetically engineered vaccine should be feasible. ILRI scientists intend to use the genetic data provided by TIGR to produce what is known as a 'sub-unit' vaccine. These are built with one or more protein molecules of the parasite, in comparison to using the whole live parasite, and are potentially more potent, cheaper and more easily stored than conventional vaccines. If successful, this novel vaccine would inject up to US$300 million annually into the economies of the 11 countries where East Coast fever is currently widespread.

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