Parasites of Fish from the Great Lakes - Great Lakes Fishery ...

their plerocercoids (Duguid and Sheppard 1944; Hoffman and Dunbar 1961), and to have caused
a major decline of Salmo trutta and Salvelinus alpinus in Norway (Vik 1965). Ligula sp. and
Schistocephalus sp. have been known to alter the behavior of their fish hosts, making them more
susceptible to predation by piscivorous birds (Ness and Foster 1999; Loot et al. 2002). The
reports of Triaenophorus spp. infecting fish are infrequent, and the last one was by Guilford
(1954).
Nematodes
The effect of adult Cystidicola farionis in the swim bladder of coregonines and salmonines is
difficult to assess based on the small number of studies performed. However, Black (1984)
reported that lesions of the swimbladder in Salvelinus namaycush may have developed due to
chronic mechanical irritation caused by mature Cystidicola stigmatura congregating, possibly to
mate. The occurrence of large numbers of larval nematodes, such as Contracaecum sp.,
Raphidascaris acus, and Eustrongylides tubifex, in various non-intestinal sites, may lead to
inflammation, tissue damage to the liver and other viscera, and fibrosis (Williams 1967).
However, the prevalence and intensity of larval nematodes reported in Lake Michigan fish have
been low.
Acanthocephalans
Two acanthocephalan species (Echinorhynchus salmonis and Acanthocephalus dirus) infect
several fish species with high prevalences and intensities. Acanthocephalans use their proboscis
with hooks on it to attach to the inner wall of the intestine. Their attachment with their proboscis
leads to inflammation of the intestinal tract reducing the amount of surface area for nutrient
absorption and possibly reducing the state of nutrition of the fish (Bullock 1963; Pippy and
Sandeman 1967; Schmidt et al. 1974). Amin and Burrows (1977) and Muzzall and Peebles (1988)
have reported on the occurrence of gravid female E. salmonis in non-intestinal sites of Osmerus
mordax.
Echinorhynchus salmonis is the most-numerous parasite of Oncorhynchus spp. in Lake Michigan
probably because of the various pathways utilized in its life cycle. Echinorhynchus salmonis
primarily uses the crustacean Diporeia (= Pontoporeia) affinis (see Amin 1978) as an
intermediate host wherein the infective stage called the cystacanth develops. In the first pathway,
the intermediate host, Diporeia, is eaten by a fish and the cystacanth develops into an adult in the
intestine of that fish. In the second pathway, called the postcyclical pathway, the infected fish
with E. salmonis attached to its intestine is then eaten by another fish, wherein the worm attaches
to the intestine, matures, and reproduces in this second fish. Hnath (1969) demonstrated in the
laboratory that adult E. salmonis can re-establish in a new host fish that has ingested another fish
infected with adult worms. The third-transmission pathway involves a paratenic host, where a fish
that initially eats the intermediate host is not a suitable host, and as a result the cystacanth of E.
salmonis penetrates the gut wall and encysts/occurs somewhere else in the fish without
undergoing further development. This fish host becomes a paratenic host. If the paratenic host is
eaten by a suitable fish host, the acanthocephalan excysts and attaches in the intestine of the
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