Early events following challenge of cattle with tsetse infected with Trypanosoma congolense: development of the local skin reaction.

A local skin reaction (chancre) was elicited in susceptible cattle after the successful feeding of Glossina morsitans morsitans infected with one of two different cloned isolates of Trypanosoma congolense. The chancre first appeared as a small 2 to 3 mm nodule at the site of the challenge as early as day 5 and reached maximum activity by days 10 to 13 when it had developed into a raised, indurated, hot, painful swelling measuring up to 100 mm in diameter. Thereafter it declined in size and activity and by days 20 to 30 was undetectable. Histologically the lesion was characterised by an intense inflammatory reaction and a four- to 10-fold increase in total cellularity. Initially, polymorphonuclear leucocytes were numerous but these were soon replaced by a mononuclear cellular infiltrate consisting mainly of small to medium lymphocytes. Development of the chancre and detection of parasites in the skin preceded by several days parasitaemia and other clinical signs. It was concluded that the skin was acting as a focus not only for establishment of infection but also as a site for localised proliferation of the parasite before dissemination into the bloodstream. The bite of an uninfected tsetse fly produced no detectable reaction and experimental intradermal inoculation of metacyclic T congolense resulted in chancre formation followed by infection. Bloodstream forms given by the same route caused infection but failed to induce a chancre.