Studies on a Microsporidian Hyperparasite of Strigeoid Trematodes. II. Experimental Transmission

Abstract

In the first paper of this series (Cort, Hussey and Ameel, 1960) we reported on investigations made during the summers of 1956, 1957, and 1958 on the prevalence and effect on the parasitized larval trematodes of a microsporidian hyperparasite of the genus Nosema, which is found in a large proportion of the strigeoid infections in the snails of the region around the University of Michigan Biological Station near the northern tip of the southern peninsula of Michigan. We also carried out series of experiments designed to solve some of the problems involved in the transmission of this species of Nosema. As in the first paper of this series, we will refer to this microsporidian hyperparasite as Nosema sp. and will not try to determine its exact specific relationships until more details of its life cycle have been worked out. In a preliminary experiment done during the summer of 1956 we tried to find out what happened to the spores of Nosema that were ingested by snails that harbored no trematode infections. Several small laboratory raised juveniles of Physa parkeri were placed in small dishes with enormous numbers of the spores of Nosema sp. They were seen to ingest the spores during the period they were in the dishes with them, so that very large numbers must have entered their digestive tracts. Examinations were made of 2 of these snails, one after 36 hours and the other after 72 hours. Although their tissues were carefully teased apart and examined under the high power of the microscope, no trace of the spores or of anything recognizable as developmental stages of the microsporidian was found. Two more of the snails were killed and fixed for sectioning, one at 36 and the other at 72 hours after exposure to the spores. In examination under high power of the microscope of sections of the whole bodies of these snails no trace of the spores or of anything recognizable as a stage of Nosema sp. was found. Rather late in the summer of 1956 an experiment was performed in which snails harboring larval strigeoids were exposed to very large numbers of the spores of Nosema sp. During the summer of 1957 four more experiments of this same type were carried out, in which it was determined how long it took experimental infections to produce mature spores. In one of these experiments it was demonstrated that the metacercariae (tetracotyles) of Cotylurus flabelliformis could be infected with Nosema sp. Two further experiments in the summer of 1958 showed that the spores could survive at least a year and that several different species of larval strigeoids could be hyperinfected experimentally with this microsporidian.