Growth rates of suramin-sensitive and resistantTrypanosoma evansi

Abstract

In the presence of a trypanocidal drug, trypanosomes which have acquired resistance to that drug have an obvious selective advantage over their drug-sensitive counterparts. In the absence of the drug, the situation probably changes and drugresistant parasites are selected against, otherwise resistance wouldspread much faster than has been observed (Mutugi, 1993). In the present work, the relationship between growth rates of stocks of Trypanosoma evansi and their resistance to suramin was determined in rats. This was to investigate whether differential growth rates of suramin-resistant and sensitive trypanosomes could limit the spread of suramin resistance.One month old male Wistar albino rats weighing between 150 to 200 g were used. The suramin sensitivities of 3 cameline stocks of T. evansi were determined by titration experiments. KETRI 2454 and KETRI 2476 were resistant, in mice, to suramin at a dose rate of 0.005mg/kg but sensitive at 0-01 mg/kg. KETRI 3136 was resistant, in mice, to suramin at 160mg/kg. Two clones from each of stocks of KETR12454 and KETRI 2476 were also included in the study (Table I). By exposing trypanosomes to sub-curative suramin doses in mice, resistance to suramin had been induced in one of the clones from each stock, while the other was its unmodified parent. Thus, KETRI 2454 CL9 was a strain in which resistance was induced starting with KETRI 2454 CL1, a clone of the field isolate KETRI 2454; while KETRI 2476 CL22 was an induced resistant strain from KETRI 2476 CLI, a clone of KETRI 2476.Each stock was infected into groups of 5 rats by intraperitoneal (i/p) injection with approximately 1 x 106 trypanosomes and the onset of parasitaemia in each rat was determined using the haematocrit centrifugation technique (Woo, 1970). Starting the following day, the rats were bled 5 times daily (at 09.00 h, 11.00 h, 13.00 h, 15.00 h and 17.00 h) for 9 days. Parasitaemia was determined by haemocytometer counting. Growth curves of each stock of trypanosomes were obtained by plotting the log trypanosome numbers against the time of bleeding and the increase of parasitaemia on the first day of the first parasitaemic wave was calculated, on the assumption that it would be the time when parasites had least interference from the host's immune response. In each rat, the growth rate of trypanosomes in terms of generation time was calculated using the formula of Topley and Wilson (1955). The mean generation time of each trypanosome stock was then determined and compared using either the Kruskal-Wallis or Mann-Whitney tests. A number of major assumptions were made; namely, that parasites were evenly distributed in the blood, all trypanosomes were capable of multiplying, trypanosome death rate was constant in all the stocks and errors of trypanosome counting were constant.