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Susceptibility of the Snail Biomphalaria Biossyi to Infection with Certain Strains of Schistosoma Mansoni¹

1. Schistosoma mansoni eggs in gradually diluted liver, fecal or intestinal cultures at 23° to 25°C. continued to hatch until the end of the second day after setting up the apparatus. A few eggs in the intestinal cultures hatched as late as the third day. Thus, under these conditions, eggs of S. mansoni remained viable for a relatively long period.

2. When exposed to miracidia, an empty snail shell as well as fine gravel appeared to be attacked as readily as was a live snail, apparently indicating that attack is confined not merely to the live snail but may include other objects encountered by the miracidia during random movement.

3. A total of 254 laboratory-reared Egyptian snails, Biomphalaria boissyi, belonging to well-determined age groups remained uninfected when exposed to miracidia of the Puerto Rican strain of S. mansoni. However, some of these snails became infected when re-exposed to the Egyptian strain of the parasite. Thus, exposing them to the Puerto Rican strain did not give them immunity against infection with the Egyptian strain of S. mansoni, to which they are the known intermediate host.

4. These experiments strongly support the view that S. mansoni endemic in one area may be physiologically distinct from the S. mansoni endemic in another region.

5. Biomphalaria boissyi snails were susceptible to infection with the Egyptian strain of S. mansoni at all ages ranging from two weeks to five months. Differences were slight between the number of snails that shed cercariae at the various ages; hence, apparently the age of the snails has little or no influence on their susceptibility to infection.

6. Dwarfed specimens of B. boissyi produced by overcrowded conditions in the aquaria showed considerably less susceptibility to infection with S. mansoni; also, the incubation period of the parasite in the snail was increased to 49–51 days as compared to 28–34 days among normal individuals.

¹ From the Museum of Zoology, University of Michigan, Ann Arbor, Michigan.