Comparative Parasitology 67(2) 2000 - Peru State College

Discussion
Twelve parasite species (2 Cestoda, 1 Monogenea,
1 Nematoda, 1 Acanthocephala, 1 Copepoda,
1 Myxozoa, 1 Mastigophora, 4 Ciliophora)
were found in 635 trout examined from 12
farms in the present study. Parasites of these
trout and their infection values varied between
ponds and years, a variability characteristic of
wild trout populations as well. Most if not all of
these parasite species have been found in wild
trout from Michigan environments (Muzzall,
1984, 1986; Hernandez and Muzzall, 1998). Digenetic
trematodes, however, found in wild trout
by Muzzall (1984, 1986), did not infect trout
from culture ponds.
Of the parasitic species found in the present
study, E. salvelini, A. dims, S. edwardsii, and
M. cerebralis had prevalences of 50% or more
in at least 1 pond and deserve further discussion.
Eubothrium salvelini was a common parasite of
rainbow and brook trout. It utilizes copepods as
intermediate hosts and commonly infects wild
salmonids in inland waters (Hernandez and
Muzzall, 1998) as well as in the Great Lakes
(Muzzall, 1993, 1995a, b). Muzzall (1984)
found immature Eubothrium sp. in brook trout
from a Michigan creek. Boyce (1969) reported
that E. salvelini reduced the growth, swimming
performance, and survival of salmon. Smith and
Margolis (1970) suggested that this cestode
caused indirect damage to young salmonids.
Hendee in 1980 believed it reduced the growth
of brook trout in the state of New Hampshire,
U.S.A. (in Hoffman, 1999).
Acanthocephalus dirus had the highest prevalences,
mean intensities, and mean abundances
of all parasites found. It is widespread in Michigan
trout farms and is common in some natural
environments of Michigan (Muzzall, 1984).
Muzzall (1984) also reported that the isopod,
Caecidotea intermedius Forbes, 1876, was the
intermediate host for this parasite in the Rogue
River. In the present study, some individuals of
all 3 trout species infected with 100 worms or
more from 3 farms appeared emaciated, and the
head appeared large for the size of the fish. Bullock
(1963) demonstrated that the most pronounced
effects of A. dirus (=A. jacksoni) in
rainbow and brook trout in a New Hampshire
hatchery were damage to the intestinal epithelium
and proliferation of connective tissue, leading
to malnutrition and emaciation. Furthermore,
MUZZALL—PARASITES OF TROUT 183
he stated (p. 33) that "this worm seriously impairs
the health of the fish." Allison (1954) discussed
the advancements in prevention and
treatment of parasitic diseases of fish and listed
13 parasitic genera that warranted discussion.
However, A. dirus was not listed, and acanthocephalans
in general receive little attention in
hatchery manuals about fish diseases. It is not
known if A. dirus was common when Allison
wrote his article or if it has become increasingly
common in Michigan.
The presence of S. edwardsii on brook trout
and its absence from rainbow and brown trout
were not unexpected, because it parasitizes only
the former species (Kabata, 1969). Some brook
trout infested with S. edwardsii had 1 or both
opercula folded underneath itself, and the distal
portions of many gill filaments showed hyperplasia
and clubbing. These characteristics also
occurred on uninfected trout, suggesting previous
infestation by this parasite. This copepod
has a direct life cycle and is a common parasite
of brook trout in Michigan (Allison and Latta,
1969; Muzzall, 1984, 1986). The mean intensities
of S. edwardsii are higher than those found
on trout in Michigan lotic environments but are
comparable to the high mean intensities found
on brook trout in Michigan lakes by Allison and
Latta (1969). Many studies on Salmincola spp.
suggest that they debilitate their hosts but may
not be direct causes of trout mortality. Allison
and Latta (1969) found no relationship between
S. edwardsii and brook trout mortality in Michigan
lakes.
Owners of 2 Michigan farms told me that they
had seen a parasitic copepod on the gills of rainbow
trout in their ponds. However, in this study,
none was found infesting rainbow trout. A copepod
that infests rainbow trout is Salmincola
californiensis (Dana, 1853), which is native to
streams in the Pacific Northwest, U.S.A., and
Canada (Kabata, 1969). Hoffman (1984) reported
on its eastward movement in North America,
being transferred on live fish and with shipments
of trout eggs. Sutherland and Wittrock (1985)
believed that 5. californiensis entered central
Iowa through the importation of infested rainbow
trout from a Missouri farm. They found a
mean intensity of 4.6 adults and suggested that
this copepod may be responsible for host mortality
if fish are sufficiently stressed. Perhaps this
species has made its way to Michigan but is infrequent
on rainbow trout in this state.
Copyright © 2011, The Helminthological Society of Washington