The relationship of Besnoitia encountered in blue wildebeest and impala to B. besnoiti of cattle was investigated by studying the susceptibility of rabbits, cattle and sheep to infection with antelope strains, and their subsequent immunity to a challenge with bovine strains. Rabbits were susceptible to infection and a strain from blue wildebeest has been maintained through 27 serial passages by subinoculation of blood during the reactions. Reactions were considerably different from those that have been experienced with bovine strains. The typical skin lesions did not develop; it was much more difficult to find proliferative forms microscopically in blood smears, even in fatal cases but they were plentiful in internal organs of the latter. No cysts were found in the skin or other tissues. Initially the reactions were very mild, but the strain appeared to adapt itself to the new host and eventually became highly pathogenic. Cattle were also susceptible to infection with antelope strains, but developed mild reactions. Some showed a febrile response, whereas in others immunity to challenge was the only indication of infect ion. Proliferative organisms were seen in blood smears of only one of the eight animals infected and no cysts could be found in any of them. Sheep were infected with a strain from blue wildebeest. They developed fairly severe reactions from which all recovered. No proliferative organisms were seen in blood smears, but small numbers of cysts were found in the peripheral veins and nasal mucosa. In spite of the abovementioned distinctiveness in biological behaviour, infection with these strains from antelopes conveyed an immunity to challenge with bovine strains in the three host-species used. This indicates a close immunogenic, and hence taxonomic relationship between the three strains. It is therefore suggested that the parasites of blue wildebeest, impala and cattle be regarded as distinct strains or biological races of B. besnoiti.
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