Snails such as Cyclops and Daphnia spp are common intermediate hosts for the parasite Philometra sp, a nematode that is pathogenic for guppies and other aquarium fish. These blood-red worms can be seen in the swollen abdominal cavity and protruding from the anus of affected fish (red worm disease).
Bluefish along the east coast of the United States appear to be heavily infected with the ovarian nematode Philometra saltatrix. Intensity and prevalence of Philometra saltatrix cycle seasonally and appear to be synchronous with the bluefish spawning cycle.
The life cycle of Philometra saltatrix is unknown and, therefore, it is not clear whether initial infection coincides with the spawning season or if nematodes are acquired at an earlier time and reside in some other host tissue site before migrating to the ovaries during the spawning season, perhaps stimulated by hormonal cues from the host. Prevalence and intensity were much lower during the spring spawning season. The rapid decline in both prevalence and intensity after the summer spawning period indicates that Philometra saltatrix are released or migrate out of the host fish synchronously with the release of fish eggs. We speculate that the reproduction cycle of the nematode is closely matched to that of the host bluefish.
Other studies have reported that infection occurs after first host maturity and that female nematodes have only been observed in the gonads of females .
Despite some studies showing deleterious effects of other parasites on fish ovaries, studies on the effects of philometrids on fish ovaries remain scarce.
Histopathological changes associated with philometrid infection in bluefish are significant. The hemorrhaging, edema, inflammation, atresia, and necrosis observed most likely reduce oocyte number and quality, leading to lower fecundity. The presence of erythrocytes in the guts of nematodes indicates that the parasites were feeding on the blood of the host; this diversion of nutrients to the parasite may exacerbate the impacts of the worm on ovarian tissue. Furthermore, intense infections can reduce the effective volume of the ovary and thus lead to lower fecundity.
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