Schistosoma mansoni: temporal distribution of radioselenium-labelled schistosomula in lungs of mice during the first two weeks of infection.

The number of schistosomula in lungs was determined by compressed organ autoradiography at intervals up to 14 days after exposure of mice to 75Se-labelled cercariae by tail immersion. Probit analysis of compressed lung autoradiogram focus counts, expressed as percentages of initial infection level, yielded estimates of the average time of arrival, peak accumulation in the lungs and average time of departure of schistosomula: 4.5 +/- 0.87, 6.3 +/- 0.45 and 11 +/- 0.58 days, respectively. At peak accumulation 92 +/- 3.5% of the initial number of schistosomula were found in the lungs. It thus appears that little or no significant attrition of schistosomula occurred in the skin and, instead, that most of the 50-70% of penetrant cercariae that fail to reach adulthood are lost somewhere between the pulmonary and hepatic phases of development. Loss of 75Se label from schistosomula during the first 14 days was exponential, with an average half-life of 4.5 +/- 0.81 days. However, the high sensitivity of autoradiography tended to compensate for this rather rapid rate of label loss. It was pointed out that autoradiographic detection of schistosomula as discrete loci of radioactivity can also be expected to overcome the problem posed by the accumulation in such tissues as liver and kidney of 75Se label that has become separated from larvae.