Trypanosoma evansi: measurement of pyruvate production as an indicator of the drug sensitivity of isolates in vitro.

In a previous study, three in vitro methods for the assessment of drug sensitivity among Trypanosoma evansi isolates were compared--a direct counting method, pyruvate production method and uptake of radiolabelled hypoxanthine. The pyruvate assay system, which measures the amount of pyruvate in the supernatant of growing populations of trypanosomes by a spectrophotometric method, was selected for further investigation with regard to its suitability for field studies. The effect of initial seeding density and incubation time on the growth of three stocks of T. evansi--TREU 1840 and TREU 1981 (suramin sensitive) and TREU 2136 (suramin resistant)--and drug sensitivities revealed by the pyruvate assay and direct counting were examined to optimise assay conditions. Maximum densities and pyruvate production achieved were not affected by varying the initial seeding densities in the range of 5 x 10(4)-5 x 10(5)/ml and had been reached after 48 hours incubation with one exception: Pyruvate levels continued to increase up to 72 hours in the suramin resistant stock. However, inhibition curves were affected by initial seeding density and incubation period. Results suggested that an initial seeding density of 1 x 10(5)/ml and an incubation time of 48 hours are optimal for the assay. Using these assay conditions, the isolates were screened against suramin, quinapyramine sulphate and Cymelarsan, the trypanocides used most commonly against T. evansi. This assay proved to be a relatively simple and cheap technique applicable to screening large numbers of isolates of differing sensitivities to trypanocidal drugs.