Schistosoma mansoni: autoradiographic tracking studies of isotopically-labelled challenge parasites in naive and vaccinated CBA/Ca mice

The migration of isotopically-labelled challenge parasites of Schistosoma mansoni in naive CBA/Ca mice, and CBA/Ca mice vaccinated 4 weeks previously with about 600 radiation-attenuated cercariae, has been followed by means of compressed organ autoradiography. In naive mice, only 16% of the challenge parasites failed to migrate from the skin to the lungs, whereas up to half of the individuals that succeeded in reaching the pulmonary vasculature did not move on to the liver. The tracking technique thus revealed a total loss of 58% of the challenge parasites, which correlated well with the fact that only 50% of the challenge was recovered as adult worms by retrograde perfusion of the hepatic portal system. Challenge migration in vaccinated mice initially proceeded more slowly than in naive mice, but peak numbers of foci were eventually recorded in the lungs on the same day in both groups of individuals. We did not therefore recognize a delay in parasite migration in vaccinated mice. In the present experiments, 58.5% of the challenge failed to reach the lungs of vaccinated rodents, and 25% of those parasites that did attain the pulmonary vasculature were not recruited to the liver. The tracking technique thus accounted for a total loss of 83.5% of the parasites, which again correlated well with the fact that we recovered only 22% of the challenge as adult worms at portal perfusion. The data presented here prove conclusively that the major phase of immune-dependent challenge elimination in vaccinated CBA/Ca mice occurs in the cutaneous tissues, and that only a small proportion of the parasites are lost in the lungs. These data are entirely consistent with those we have published elsewhere for the CBA/Ca mouse using a multiplicity of different techniques; they differ however, from results reported by others for the C57 Black strain of mouse. Possible reasons for these discrepancies are discussed.