The production of heartwater vaccine.

It has long been recognized that animals which re­ cover from heartwater acquire an immunity to the disease. Based on this, numerous attempts at producing a vaccine have been made. The early trials were generally unsuccessful and are adquately described by Alexander (1931 ). It was in the early years of this century that Hutcheon (1902) and Spreull (1904) considered the use of blood of animals suffering from heartwater as an in­ oculum. It was, however, only in 1931 that Alexander used the correct combination of inoculum (infected blood) and route of administration (intravenous) . Neitz (1940), through the discovery that heartwater could be treated with sulphonamides , and Neitz & Alexander (1941) through their observations that young calves and lambs are relatively insusceptible to heartwater, deve­ loped a feasible, although cumbersome, method of vac­ cination. Later, Bezuidenhout (198 I) introduced the use of homogenized Cowdria ruminantium infected Amblyomma hebraeum nymphs as inoculum in an attempt to reduce the production costs of the vaccine. This did not , however, solve the problems caused by these both being live, virulent vaccines. Althou~h infected brain material has been found to be infective 1f administered subcutaneously (llemobade & Blotkamp, 1978), only blood and nymph suspension have ever been produced and used commercially . The method of production of the blood vaccines has been described in relative detail (Anon., 1984; Arnold & Kanhai, 1979). Uilenberg (1983) briefly reviewed both methods of production and their limitations. PREsENT-DAY TEcHNIQUES ~e techniques ~sed to produce the blood and n~mph vaccme~ are essentially .the sa~e. In fact, i\is only m the p~oductwn of the workmg antigen that the 2 techniques d1ffer. F?r th~s reason the production techniques will be dealt with simultaneously and only the differences emphasized. Stock antigen The production of the stock antigen involves 3 distinct steps: (a) The isolation of strains or isolate A number of ways by which field strains can be iso­ lated have been listed (Anon, 1984). These include the collection of blood from suspected cases of heartwater, pooling of blood from animals in endemic areas and the use of brain emulsions from animals which have died of heartwater, as described by Ilemobade & Blotkamp (1978). Other sources which have been successfully em­ ployed for this purpose are lymph nodes (Du Plessis &