Dose and stage dependency for the development of local skin reactions caused by Trypanosoma congolense in goats.

Intradermal inoculation of metacyclic forms of Trypanosoma congolense propagated in vitro caused skin reactions in goats similar to the local skin reaction (chancre) induced by the bite of an infected tsetse fly. The onset, size and duration of these local skin reactions were dose-dependent. Whereas one cultured metacyclic T. congolense was sufficient to cause a local skin reaction in a goat, over 10(7) bloodstream forms of T. congolense were necessary to elicit a detectable skin reaction and while T. congolense parasites present in lymph did not cause local skin reactions, trypanosomes collected from oedematous fluid of the chancre did. - Using non-dividing irradiated bloodstream forms it was estimated that 10(8) T. congolense were required to induce a detectable local skin reaction. - Intradermal needle inoculation of procyclic forms (uncoated trypomastigotes) of T. congolense propagated in vitro induced an intense inflammatory response which was similar to that found in the early phases of the reaction elicited by metacyclic trypanosomes. This suggests that the uncoated trypomastigotes which are known to be present in the saliva of infected tsetse may play a role in the initial development of the chancre. - The data obtained for the local skin reaction suggest the presence of an intracutaneous dividing stage of T. congolense which is intermediate between the metacyclic and bloodstream forms.