Chemoprophylaxis Against Bovine Trypanosomiasis

Resistance to diminazine aceturate by Trypanosoma congolense from cattle in the Zambezi Valley of Zimbabwe

The susceptibility of 14 stocks of Trypanosoma congolense, recently isolated from cattle, to therapeutic doses of diminazene aceturate and to isometamidium chloride was assessed in laboratory mice. Eight isolates were readily susceptible to the normal therapeutic dose of diminazene, two were resistant to the drug at 14 mg kg-1, and four were totally resistant at 28 mg kg-1. All the isolates were susceptible to isometamidium chloride at 0.5 mg kg-1. These observations highlight the need for regular evaluation of drugs used in the control of trypanosomosis.

Trypanosoma congolense: manifestation of resistance to Berenil and Samorin in cloned trypanosomes isolated from Zambian cattle

Four Trypanosoma congolense clones derived from a Mumbwa field isolate proved to be resistant to Berenil with a minimum curative dose (MCD) value of 40 mg/kg and to Samorin with an MCD of 4 mg/kg for mice. Two other clones, one being resistant to Berenil with an MCD of 45 mg/kg but susceptible to 1 mg/kg Samorin, and the other being resistant to Samorin with MCD of 16 mg/kg but susceptible to 7 mg/kg Berenil, were experimentally rendered resistant to each of the respective drugs they were susceptible to by subcurative treatments in mice. The original trypanosome strains and their derivative clones were then screened for their sensitivity to Berenil or Samorin. Three clones derived from the Mumbwa isolate were resistant to Berenil, with MCD's of 14 to 28 mg/kg, and to Samorin, with MCD's of 4 mg/kg. A single Mumbwa derivative clone was relatively sensitive to both Berenil with an MCD of 7 mg/kg and to Samorin with an MCD of 2 mg/kg. The reciprocal drug induction results confirmed that although trypanosomes can acquire tolerance to both Berenil and Samorin, no cross-resistance between the two was evident.

The cyclical transmission of strains of Trypanosoma congolense and T. vivax resistant to normal therapeutic doses of trypanocidal drugs

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.