Histopathological studies were made on cattle and rabbits, artificially infected with either proliferative organisms of low and high passage level bovine strains of Besnoitia besnoiti (Marotel, 1912) grown in tissue culture and in rabbits, or cyst organisms from a natural bovine case of besnoitiosis. Rabbits similarly infected with either proliferative organisms of a low level rabbit-passaged blue wildebeest [Connochaetes taurinus (Burchell, 1823)] strain or cyst organisms from naturally infected blue wildebeest and impala [Aepyceros melampus (Lichtenstein, 1812)] were likewise used for comparative studies. Degenerative and necrotic vascular lesions, vasculitis and thrombosis, mainly of the medium and smaller veins and some arteries, were the most outstanding changes in the acute stages of the disease. These changes coincided with the parasitization of certain cells in the vessels such as the endothelium, where the organisms proliferate before the onset of the cystic stage. These basic lesions were responsible for oedema, degenerative changes and even infarction, particularly in the testes and skin. A histiocytic reaction and mild eosinophil infiltration were some of the other characteristic features. The cystic stage in cattle apparently developed in enlarged histiocytes, which were recognizable 11 days after infection. These host cells became multinuclear and seemed to be responsible for the production of the cyst wall. The cysts reached maturity 71 days after infection. Reactions to cysts apparently commenced before any degenerative changes in the organisms were detectable and could have been preceded by such changes in the cyst wall. The antelope strains of B. besnoiti were only very mildly pathogenic to rabbits, but passage during the acute stage of the disease in this host increased the pathogenicity considerably. Rabbits which were infected with bovine strains developed severe testicular and skin lesions, but these lesions were either absent or relatively mild in rabbits infected with antelope strains. In the latter, lesions were usually confined to internal tissues and organs such as the myocardium, gut and lungs and, more rarely, even in the adrenal, liver, kidneys and urinary bladder. However, cysts were rarely encountered in both these groups.
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