The digenean parasite, Plagiorchis elegans can establish infections in the incompatible snail, Biomphalaria glabrata , a vector of human schistosomiasis. Although embryonic development is arrested at the sporocyst stage, infection with a single parasite egg reduced reproductive success of this incompatible host to 64%. Heavier doses reduced this to 45%. Biomphalaria glabrata quickly acquired large numbers of parasites by ad libitum browsing on egg-contaminated substrates. Age of the host at exposure affected subsequent reproductive success and survival. Snails exposed as young (3mm), produced 54% fewer eggs, and suffered relatively high mortality. Adults (9mm) were affected only marginally. Plagiorchis elegans shares its ability to establish truncated infections in incompatible hosts with at least one other plagiorchiid. Haematoloechus medioplexus castrated the snail Stagnicola elodes, but not B. glabrata. Findings are discussed in the context of using incompatible digenean parasites as agents in the biological control of snails and snail-borne diseases, and ecological consequences of these infections.

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Graduation date: 2001

The host/parasite association between the pulmonate snail Stagnicola elodes and the digenean Plagiorchis elegans was examined with a view to identifying some of the biotic and abiotic factors that govern parasite egg development, infectivity and survival, the dynamics of cercarial production, as well as host susceptibility to infection, growth, reproduction and mortality. Plagiorchis elegans eggs passed with the feces of the hamster (Mesocricetus auratus) experimental definitive host were unembryonated. Between 0 and 30 C, the rate of embryonation continued to increase with temperature, whereas infectivity and survival were optimal between 10 to 20 C. Light adversely affected embryonation. Parasite acquisition increased with the intensity of exposure and sexual maturity. Infection invariably enhanced pre-patent host growth in a dose-dependent manner. Cercarial production increased with time and reached a plateau. Snails infected prior to sexual maturity released more cercariae than mature individuals. Heavily infected snails tended to die prematurely regardless of age at infection and parasite dose, thereby reducing their total production of cercariae to levels below those of more lightly infected individuals. Even light infections castrated the snail host. Snails infected when immature never reproduced, whereas reproductive snails ceased egg production within days of infection. Similar effects were observed in the incompatible snail host Biomphalaria glabrata, although no cercariae were ever produced. Both enhanced growth and parasitic castration are attributable to the mother sporocyst.

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The impact of a primary infection with the incompatible digenean Plagiorchis elegans to groups composed of young, juvenile and adult Biomphalaria glabrata and a challenge infection with the compatible parasite Schistosoma mansoni was examined. Egg production of young B. glabrata was significantly reduced by P. elegans at the higher exposures of 16 eggs per snail, while egg production of adult B. glabrata was reduced at exposures of 8 and 16 eggs per snail. Egg production of juvenile B. glabrata snails was not significantly affected by any of the levels of P. elegans exposures. The survivorship of juvenile B. glabrata snails was significantly lowered by an exposure to 16 P. elegans eggs per snail. The survivorship of adult snails was significantly reduced by exposures to both 8 and 16 P. elegans eggs per snail. Interestingly enough, the survivorship of young B. glabrata snails was not significantly affected by P. elegans exposure. Snails harboring small P. elegans infections acquired a resistance to Schistosoma mansoni infection. Infection with P. elegans did not have any significant effect on the egg production of the young and adult sympatric snails, Helisoma trivolvis trivolvis. The survivorship of adult H. trivolvis trivolvis was not significantly affected by exposure to P. elegans. This being said, young H. t. trivolvis that were exposed to P. elegans had a higher survivorship than did control snails. Exposed Bulinus truncatus snails laid up to 50% fewer eggs for the first five weeks due to P. elegans infection. There was however no effect on the survivorship of the snails.

No description available.