The use of 75-seleno-methionine labelled Trypanosoma brucei to measure parasite replication in vivo.

The suitability of 75-Se-labelled trypanosomes for the measurement of trypanosome replication rates in vivo was investigated. The principle used to estimate the doubling time of the circulating parasites was the decrease in specific activity of 10(-7) parasites and this was determined both for parasites contained in a whole blood sample and for parasites separated from the blood sample by DEAE-cellulose chromatography. When two stocks of T. brucei (TrEU 226, TrEU 667) were compared it was found that the parasitemic profiles of radio-labelled trypanosomes transfered into naive mice were essentially the same as those of unlabelled parasites of each stock. Furthermore it was found that in an acute infection (TrEU 226) there was a rapid and continuous fall in the specific activity implying a constant doubling time. However, in the more chronic relapsing infection (TrEU 667) a biphasic decrease in the specific activity occurred which although initially similar to that of the acute infection changed to a much slower rate of replication at the time of peak parasitemia. The results indicate that radiolabelled trypanosomes can be used to measure parasite replication rates in vivo, and may therefore be a valuable method for investigating the factors governing parasite growth in the circulation.