A strain of Trypanosoma congolense directly resistant to Berenil

Long-term occurrence of Trypanosoma congolense resistant to diminazene, isometamidium and homidium in cattle at Ghibe, Ethiopia

Ten trypanosome isolates were collected at random from cattle at Ghibe, Ethiopia, in February 1993 and all shown to be savannah-type Trypanosoma congolense. When inoculated into naïve Boran (Bos indicus) calves, all 10 isolates were resistant to diminazene aceturate (Berenil), isometamidium chloride (Samorin) and homidium chloride (Novidium) at doses of 7.0 mg/kg body weight (b.w.), 0.5 mg/kg b.w. and 1.0 mg/kg b.w., respectively. In order to determine whether this multiple-drug resistance was expressed by individual trypanosomes, clones were derived from two of the isolates and characterised in mice for their sensitivity to the three compounds; by comparison to drug-sensitive populations, the two clones expressed high levels of resistance to all 3 trypanocides. In experiments to characterise the uptake kinetics of [14C]-Samorin, the maximal rates of uptake (Vmax) for 4 Ghibe isolates ranged from 9.2 to 15.0 ng/10(8) trypanosomes/min. In contrast, Vmax for the isometamidium-sensitive clone T. congolense IL 1180 was 86.7 +/- 8.6 ng/10(8) trypanosomes/min. Lastly, molecular karyotypes were determined for eight isolates: seven different chromosome profiles were observed. These data indicate that in February 1993 there was a high prevalence of drug-resistant trypanosome populations with different chromosome profiles in cattle at Ghibe. Since a similar situation existed at the same site in July 1989, this suggests that the drug-resistance phenotype of trypanosomes at Ghibe had not altered over a 4 year period.

Pharmacology of diminazene: a review

Chemotherapy for trypanosomiasis in domestic livestock depends on only a few compounds, of which several are chemically closely related. Of these compounds, the most widely used therapeutic agent in cattle, sheep and goats is diminazene aceturate. Diminazene was first described in 1955. Subsequently, a substantial body of data has been generated on various pharmacological aspects of the compound. In this review, we consider the current status of knowledge concerning the therapeutic spectrum of diminazene, resistance to diminazene in trypanosomes, and combination therapeutic regimens in which diminazene has been administered together with other compounds. Analytical techniques for diminazene, the pharmacokinetics of diminazene, data concerning diminazene's toxicity, and the different molecular mechanisms by which diminazene may exhibit trypanocidal action are also considered.

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.

Trypanosoma congolense: drug resistance during cyclical transmissions in tsetse flies and syringe passages in mice

A drug-resistant Trypanosoma congolense strain with predetermined curative doses (CD50 and CD90) of samorin at 13.9 +/- 1.02 and 20.3 +/- 1.13 mg/kg body weight, respectively, was cyclically transmitted through tsetse flies and by syringe passages in mice in the absence of drug pressure. The changing levels of drug sensitivity were determined after every 3rd cyclic and 5th syringe passage intervals. It was noted that when the strain was maintained in tsetse flies through 12 cyclical transmissions, the CD50 and CD90 dropped slightly from 13.9 to 11.9 +/- 1.06 and from 20.3 to 18.0 +/- 1.08 mg/kg body weight, respectively. This decrease in the level of resistance was not significant (P greater than 0.05). However, when the trypanosomes were maintained by syringe passages in mice, there was a significant reduction (P less than 0.05) in the degree of resistance (CD50 from 13.9 to 11.4 +/- 1.07 and CD90 from 20.3 to 16.7 +/- 1.16 mg/kg), by the 15th syringe passage.

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.