The cyclical transmission of strains ofTrypanosoma congolenseandT. vivaxresistant to normal therapeutic doses of trypanocidal drugs

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Abstract
Two strains of T. congolense and two strains ofT. vivax,resistant to normal therapeutic doses of trypanocidal drugs, were transmitted byG. morsitansandG. tachinoidesthrough series of cattle in the absence of drugs to assess the transmissibility of the trypanosomes by tsetse flies and the stability of drug resistance in the organisms during cyclical development.The characteristics of both strains ofT. vivaxand one strain ofT. congolensewere typical of the species, but the infectivity of the second strain ofT. congolensefor laboratory animals was abnormally low.The transmissibility of the trypanosomes was assessed on the bases of the infection rates in tsetse flies used to transmit the strains and the periods required for the development of the trypanosomes in the flies and bovine hosts. The transmissibilities of both strains ofT. vivaxand the first strain ofT. congolensewere normal by comparison with data obtained by cyclically transmitting non-resistant trypanosomes of the same species, but the period required for the development of the second strain ofT. congolensein tsetse flies was abnormally long.There was no loss of drug resistance in any of the strains examined in the experiments. A strain ofT. congolenseretained resistance to diminazene aceturate at a dose rate of 7-0 mg/kg, homidium chloride at 2-0 mg/kg, quinapyramine sulphate at 4-4 mg/kg and isometamidium chloride at 0-5 mg/kg during a series of eight transmissions by tsetse flies in a period of 376 days.