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The Influence of Host-Parasite Dispersion upon the Capacity of Schistosoma Mansoni Miracidia to Infect Australorbis Glabratus^*

The capacity of the miracidium of Schistosoma mansoni to locate and infect Australorbis glabratus was investigated under several conditions of host-parasite dispersion. Experiments usually involved exposing isolated snails to one miracidium each in vessels of different dimensions. Infection of snails was assessed 13 to 15 days post-exposure by detecting daughter sporocysts in squash preparations of the hosts. Aspects of miracidial behavior were also explored.

Most of the findings on the effects of host-parasite dispersion do not lend themselves to terse summarization. It is sufficient to note that miracidia were successful in locating and infecting hosts situated at distances of 86 cm on the horizontal, at distances of 33 cm downwards, and that there is reason to believe that these do not represent the limits of miracidial scanning capacity. Newly-emerged miracidia were found capable of a velocity of about 690 to 750 cm per hour. Study of known miracidial tropisms disclosed that negative geotropism is a stronger influence upon their behavior than is positive phototropism. However, the tropisms do not elicit an absolute response from all miracidia even when acting together. Other evidence indicated that neither miracidia nor snails become randomly dispersed in a vessel but that both organisms are found more frequently near the outer margins. This suggests that the perimeter of a vessel is an important parameter in assessing miracidium-snail interactions. Existing evidence that snails may "attract" miracidia has been reviewed and found wanting.

The data developed in this study suggest several working hypotheses: (a) Under ordinary field conditions, water depth is probably not an important barrier to host-location by miracidia; (b) certain submerged "margins" may represent the sites where miracidium-snail interactions are likeliest to occur; (c) in view of the marked scanning capacity of the miracidium, the threshold below which snails may not become infected will vary according to the degree of miracidial pressure; and (d) in regions of high endemicity such density thresholds may be close to the extinction point of the snail populations.

^* This study was supported in part by Grant E-513-C from the National Institutes of Health, U. S. Public Health Service, and by a Research Career Development Award from the Public Health Service to the senior author.