Search

The effect of the number of trypanosomes in the initial inoculum on the minimum curative dose, was determined for an experimental infection of Trypanosoma congolense in mice treated with isometamidium. Mice were infected by the intravenous route and were then treated three hours later by intraperitoneal injection. The minimum curative dose was shown to be dependent on the size of the initial inoculum, with a difference of a factor of 7.5 as the initial inoculum was increased from 10(3) to 10(6) trypanosomes per mouse. It is concluded that this may be a significant variable for in vivo drug sensitivity test, and may also have implications for treatment of infections in the field.

A study was undertaken in goats to investigate the ability of two unrelated stocks of Trypanosoma congolense, one of which is highly sensitive to isometamidium chloride and one which is drug-resistant, to become established in the presence of an existing infection with the other stock. The goats, which were initially infected with the sensitive strain and were then challenged with the resistant strain, were cured by treatment at 0.1 mg kg-1 isometamidium, indicating that the resistant stock did not establish an infection. Goats initially infected with the resistant stock, which were then challenged with the sensitive stock, experienced temporary remission of infection followed by relapse after treatment at 0.1 mg kg-1 isometamidium. In contrast, the goat infected only with the resistant stock remained parasitaemic following treatment at 0.1 mg kg-1. This suggests that superinfection with the sensitive stock resulted in the establishment of infection, which suppressed the resistant stock to below the limit of detection of the method used. These observations suggest that isometamidium-resistant stocks may be less viable than sensitive strains, and could explain the relative scarcity of isometamidium resistant in the field.

The sensitivities of 3 strains of Trypanosoma congolense to isometamidium chloride (Samorin) were determined in mice and cattle, with the objective of evaluating sensitivity testing in mice as a means of predicting curative doses in cattle. Comparison of mouse effective dose 80% (ED80) or curative dose 80% (CD80) values with cattle minimum curative dose (MCD) values demonstrated a wide variation between trypanosome strains. Although a mouse test may give a broad indication of the sensitivity of a strain, it cannot be used to predict curative doses for cattle. It was concluded that care should be exercised in extrapolating the results of a mouse test to cattle.