Influence of Larval Trematode Infections in Snails on Their Second Intermediate Host Relations to the Strigeid Trematode, Cotylurus flabelliformis (Faust, 1917)

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Abstract
This paper brings together all the data obtained during the summers of 1932 to 1941 at the Univ. of Michigan Biol. Station on the influence of larval trematode infections in lymnaeid snails on their 2d intermediate host relations to the strigeid trematode, C. flabelliformis. Whenever the cercariae of C. flabelliformis are able to get into the germinal sacs of trematodes parasitic in snails the metacercariae develop normally to the encysted stage as hyperparasites. In fact, they develop more rapidly in this location than in their normal habitat, the hermaphroditic gland. In snails infected with the sporocysts of C. flabelliformis there is a specific immunity produced against the cercariae of this sp. which prevents all but a few from penetrating. In other words, snails serving as intermediate hosts for this trematode sp. cannot serve effectively as 2d intermediate hosts for the development of its metacercariae. In normal 2d intermediate hosts the presence of the germinal sacs of certain spp. of larval trematodes does not affect the numbers of metacercariae that develop; other spp. produce in the infected snails a non-specific immunity which greatly reduces the numbers of metacercariae that develop. Both the specific and the non-specific immunity in the snails infected with these larval trematodes appear to be caused by some change in the snails that prevents most of the cercariae from penetrating. The few that do get in find their way into the germinal sacs of the trematodes and develop normally. In snails that are abnormal 2d intermediate hosts for C. flabelliformis, the presence of the germinal sacs of larval trematodes tends to break down the resistance and allows numbers of the metacercariae to develop. In completely abnormal hosts such as physid and planorbid snails, the cercariae penetrate but fail to develop unless trematode parasites are present. In the infected snails, they penetrate into the germinal sacs and there develop normally. The same relation was found to be true in L. stagnalis and S. emarginata which are abnormal 2d intermediate hosts for the vars. of C. flabelliformis the cercariae of which develop in S. emarginata and L. stagnalis, respectively. Almost no development of metacercariae takes place in L. stagnalis when exposed to infection with the cercariae of C. flabelliformis from S. emarginata. In snails infected with the sporocysts of D. flexicaudum, however, the cercariae of this var. develop in large numbers. Also, almost no development of metacercariae takes place in juveniles of S. emarginata when exposed to infection with the cercariae of C. flabelliformis from L. stagnalis. When, however, the juveniles of this species harbor the sporocysts of certain spp. of larval trematodes, the metacercariae of this var. develop in large numbers. Adults of S. emarginata, while much more susceptible than juveniles to the var. of the cercariae of C. flabelliformis from L. stagnalis, still show a degree of abnormality. Infection with certain spp. of larval trematodes breaks this partial resistance and permits much larger numbers of the metacercariae to develop than in uninfected snails, while the presence of certain other spp. increases this resistance so that the snails are almost completely immune to the development of the metacercariae. Perhaps most surprising of all was the finding that infections with certain spp. of larval trematodes (D. flexicaudum and P. muris) produce non-specific immunity in normal 2d intermediate hosts and also increase the susceptibility of abnormal hosts.

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