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

commonly found unencysted and coiled up in the musculature and encysted in the viscera.
Endotherms (birds and mammals, including humans) become infected by eating raw or poorly
cooked fish infected with plerocercoids. Cestodes then mature in the small intestine of the
endotherm, undergo sexual reproduction, and produce eggs that are released with the feces into
the aquatic environment. Plerocercoids can be seen as “white masses” in uncooked fish but when
the flesh is cooked, some worms may not be noticed. Thorough cooking of the flesh kills the
plerocercoids.
In the Great Lakes area, there are at least four species of Diphyllobothrium (D. ditremum from
Lakes Superior, Huron, and Ontario; D. laruei from Lakes Superior, Huron, Erie, and Ontario; D.
latum from Lake Superior; and D. oblongatum from Lakes Michigan and Superior) that have
been identified infecting a variety of fishes in several families as plerocercoids. These species use
the following animals as definitive hosts (in parentheses): D. ditremum (piscivorous birds), D.
laruei (cats, dogs), D. latum (dogs, bears, humans), and D. oblongatum (gulls, terns). There are
also many reports of unidentified plerocercoids of Diphyllobothrium infecting several species of
salmonids. Species of Diphyllobothrium in the plerocercoid stage that infect fish are difficult to
identify to species. Furthermore, a few researchers believe that D. latum plerocercoids occur only
in the genera Esox, Sander and Lota.
Adams and Rausch (1997) listed 13 species of Diphyllobothrium that are infective to humans. Of
these species, Diphyllobothrium latum is probably the only species in the Great Lakes that could
infect humans, and all reports of human infections have been associated with Lake Superior and
associated inland waters. The first report of D. latum infecting humans in this area is that of
Nickerson (1906) who reported it from a child. Vergeer (1928) reported that D. latum was present
in fish in the inland waters of the Keweenaw Peninsula of Michigan’s Upper Peninsula and
nearby parts of Lake Superior. This information served as the basis for indicating that northern
Michigan and the lake region of Michigan is an endemic area for D. latum mentioned in several
older medical parasitology textbooks as well as in a nontechnical magazine (Porter 1978). Peters
et al. (1978) specifically examined 62 Esox lucius, 35 Sander canadense, and 3 Sander vitreus
from the same collection area of Vergeer (1928), as well as 38 Lota lota from the Portage
waterway in Michigan’s Upper Peninsula and from the Otter River near Elo, Michigan for
Diphyllobothrium latum. All fish examined by Peters et al. (1978) were negative for D. latum.
Peters et al. (1978) reported that the most-recent record of human diphyllobothriasis diagnosed at
the Houghton, MI laboratory of the Michigan Department of Health was in 1954. Peters et al.
(1978) concluded “…the decline and probable disappearance of D. latum from this area is most
likely due to culinary changes, resulting in few, if any, pike or other vector fish being eaten
without thorough cooking.” We concur with the conclusion of Peters et al. (1978) and believe
that, if D. latum is present in the Lake Superior area, it is very infrequent. Furthermore, Rogers
(1975) did not find D. latum in a parasite survey of black bears (Ursus americanus) that included
upper Michigan.
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