Therapeutic activity of isometamidium chloride in Boran cattle against a tsetse-transmitted clone of Trypanosoma congolense with a low level of drug resistance

Response of Trypanosoma vivax and Trypanosoma congolense in zebu cattle in North Cameroon to prophylactic treatment with two formulations of isometamidium

We tested the efficacy of two formulations of isometamidium in a tsetse-infested farm in North Cameroon from 20 August 2000 to 5 January 2001. A total of 90 adult cattle were used in three groups of 30 each corresponding to two treated and one untreated control. Drug efficacies were evaluated in terms of reduction of parasite incidence in the host's blood, maintenance of packed-cell volume (PCV) and weight gains. Both drugs reduced the incidence of parasites even though re-infections 2 weeks after treatment were common. PCV values were similar in both treated groups but higher than in the untreated control. Body weight changes followed a similar trend with the control losing weight from a mean of 427+/-119kg at the beginning to 398+/-93kg in 4 months. Weights increased from 375+/-76 and 396+/-110 to 396+/-69 and 418+/-112kg in the Veridium and Trypamidium groups, respectively. Efficacy was similar between the two formulations of isometamidium in the prophylaxis of bovine trypanosomosis. However, the presence of parasites in some animals barely 2 weeks after treatment suggested that either infections were not cleared or residual drug effects were not sufficient to prevent re-infections.

Resistance to therapeutic treatment with macrocyclic lactone anthelmintics in Ostertagia circumcincta

Eighty-eight lambs were allocated to one of four groups which were then dosed with 10,000 infective-stage larvae (L3) of one of four populations of Ostertagia circumcincta; the first (S) was an isolate known to be anthelmintic-susceptible; the second (OR) was a multiple anthelmintic-resistant isolate which had been recovered from the field following therapeutic failure of both ivermectin and moxidectin and subsequently maintained in the laboratory without further anthelmintic selection. The third (R) was derived from OR but had been passaged for five generations in the laboratory with each generation being screened with all three broad-spectrum drench families; the fourth (R x S) was an F1 cross between the S and R isolates. On patency, each of the four infection groups was sub-divided into five treatment groups, one of which received no anthelmintic while the others were administered either oral ivermectin (IVM-oral), controlled-release capsules containing ivermectin (IVM-CRCs), oral moxidectin (MOX-oral) or injectable MOX (MOX-inj). Neither formulation of IVM reduced FEC in the R, R x S and OR infected lambs compared to their untreated controls, but significant reductions were observed in all cases following MOX-oral or MOX-inj treatment. Similarly, neither IVM formulation significantly reduced the numbers of R or R x S worms compared to their untreated controls, although the numbers of OR worms were reduced in both cases (P<0.05). Direct comparisons of efficacy across the isolates, however, indicated that neither formulation was any more effective against R x S or OR worms than against the more highly selected R worms. In contrast, both MOX formulations significantly reduced worm numbers of all the resistant isolates compared to their respective untreated controls; furthermore, worm burdens of R x S were reduced significantly more than burdens of R (P<0.05). Reductions in OR burdens, which were intermediate between the two, did not differ significantly from either. The results are consistent with published work on Haemonchus contortus, which suggests that macrocyclic lactone (ML) resistance is expressed as a dominant trait under treatment with IVM. However, these data differ from the H. contortus studies in suggesting that ML resistance in O. circumcincta may effectively be rendered incompletely dominant or recessive by treatment with MOX.

Chemotherapy and delivery systems: haemoparasites

Chemotherapy of haemoparasitic diseases in domestic animals is dependent on a limited number of compounds, many of which are chemically closely related. In this review, a summary is given of each of the drugs currently available for treatment and prophylaxis of trypanosomosis and the tick-borne diseases theileriosis, babesiosis, anaplasmosis and cowdriosis. In contrast to the situation with the drugs used for tick-borne diseases, drug resistance appears to be becoming an increasing problem associated with the compounds used for trypanosomosis. The literature that has been reviewed, therefore, is that which relates to the methods used to identify and quantify drug resistance in trypanosome populations, reports of resistance to trypanocides, and cross-resistance between trypanosome populations, reports of resistance to trypanocides, and cross-resistance between trypanocides. The possible reason(s) for the apparent lack of development of resistance to the compounds used for treatment of tick-borne diseases is also discussed. Local toxicity at the site of injection is a problem that is particularly associated with many of the trypanocides when used on a long-term basis in individual animals. Various alternative preparations of the currently used trypanocides therefore have been evaluated in an attempt to reduce this toxicity, and are summarised. Finally, future developments in haemoparasitic chemotherapy are considered and, for trypanosomosis, highlight the importance of integrating chemotherapeutic and chemoprophylactic programmes with control of the vector when drug resistance becomes a significant constraint.

Alterations in drug transport in resistant Trypanosoma congolense

The transport of isometamidium chloride (Samorin) in Trypanosoma congolense which were either sensitive or resistant to this widely used trypanocide was studied in vitro. Significantly lower amounts of drug were accumulated over time by resistant than by sensitive trypanosomes. While no direct evidence could be obtained, indirect observations implied the involvement of an increased efflux of drug from the resistant trypanosomes. In both the resistant and sensitive parasites, drug transport was found to be mediated by an energy-dependent, specific process, presumably receptor-mediated. However, the specificity of the putative receptors was altered in the drug-resistant parasites. It is proposed that an alteration or replacement of a specific receptor in isometamidium chloride-resistant T. congolense results in an increased efflux of the drug and that this increased efflux at least partially mediates the reduction in sensitivity to the compound.