The initial stage of infection with cyclically-transmitted Trypanosoma congolense in rabbits, calves and sheep

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Glossina morsitans infected with a stock of Trypanosoma congolense were used to infect rabbits, calves and a sheep. Local skin reactions developing after 6 to 10 days at the sites of infected tsetse bites were removed surgically or post mortem, and the tissues processed and examined for trypanosome distribution and pathological change by light, electron and fluorescence microscopy. Local reactions from rabbits were examined at intervals between 7 and 19 days after infection and from calves between 8 and 30 days after infection. A single reaction from a sheep was examined 10 days after infection. Observations were also made on the presence of trypanosomes in lymph nodes and blood from the infected animals. Substantial numbers of trypanosomes were located in the collagen of the deep dermis and near the bases of hair follicles following infection in all three host species. The parasites were numerous in reactions examined between days 7 and 12 and calf tissues were most heavily infected. Trypanosomes were also seen in lymphatic vessels of the skin and in superficial lymph nodes draining local reaction sites up to day 10. Although many trypanosomes in local reactions were destroyed by extracellular lysis, a few parasites were detectable in the dermal collagen until day 19 in rabbits and day 30 in calves. The multiplication and development of trypanosomes in each host was accompanied by focal disorganization and degeneration of infected areas of dermal collagen. Reactions examined 7 to 15 days after infection showed acute inflammatory and exudative changes with extensive mononuclear cell infiltration particularly in areas showing collagen necrosis and in the vicinity of dermal blood vessels. Macroscopically, reactions left in situ gradually disappeared by about day 16 in rabbits and day 21 in calves. Histological examinations confirmed that inflammatory changes subsided between days 14 and 21, leaving extensive areas of newly formed collagen and persistent perivascular and tissue foci of mononuclear cells.