Tran 35S-labeling of cercariae of Schistosoma mansoni: a less expensive tool for tracing in vivo migration of schistosomula in mice

Abstract

The ability of [ 35S]methionine (Tran 35S-label) to label cercariae of Schistosoma mansoni in situ by exposing Biomphalaria glabrata snails to the radioisotope was critically evaluated in this study. In the present study S. mansoni infected snails were exposed to the Tran 35S-label 1 week after they started shedding cercariae and were found to release nearly 100% radio-labeled cercariae within 72 h. These radio-labeled cercariae could then be detected by autoradiographic techniques. Efficacy of the labeling was tested by calculating the percentage of Tran 35S-labeled cercariae that has emerged from labeled snails at different time intervals after exposure to the label. These results showed that one time labeling of snails with 25 μCi/ml at 27 °C for 24 h was sufficient to release radio-labeled cercariae continuously for 32 days. More than 90% of cercariae released after 72 h of labeling the snails were radio-labeled. Approximately 2/3 of the label was associated with the body of the cercaria and the rest was found in the tail. Analysis of the optimum conditions (duration of exposure to the label and the concentration of the radioisotope) required to obtain maximum numbers of radio-labeled cercariae showed that exposure of infected snails in distilled water (approximately 2 ml/snail) containing 6.25 μCi/ml of Tran 35S-label for 2 h consistently yielded 99 ±4% of radio-labeled cercariae especially when the snails were shed 72 h later. The use of such Tran 35S-labeled cercariae in tracer studies were evaluated in vivo. These results showed that migration of schistosomula could be traced in vivo in various tissues (the skin, lungs and liver) of mice by compressed organ autoradiographic techniques. Although a significant amount of the parasite-associated radioactivity was lost during the in vivo development and migration of schistosomula through various tissues of the mice, there was sufficient radioactivity present in all the parasites recovered on day 21 after infection, to be identified by autoradiographic techniques. These studies thus show that Tran 35S-labeled cercariae could be used successfully to trace migration and distribution of schistosomula of S. mansoni in various organs of mice. The ease of availability of the radio-label and its lesser cost makes Tran [ 35S]methionine a more suitable radioisotope for labeling cercariae of S. mansoni than other labeling methods.