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Biological Studies on the Miracidium of Schistosoma Mansoni

1. A total of 1997 normal looking eggs of Schistosoma mansoni, representing 63 different fresh stool samples from a single patient, were isolated in rain water, each to a vial, and studied in respect to hatchability and in respect to the longevity and infectivity of the liberated miracidia.

2. The data on hatching demonstrate the following:

a. Hatchability varied markedly from day to day, but the overall average was high, 83.6 per cent.

b. Hatching was spread over a relatively long time. Sixty-five per cent of the eggs that hatched did so during the first 8 hours in water; an additional 24 per cent hatched during the next 16 hours. Of the rest, 9.5 per cent hatched on the second, and 1.5 per cent on the third day.

c. Light acted as a definite stimulus to hatching.

3. Mortality among groups of miracidia held for different hour-intervals increased with advancing age. At nine hours, it reached 100 per cent.

4. Only one out of a batch of 112 miracidia attained a life span of eight hours and forty-two minutes. The average span of life was five to six hours. Further trials using river water gave practically the same results. The life expectancy of the miracidium of S. mansoni is therefore much shorter than previously supposed.

5. About three-fourths of the miracidia which came in contact with the snail immediately after hatching succeeded in accomplishing penetration. Infectivity remained about the same, irrespective of age, for as long as the larvae retained their vitality. Among those miracidia which failed to penetrate, some did so after repeated attacks while others remained indifferent to the snail throughout.

6. The proportion of miracidia which ultimately accomplished penetration (the overall infectivity rate) was found to be about two-thirds among those larvae that came in contact with the snail within two hours of free life. It gradually decreased to zero within eight to nine hours of free life.

7. The present study strongly suggests that even limited water pollution may bring about ample infection of snails.

¹ From the Department of Medical Zoology, School of Tropical Medicine, San Juan, Puerto Rico.