Relapse of monomorphic and pleomorphic Trypanosoma brucei infections in the mouse after chemotherapy.

Infections in mice were initiated with trypomastigotes from two lines of Trypanosoma brucei derived from the same primary isolated. Infections with one line were initiated by inoculation of metacyclic trypomastigotes from infected tsetse flies and the resulting infections were pleomorphic. The other line had been passaged 32 times in rodents and inoculation of bloodstream trypomastigotes gave rise to monomorphic infections. In both infections there were high levels of parasitaemia until death up to 4 weeks later if the infection was untreated. It was shown that after chemotherapy with 40 mg/kg diminazene aceturate (Berenil) relapses occurred in both types of infection after an aparasitaemic period of 2--3 weeks. Further, it was shown that 3 days after chemotherapy, brain tissue but neither spleen, liver nor blood was capable of transferring infection to normal recipient mice. There were two major differences in the response of the two infections to chemotherapy. First, treatment of the pleomorphic infection as soon as day 6 after infection resulted in a subsequent relapse while the monomorphic infection had to be at least 12 days old at the time of treatment before occurred. Second, following treatment of the pleomorphic, but not of the monomorphic infection there was an early transient recrudescence of low numbers of trypanosomes which were found to be non-infective to recipient mice. The early transient relapse was followed by a further aparasitaemic period and then the late continuous relapse characterised by large numbers of infective trypanosomes.