A method for the in vivo labelling of Schistosoma mansoni and S. intercalatum cercariae with radioselenium

Summary

In order to develop a standard technique for in vivo labelling of Schistosoma mansoni and S. intercalatum cercariae by incubation of infected host snails with radioselenium, studies were conducted on the relationship between mode of administration, dose size (μCi), and time of administration of the radioisotope on the one hand and the production of cercariae and the amount of cercaria-bound radioactivity obtained on the other.

The ability to take up radioactivity by B. glabrata snails was influenced by the following factors: living/heat-killed snails, size of snails, length of exposure period, volume of exposure medium, and the amount of radioactivity per snail in the exposure medium. A linear proportionality existed between the amount of radioactivity in the exposure medium and the subsequent snail-bound radioactivity. The rate of uptake and the final level of snail-bound radioactivity reached was increased by reducing the volume of the exposure medium to a minimum. Thus, at exposure to ⁷⁵Se-methionine in 1 ml exposure medium the maximum snail-bound radioactivity (70–85% of available amount) was obtained after 5 h. Therefore, the labelling procedure used comprised an individual exposure of snails in 1 ml for 20 h to radioselenium. Irrespective of the exposure level (μCi) used the labelling of infected B. glabrata prior to or 3 days after the start of shedding of cercariae resulted in a significant interference with the production of cercariae. The same interference was observed when snails were labelled 12 days after patency using exposure levels at or above 6.6 μCi/snail. However, a dose level of or below 5.5 μCi/snail administered 12 days after the start of shedding cercariae did not interfere with the host-parasite relationship, i.e. a normal and continuous production of labelled cercariae was obtained during a period of at least 54 days.

The maximum cercaria-bound radioactivity obtained, using non-influential dose levels, was obtained 3 days after exposure of snails to the isotope. The maximum amount obtained was roughly comparable when exposure levels of 3.0–19.5 μCi per snail were used after the start of shedding of cercariae.

The labelling procedure did not interfere with the biological characteristics of the labelled larvae. Thus their behavioural activity, longevity, and infectivity to mice were comparable with those of unlabelled cercariae.

The possible application of labelled larval schistosomes to various problems in research on schistosomiasis is discussed.