Circular

COMMON PARASITES
OF FISHES

U.S. Department of the Interior
Fish and Wildlife Service

kVUOUi' HULE, ^Jii^^,

COMMON PARASITES OF FISHES

UNITED STATES DEPARTMENT OF THE INTERIOR

Stewart L. Udall, Secretary-
Fish and Wildlife Service

Clarence F. Pautzke, Commissioner

CIRCULAR 144
1962

CONTENTS

Page

Int reduction 1

FRESH-WATER PARASITES 3

of the body surfaces and gills 3

of the body muscles 6

of the viscera 8

SALT-WATER PARASITES 10

of the body surfaces and gills 10

of the body mu scles 13

of the viscera 14

Suggested references 15

COMMON PARASITES OF FISHES

Glenn L. Hoffnnan, Bureau of Sport Fisheries and Wildlife, Leetown, W. Va.

and
Carl J. Sindermann, Bureau of Comnnercial Fisheries, Boothbay Harbor, Maine

Fish taken conamercially, for sport, or
raised by fish fanciers are sometimes found
to be abnormal due to injury, deformity,
disease, or the presence of parasites. This
paper is concerned with connnnon parasites
of fishes and their recognition. Life cycles
and biology are mentioned briefly, and
references to more detailed literature are
included.

Parasites nnay be defined as "animals
that live on or in another animal, the host,
at the expense of that animal." There are
many kinds of parasites which may be
found on the surfaces of fish, including the
gills, as well as in the flesh and internal
organs (fig. 1). Those on the surface are
known as external parasites, and those that live
inside are called internal parasites. Several
types may occur together in a single fish.
Some parasites are fairly large (up to
several inches) and can be seen easily,
but many are microscopic. Scientists who
work on parasites describe the organisms,
study the life cycles (some have several
intermediate hosts), survey the kinds and
numbers found in fish, study damage done
to fish, and attempt to control them. Re-
search is carried on by the U. S. Fish
and Wildlife Service and some colleges and
universities.

Under natural conditions in both fresh
and salt water, nnost of the parasites pro-
duced are lost to enemies or to the expanse
of water before they can infect fish. Ap-
parently a light parasitic infection does
little harm to the host. Under crowded
conditions or inadequate water and oxygen
supply, however, fish may become heavily
parasitized; in such cases, more
damage is done and the fish may even
die. Such conditions sometimes occur
in hatcheries, causing heavy loss of
fish.

Much of the damage to fish appears to be
mechanical. Parasites may injure tissues
and blood vessels by their burrowing, or
block blood vessels entirely with their
bodies or their eggs. Some actually eat
skin, other tissues, mucus, or body fluids.
Some parasites are known to release toxic
materials in other hosts, but this has never
been demonstrated in fish. Probably fish
weakened by parasites are easily captured
by predators.

Fishermen who find parasites when they
clean their catch frequently discard the
fish. This is an unnecessary waste. Although
worms and other parasitic fornns are un-
sightly, none of them can possibly harm
humans if the fish is thoroughly cooked.
Freezing and hot smoking of the fish will
also kill nnost parasites, but sonne may
remain alive in brine for a month. A few
parasites may develop in man if the fish
containing them is eaten raw. Best known
in this category is the broad fish tapeworm
(Diphyllobothrium latum), whose larvae may be
found in the muscles and among the viscera
of pike and perch, particularly in our north-
ern lakes. Most fish parasites will not live
in man.

The commercial fishing industry suffers
great losses each year because of parasit-
ized fish which are condemned for human
consumption and can be sold only for
animal feed at reduced price. In Minnesota,
for example, tullibees (ciscos) are infected
with tapeworm cysts (Triaenophoras) \vhichdo
not infect humans but are unsightly. The
annual catchof 2-l/2 to 3 million pounds, if
uninfected, would find ready markets for
human use, but as much as 1,500,000 pounds
have been condemned by governmental
agencies. Sinnilar losses occur in marine
fisheries.

At present there is no known control for
nnost parasites except in relatively small
bodies of water such as fish hatcheries,
small ponds, and aquaria. References to
the methods of treatment of fish parasites
nnay be found in Hoffman (1959).

The major parasite groups discussed in
this report are listed and explained
below:

Protozoa - single-celled animals, usually
microscopic. Parasitic groups of pro-
tozoa important to fishes include
Ciliata, Flagellata, Myxosporidia, and
Microsporidia; may be either external
or internal parasites. While the in-
dividual protozoan is usually micro-
scopic, aggregations nnay cause effects
recognizable by the naked eye.

Monogenetic trematodes - External flatworms,
usually small, commonly calledflukes,
often found on the gills; are called
monogenetic because they complete
their life-cycle on one host (mono =
single, genetic - origin). Posterior or-
gan of attachment (haptor) well de-
veloped with chitinous clamps and
hooks.

Digenetic trematodes - Internal flatworms,
often very small, but occasionally large
enough to be seen easily; are called
digenetic because at least two hosts are
needed for their life-cycle (di = two,
genetic = origin). Larva (metacercaria)
or adult may be found in fish, first
internnediate host is snail or clann;
possess oral and ventral suckers; eggs
present in adults which may be found
in alimentary tract, occasionally else-
where; metacercariae in various or-
gans (skin, muscle, mesenteries,
etc.).

Cestodes - Tapeworms, are also flatwornns,
but are distinct from "flukes" in that
the adult worm is usually composed of
a head (scolex) and many egg-producing
segments. Found as adults or larvae
in fish; intermediate stages in "water
fleas".

Nematodes - Unsegmented roundworms (ac-
tually cylindrical) which occur as lar-
vae in tissues, and as adults in the
alimentary tract, occasionally else-
where. Adult female contains eggs,
larva does not.

Acanthocephala - "thorny-headed" worms,
which occur as larvae or adults in
fishes.

Parasitic copepods - Small highly special-
ized crustaceans related to crayfish
and crabs, often called "fish lice,"
usually found on external surfaces;
sonnetimes embedded, sometimes
loosely attached; shape may be louse-
like to wormlike.

Leeches - External worms recognizable
by their external segmented appear-
ance, frequently bright coloration, and
a large sucker at the posterior end.

Glochidia - Fresh-water clam larvae in
cysts on gills or fins of fish.

Lampreys - Primitive fishlike vertebrates
and the largest of the fish parasites,
ranging up to 3 feet in length. They
may be recognized by the large cir-
cular mouth containing many ro%vs of
thornlike teeth.

In most instances marine and freshwater
fish have different parasites, therefore,
they are discussed separately under FRESH-
WATER PARASITES and SALT-WATER
PARASITES.

The following plan of organization is
followed \inder each category:

(1) Parasites of the body surfaces and
gills

(2) Parasites of the body muscles

(3) Parasites of the viscera

A sinnplified drawing (fig. 1) of an opened
fish is included to show the various organs.

LIVER

STOMACH

SWIM BLADDER
KIDNEV

DORSAL FIN

ANAL FIN

INTESTINE
HEART

'ENTRAL FIN
PVLORC CAECA

Figure 1.- -Drawing of Lepomis cyaneltus, (from Hile, 1960, U. S. Fish and Wildlife Service. Fishery Leaflet 132).

FRESH-WATER PARASITES
Parasites of the Body Surfaces and Gills

1. Fun^. White cottony growths on wounds
or ulcerations are usually fungi of the
genera Saprolegnia and Achlya. The filaments
can be seen easily under the microscope.
The fungus often develops as a secondary
infection following wounding or ulceration
due to other agents.

2. Protozoa. These are the smallest ani-
mals. They consist of a single cell, and
nearly all are too small to be seen with-
out a microscope. Some protozoa fornn
\vhitish cysts containing many spores. Such
cysts are large enough to resemble worm
larvae superficially. They may be found
in many organs including the skin and gills.

Often in aquaria and fish hatcheries, a
condition develops which is known as "Ich"
or "white spot." The fish are seriously
ctffected, and many little white spots can
be seen - some of them moving slowly over
the fish. These are individual protozoa,
Ichthyophthirius muUifilis, the largest protozoan

to be found on fish. Under the microscope
the most striking characteristics that can
be seen are the constantly beating hair-
like cilia, which completely cover the para-
site (fig. 2.).

All other protozoa that live on the skin
of fish are very small in comparison to
Ichthyophthirius. Although some of them are
serious fish disease agents, none is harm-
ful to man.

Myxosporidian cysts containing many
spores (fig. 3) are whitish and usually
large enough to be seen easily. They are
found in many organs including the skin
and gills.

3. Internal flukes (digenetic trematodes).
These small worms are flat, leaflike forms
from 1/250- to l/S-inch long with two
suckers. The trematodes most frequently
seen are larvae ("grubs") encysted in the
skin or flesh. The largest of these larvae
is the yello^v grub, Clinostomum marginatum (fig.
4). The cyst is yellowish, about 1/8 inch
in diameter, and is usually seen in the
gills and at the bases of the fins but nnay

Figure 2.-- Ichthyophthirius multifilis, ciliated protozoan, the
"Ich" of aquarium and fish hatchery fish.

*- *

Figure Z,- - Myxobolus spore from white cysts.

KiKure JT, — Vrllow grubs in lltwh of yt-Uow pcTch. (Courlray of
llif Nf>v York loiuuTvutiiiii I >Fpartiiirut '

Figure 4.-- Clinostomum marginatum "yellow grubs" in the flesh
of yellow perch. (Courtesy of the New York Conservation De-
partment).

produces eggs which pass out in the feces
into the water. After a suitable period,
these hatch into a microscopic free-swim-
ming larva (miracidium) which has about
8 hours to find the right kind of snail. If it
succeeds, it burrows in and continues to
develop. It produces many of the next stage
larvae (cercariae) which burrow out of the
snail. To survive, cercariae have about a
day in which to contact a fish and burrow
into the tissue. In the fish, the worms may
migrate a short distance before reaching
their final site where they secrete a cyst wall
about themselves and become the "yellow
grubs", completing the cycle.

Another type of fluke that is often noticed
is the "black grub" (fig. 6), a very small
larva enclosed in a black cyst about l/l6
inch in diameter. These may be the larvae
of any one of many trematodes but the most
common is known as Neascus. The life cycles
are similar to that of the "yellow grub."
Different fish, birds, and snails may be
involved.

occur almost anywhere, including intern-
ally. If the cyst is removed and opened, the
larva can -be seen. The life history (fig. 5)
is typical for the group. When the fish is
eaten by a heron the larva is digested free
from the cyst and continues development
in the intestine of the bird. Most trematodes
develop to maturity in the intestine of the
host, but the "yellow grub" migrates to the
esophagus where it matures. The adult

4. Gill flukes (monogenetic trematodes).
Most of these are microscopic, but one of
the larger ones, Discocotyle salmonis (fig. 7)
often occurs on the gills of trout and salmon.
It is about 1/4 inch long and attaches by
its large rear suckerlike attachment organ
to the gill of the fish. Sometimes these
flukes become so numerous that the fish
are seriously affected. Another very com-
mon monogenetic trematode, Gyrodactylus,

HERON BECOMES INFECTED BY EATING FISH
WORM BECOMES ADULT IN MOTTTH CAVITT

EGG PASSES OUT IN FECES OR SALIVA

BODY OF CERCARIA
PENETRATES FISH,
BECOMES YELLOW
GRtJB

MIRACIDIDM PENETRATES
HELISOMA SNAIL

MOTHER SPOROCTST

- - LARVAL STAGES IH SNAIL

CERCARIA EMERCES FROM SNAIL
Figure 5.--Life cycle of the "yellow grub," Clinostomum marginatum. Outer circle (solid line) includes the worm stages. Inner circle

(broken line) includes the hosts.

is usually found in greater numbers on the
body than on the gills; most of the others
occur on the gills only.

5. Parasitic copepods. In some ponds and
hatcheries, the "anchor worm" copepod
CLemoeoj flourishes. It protrudes wormlike
from the fish, usually at the base of a fin,
with its head buried in the fish. The visible
portion is cylindrical, whitish, and about
1/2 inch long (fig. 8). There are often two
egg sacs extending from the end of the
creature. With care, the head can be dis-
sected out of the fish, and one can then see
that some of the forward appendages have

become modified into a very efficient "an-
chor." This parasite goes through several
microscopic developmental stages usually
on the gills of fish other than the one on
which it finally matures. Because it can do
considerable damage to fish if present in
large numbers, it should be eradicated from
fish rearing ponds and hatcheries, and
parasitized fish should not be used for
stocking.

Sometimes "fish lice" (Argulus) may be
seen crawling over a fish. They are among
the largest of external parasites and can be
seen easily. They are round to oval when

Figure 7.-- DiscocotyU salmonis, an ectoparasitic trematode
from the gills of trout (after Price).

Figures, — Crossiphiala bulboglossa,i Neascus metacercaria
of one of the several "black spot grubs" of fish in the fathead
minnow, Pimephales promelas.

seen from above and are flattened from top
to bottom. Four pairs of swimming legs
extend from their sides (fig. 9). The eggs
are borne in sacs at the end of the female.
The eggs drop off when developed, hatch,
and the larvae must eventually find a fish
or perish. There are several larval stages
which precede final development of the adult.
Other f ornns , Ergaailus , Achtheres, and Salmincola,
are usually found attached to the gills.

6. Glochidia. These small larvae of some
of the large fresh-water clams clamp onto
the gills or fins of fish where they remain
for 10 to 20 days. Some of the fish tissue
grows up and over the glochidiunn (larva)
forming a small translucent cyst about
1/8 inch in diameter. If nunnerous, they

cause considerable
fish.

damage to the

7. Bloodsuckers (leeches). Except for
lampreys, these are the largest external
parasites, sometimes reaching an inch in
length. There are suckers at each end of
the flattened, segmented body. The front
sucker contains the mouth with which the
parasite rasps a small hole in the fish skin
to obtain its meal of blood (fig. 10). Leeches
often attach near the bases of or on the fins.
They transmit certain protozoan parasites
from fish to fish through their feeding. The
kinds of leeches that parasitize fish do
not attack humans.

Parasites of the Body Muscles

1. Protozoa. Myxosporidian cysts contain-
ing many spores (fig. 3) are whitish and

Figure 8.-- Lemaea tortua, the "anchor worm" of fish (after
Wilson).

large enough to be seen easily. They are
found in many organs but some species
found in the muscle produce a large ugly-
looking "boil." These are not infective for
man.

2. Flukes. Some of the "grubs" discussed
in the previous section may be found in the
nnuscle.

Figure d.--Argutus irilineaius, the "fish louse" (after Guberlet).

3. Tapeworms. A tapew^orm that is con-
sidered important because it may infect man
through eating improperly cooked infected
fresh-water fish is the broad fish tape-
worm,/;i;)Ay/io6o«^rium Zoium. The white larva
(plerocercoid) may be found among the
viscera as well as in the flesh, and may be
up to an inch in length, but has no other
outstanding characteristics. Normally the
parasite ■will attain maturity in the intestine
of bears, man, dogs, and perhaps other
animals if they eat infected fish (fig. 11).
It grows into a large worm - up to 30 feet
in length - and produces millions of eggs
which pass out with the feces of the animal.
If the eggs fall or are washed into the lake,
a small swimming larva (coracidium)
hatches out. This very active creature
develops into the next larval stage (pro-
cercoid) when eaten by certain species of
"water fleas" (copepods). Small fish feed
on the infected copepods, and if a pike or
perch eats such fish, the larva will continue
to develop. Humans beconne infected by
eating raw fish containing the larval stages.

4. Roundworms (nematodes). The largest
one seen in muscle is Kustrongylides (fig. 12).
It is a red worm coiled up in a cyst about
1/4 inch in diameter and is sometimes
seen while dressing a fish. It may also be
found in the body cavity. The life cycle is
not entirely known, but it is probable that
a bird is the final host.

Figure 10.-- Pis cicola salmositica, a leech or "blood-sucker"
(after Meyer).

Parasites of the Viscera

1. Protozoa. Internal fish protozoa are
usually not seen by the casual observer
except for the myxosporidian cysts dis-
cussed in previous sections.

2. Flukes. "White grubs" are sometimes
found in the visceral organs. These are

somewhat similar to the "yellow grubs"
discussed previously, and are not harmful
to man. Adult flukes of fish occur in the
intestine and stomach, but are usually small
and not seen.

3. Tapeworms (cestodes). Two forms of
these are seen in fish. One, the adult,
lives in the intestine of fish. It is flat like
a tape, and often several inches long. It
is this stage that is seen when the intestine
is accidentally cut or torn during cleaning
of the fish. The other form is the larva
(plerocercoid), which may or may not be
the same species as the adult seen in the
fish. Larvae are smaller, nonsegmented,
and may occur among the internal organs
as well as in the flesh of the fish. Cal-
careous corpuscles (round concretions of
lime) can be seen microscopically in all
tapeworm larvae; this is sometimes a very
helpful characteristic for identifying a lar-
val tapeworm parasite. The adult from
fish is never harmful to man, but some of
the larvae, if eaten raw, can develop in
man. We have previously discussed the
best known one in this group, the broad
fish tapeworm (p. 7).

One of the most noteworthy fish tape-
worms is the bass tapeworm, Proteocephalus
ambloplitis (fig. 13). The larvae (plerocer-
coids) often cause sterility in black bass.
The adult lives in the intestine of black
bass and produces large numbers of eggs
which pass out with the feces of the fish
into the water. The small larva in the egg
will develop to the next stage larva (pro-
cercoid) if eaten by the proper kind of
copepod. After development in the copepod,
this larva invades small fish which eat the
infected copepods. The larva is freed in the
intestine of the small fish, burrows through
the intestinal wall, and wanders among the
internal organs. When large numbers of
these larvae (plerocercoids) are present
they cause considerable damage to the fish.
Small bass, once infected, will retain the
larvae for a long period of time. Larger
bass accumulate larvae by eating small
fish that have recently fed on infected cope-
pods. Larvae, if still in the stomach or
intestine of the small fish when eaten by the
larger fish, will migrate through the intes-
tinal wall and into the visceral cavity of

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ADULT WORM IN SWAIX lUTESTINE
CF BEAR, MAN, DOG

MAN IS INFECTED BT EATING
MPROPERXT COOKED INFECTED FISH

EGGS ARE PASSED IN FECES
AND REACH WATER

NORTHERN PIKE, WALLEYE PIKE, PERCH, TROTTT
EATS INFECTED COPEPOD OR SMALL FISH

.^

PLEROCERCOID LARVA
DEVELOPS IN MUSCLE

OR VISCERA OF FISH

:^\ <■

SMALL FISH EATS
IHFECTH) COPEPOD

EGGS HATCH IN WATER AND
REIEASE CORACIDIDM

CORACTrrtIM IS INGESTED BY
WATER FIEA (COPEPOD)

PROCERCOID LARVA DEVELOPS
IN COPEPCJ)

Figure ll.--Life cycle of the broad fish tapeworm, Diphyllobothrium latum. Outer circle (soLd line) includes the worm stages. Inner

circle (broken line) includes the hosts.

the larger fish. The damage done by the
larvae moving among the reproductive or-
gans may render the fish sterile. The life
cycle is completed when a larger bass eats
an infected smaller fish in which the larvae
have had time to become established in the
visceral cavity. The larva is then freed
in the intestine of the larger bass and
grows into the adult tapeworm.

The large larva of another tapeworm,
Ligula intestinalis, is often seen in the body
cavity of minnows and suckers (fig. 14).
Sometimes it becomes so large in small

fish that it causes the body wall to burst,
releasing the worm. This worm develops
into an adult in the intestine of fish-eating
birds.

4. Roundworms (nematodes). The red worm,
Eustrongylides (fig. 12 and p. 7) may occur
among the viscera. Some smaller larval
nematodes may also be present. A very long,
thin nematode, Philonema, causes serious
damage to the reproductive organs of sal-
monid fish. Usually the adult nematodes in
the intestine are not noticed because of their
small size. None of these is harmful to
man.

Figure 12.

-The "red worm," larval nematode, Eustrongylides,
from the flesh of fish.

5. Thomy-headed worms (Acanthocephala).
These cylindrical worms have rows of hooks
on their heads which become embedded in
the intestinal wall of the fish. Unless large
numbers are present no harm is apparent.

SALT-WATER PARASITES

Parasites of the Body Surfaces and Gills

When a fish is first taken from the water
it may be carrying a variety of external
parasites or external evidences of disease.
The mouth and gill chambers are favored
sites for certain parasites because these
areas afford protection and are in close
proximity to the host's blood supply.

1. Fungi and ■protozoa ulcerations of the skin
may be due to underlying protozoan or
fungus infection of the flesh (as in young
sea herring). Such infections kill tissues
and cause the skin to slough, creating the
external ulcers. Two groups of the protozoa
(Myxosporidia and Microsporidia) may in-
vade the muscles, producing ulcers of the
skin (fig. 15). Fungus organisms, such as
Ichthyosporidium hoferi, rnay also cause ulcers.

2. Trematodes or flukes may be found on
the body surfaces of the larger marine
fishes such as halibut, sharks, skates, and
ocean sunfish. These worms may be leauf-

or disc-shaped and characteristically pos-
sess a conspicuous attachment organ of
hooks and/or suckers. Trematodes are also
common on the gill bars and filaments of
marine fishes, but are usually quite small
and not easily observed, unless the fish is
heavily parasitized.

3. Grubs of marine fishes --larval trema-
todes that localize beneath the skin or in
the fins--are common in inshore waters.
So-called "pigment spot" of cunner, herring,
mackerel, butterfish, and other fish is
caused by encystment of such larvae be-
neath the skin. The life cycle of the worm
(Cryptocotyle) that is responsible involves suc-
cessively a snail, a fish, and a sea gull
(fig. 16a, 16b). The adult fluke inhabits
the bird's digestive tract and sheds its
eggs with the droppings of the host. Snails
become infected by eating the worm eggs.
After a period of larval development in the
tissues of the snail, an infective stage,
known as the cercaria, emerges from
infected snails and is free -swimming vintil
it contacts the fish host, where it penetrates
the skin and encysts. The cycle is com-
pleted when fish carrying encysted larval
worms are eaten by the bird. Other larval
trematodes may also cause "pigment spot"
of marine fish, but they are not as well
understood, except that a fish-eating bird
and a snail are usually necessary for com-
pletion of the life cycle. Flounders are
often invaded by larval flukes which do not
cause pigment accumulation. Encysted lar-
val worms appear as tiny opaque white
patches in the fins and on the light under-
surface of the fish.

4. Parasitic copepods --fish lice--may be
found on external surfaces of many species
of marine fishes. These may be of various
forms. Some are temporary and retain their
mobility, moving freely from fish to fish,
while others, such as Sphyrion on the ocean
perch (redfish), are permanent tissue in-
vaders (fig. 17). Anchorlike projections of
the head of this particular copepod grow
into the flesh, often causing an \insightly
ulcer. This projection persists as a brown-
ish mass in the flesh eifter the parasite dies.

Copepods may also be found attached to the
gills and gill regions of marine fishes. An

10

C3I^

ADULT T/LPEWORll
Bi INTESTINE CF BUCK BASS

BECOIIBS ADULT WORM IF

BATKN BY BLACK BASS /

/

REMAIKS AS LAFVA IK ADULT BASS
"HIGH MAY ALSO RAT PROCERCOIDS
THAT ARE IN THE STOMACH OF FOOD FISH
\
V

COPEPOD

BECOMES PliROCERCOH) LARVA
lU VISCERA CF FISH

'^^

"RIPE" SEGMENTS CF WORM
PASS OUT WITH FECES

\

BGGS ARE FREED AND
KATEN BY COPEPOD

BECOMES PROCERCOID
LARVA IN COPEPd)

Figure 13,--Life cycle of the bass tapeworm, Proteocephalus ambloplitis. Outer circle (solid line) includes the worm staRcs. Inner

circle (brol<en line) includes the hosts.

Figure 14.--LigiJa intestinalis (tapeworm larva) in the chub, Couesius plumbeus. (C>ourtesy of the New York Conservation D^>artment).

11

Figure 15,--Skin ulcers in young herring caused by underlying
muscle infection with the myxosporidian. Kudoa dupeidae.

METACERCAfllA ENCYST

BENEATH SKIN CAUSING

PIGMENT SPOT

CERCARIAE DEVELOP IN REDIA
AND EMERGE FROM SNAIL

REDIAE DEVELOP
I SNAIL DIGESTIVE GLAND

Figure 16a.--Life cycle of CryptocotyU lingua, the worm whose larvae cause '•pigment spot" of herring and other fish,

Figure 16b.--Pigment spot of herring caused by CryptocotyU

lingua.

12

extreme example is Lemaeocera branchialis
found on cod and some other species (fig.
18). The copepod is located in the gill
chamber, but roots formed by extensions
of the body extend into the host, eventually
penetrating to the heart region. The life
cycle includes lumpfish, flatfish, and pos-
sibly others as intermediate hosts.

Parasites of the Body Muscles

Because the flesh of fish is the part that
is usually consumed by man, parasites and
diseases Eiffecting the body muscles of fish
are of primary concern. Though parasites
are killed by proper cooking, the presence
of worms or other abnormal conditions in the
flesh of food fish is esthetically and phy-
chologically disturbing, and many fish thus
affected are discarded unnecessarily.

1. Protozoa are significant and sometimes
conspicuous parasites. Myxosporidia form
either spindle-shaped white nodules up to

Figure 17.- -Parasitic copepods (Sphyrion lumpi) embedded in
redfish. Dissected copepod is shown at right.

Figure \6.-- Lemaeocera branchialis from cod. Note the
■■antlers"--anterior projections of the copepod tliat anchor it
in the flesh of the host.

1 /Z inch in length or so-called "pus pockets"
in the flesh of small herring, alewives,
and menhaden from the east coast.
Myxosporidia are also responsible for the
conditions known as "jellied swordfish" on
the Atlantic coast and "wormy halibut" on
the Pacific coast. Both are characterized
by progressive destruction and liquefaction
of the muscles, producing unsightly areas
in the flesh that must be cut out and dis-
carded, or else the entire fish may be
discarded.

Z. Fungus infections may produce muscle
abnormalities. A fungus infection of herring
on the east coast of North America produces
small yellow- white nodules. Advanced in-
fections result in extensive degeneration of
muscles, and diseased fish are difficult to
salt or smoke (fig. 19). Another symptom
of this fungus infection is the accumulation
of black pigment around spores in the flesh,
making the fish less desirable for filleting
and pickling.

3. Larval trematodes occur commonly in the
flesh of many coastal marine fishes . Con-
spicuous in this respect are young Atlantic
herring, in which larvae of the fluke
Cryptocotyle frequently localize in the mus-
cles as well as underneath the skin, causing
black pigment accumulation and the forma-
tion of a conspicuous "pigment spot." Floun-
ders are frequently invaded by another
larval trematode which appears as a small
opaque white cyst in the flesh although there
is no pigment response.

4. Larval nematodes in the flesh of marine
fishes best fulfill the popular conception of
"worms." They may occur free or encysted
in the muscles, and may become very active
when released. The so-called "codworm"
PoTocaecum has received particular attention,
especially on the Canadian east coast. This
larval roundworm encysts, sometimes in
great numbers, in the flesh of cod, smelt,
and other fishes (fig. ZO). Its life cycle is
not completely understood, but it involves
a succession of fish hosts, with the seal as
the final host for the adult worm. Other
kinds of larval roundworms may be found
in haddock and other commercial marine
fish, occasionally in great numbers, but
usually only a few in any single fish.

13

Usually they are cooked, eaten and never
noticed, but occasionally they may be seen
and may result in unnecessary waste of the
fillet or the fish. None has been demon-
strated to be harmful to humans. Usually a
fish-eating bird or mammal serves as host
for the adult worms.

5. Larval cestodes may localize in the flesh
of marine fishes, although this condition
seems less frequent than in fresh water.
Butterfish of the U. S. east coast fre-
quently have tapeworm larvae in small
(less than l/Z5-inch) white to yellow cysts
in the body muscles. Occasionally enormous
numbers of such cysts may be found in
individual fish (fig. 21). Other tapeworms
may occur as contorted opaque white
ribbons in the flesh of fish, often in suf-
ficient numbers to require discarding the
fish.

Parasites of the Viscera

Inhabitants of the visceral mass of fishes
are many, and are often apparent when fish
are dressed. Adult worms--cestodes, tre-
matodes, nematodes, and acanthocephala--
usually occupy the digestive tract, while
larval stages of members of these groups
may be foiind, usually encysted, in the gut
wall, the liver, or the supporting mem-
branes. In addition to worm parasites,
there may be protozoan and fungus infec-
tions.

1. Protozoa may occur in nodules or cysts
on and in the viscera of fish. These are
actually masses of thousands of spores,
much like the nodules or cysts in the flesh.
Conspicuous in this respect are such forms
as Glugea hertwigi, a nnicrosporidian that pro-
duces white cysts in the viscera of eastern
smelt (fig. 22). This parasite nnay occa-
sionally be sufficiently abundant to interfere
with reproduction. It varies in abundance
geographically, and may occur in over one-
quarter of all fish sampled in particular
areas.

2. Fungus infections and resulting involve-
ment of the viscera are found in such
marine fishes as the Atlantic herring,
mackerel, and flounder. A fungus disease

Figure 19.--Adult herring infected with fungus. Top--skin has
been sliced away to show normal muscle. Lower --shows ad-
vanced decay of muscles.

Figure 21.--Larval tapeworms in the flesh of bunerfish.

Figure 20,- -Smelt with codworms encysted in flesh.

Figure 22.--Visceral cysts of the microsporidianC/ugea hertwigi
in a smelt.

14

of marine fishes, caused hy Ichthyosporidium
hoferi, has received some attention in recent
years, especially in North America, where
it has periodically assunned epidemic pro-
portions in the Atlantic herring. Visceral
symptoms often include extensive white
nodules on and in the heart, liver, gonads,
and mesenteries (fig. 23).

3. Adult trematodes are connmon, but their
small size and location within the gut make
them inconspicuous. Occasionally hundreds
or even thousands of these small adult
worms may be found in the digestive tract
of a single fish. They are usually less
than 1/4 inch long, opaque white, with a
brownish patch of eggs near the center or
posterior part of the body.

4. Larval nematodes may localize in the vis-
cera, particularly in the mesenteries ad-
jacent to the digestive tract. They are
often found encysted in tightly coiled spirals
in the Atlantic and Pacific herrings, red-
fish, and many other species, and may
become very active when released--moving
in a typical whiplike manner. Numbers per
fish range from a few to many hundreds.

5. AduU nematodes may also be found in the
digestive tract, but as a rule are not con-
spicuous .

6. Larval cestodes maybe regionally abun-
dant in the viscera of many fish species.
Frequently these are larvae of tapeworms
which mature in sharks and skates or in
fish-eating birds. The larvae encyst in the
mesenteries or wall of the digestive tract
as white ovoid or club-shaped nodules, fre-
quently 1/8 to 1/2 inch in length.

7. Adult cestodes inhabit the digestive tract
of fish, and may be recognized by their

Figure 23.--Visceral nodules in herring caused by the fungus
Ichthyosporidium hoferi.

extended white ribbonlike appearance--the
ribbon being composed of many egg-pro-
ducing segments. Such worms may occa-
sionally be found extruded from the vent
of the fish after death (this often happens
with smelt, for example). Worms are usually
few in number in any single host, but may
occupy the entire length of the intestine.

SUGGESTED REFERENCES
General

Allison, Leonard N.

1950. Common diseases of fish in
Michigan. Michigan Department of
Conservation, Miscellaneous Pub-
lication No. 5, 27 p.

Bang'nam, Ralph V.

1941. Parasites of game fish. The
Aquarium Journal, vol. 14, No. 3,
p. 29-31.

1948. Parasites of freshwater fishes.
State of Ohio Department of Agricul-
ture, Bulletin No. 229, 12 p.

Cameron, T. W. M.

1945. Fish-carried parasites in Can-
ada. Canadian Journal of Compara-
tive Medicine, vol. 9, p. 245-254.

Davis, Herbert S.

1953. Culture and diseases of game
fishes. University of California
Press, Berkeley and Los Angeles,
332 p.

Haderlie, Eugene Clinton

1953. Parasites of the fresh-water
fishes of northern California. Uni-
versity of California Publications in
Zoology, vol. 57, p. 303-440.

Hargis, William J.

1958. Parasites and fisheries prob-
lems. Proceedings of the Gulf and
Caribbean Fisheries Institute. Elev-
enth Annual Session, p. 70-75.

Heller, Anita F.

1949. Parasites of cod and other ma-
rine fish from the Bay of Chaleur
region. Canada Journal of Research,
vol. 27, p. 243-264.

15

Hoffman, Glenn L.

1959. Recomnfiended treatnnent for fish
parasite diseases. U. S. Fish and
Wildlife Service, Fishery Leziflet
486, 4 p.

Hugghins, Ernest J.

1959. Parasites of fishes in South
Dakota. South Dakota Departnnent
Game, Fish and Parks Bulletin No.
484, 73 p.

Hunter, George W., Ill

1942. Studies on the parasites of fresh-
water fishes of Connecticut. Con-
necticut Geological and Natural His-
tory Survey Bulletin No. 63, p. 228-
288.

Meyer, Marvin C.

1954. The larger animal parasites of
the fresh-water fishes of Maine.
Maine Department of Inland Fisher-
ies and Game, Fishery Research and
Management Division Bulletin No. 1,
92 p.

Northcote, T. G.

1957. Common diseases and parasites
of fresh-water fishes in British Co-
lumbia. British Columbia Game
Commission, Management Publica-
tion No. 6, 25 p.

Pratt, Henry S.

1929. Parasites of fresh-waterfishes,
comprising some general considera-
tions. U. S. Department of Commerce,
Bureau of Fisheries Econonnic Cir-
cular No. 42, 10 p.

Schaperclaus, W.

1954. Fischkrankheiten. 3rd ed. Aka-
demie- Verlag, Berlin, 708 p.

Sindermann, Carl J.

1953. Parasites of fishes of North
Central Massachusetts. Massachu-
setts Division Fisheries and Game,
Fisheries Report for lakes of North
Central Massachusetts (1950), p. 4-
28.

Sindermann, Carl J., and Aaron Rosenfield

1954. Diseases of fishes of the western
North Atlantic. I. Diseases of the sea
herring. Maine Department of Sea and
Shore Fisheries Research Bulletin
No. 18, 23 p.

Van Cleave, Harley J., and Justus F.
Mueller

1934. Parasites of Oneida Lake fishes .
Part III. A biological and ecological
survey of the worm parasites. Roose-
velt Wildlife Annals, vol. 3,Nos.3&
4, p. 161-334. (Bulletin New York
State College of Forestry, vol. 7, No.
1.)

Van Duijn, C., Jr.

1956. Diseases of fishes. Water Life,
Dorset House, London, 174 p.

Viruses, bacteria, fungi

U. S. Fish and Wildlife Service, Fish-
ery Leaflets 453 - 467, 494, and 497.

Protozoa

Davis, Herbert S.

1947. Studies on the protozoan para-
sites of fresh-water fishes. U. S.
Fish and Wildlife Service, Fishery
Bulletin 41, vol. 51, p. 1-29.

1953. Culture and diseases of game
fishes. University of California
Press, Berkeley and Los Angeles,
332 p.

Kudo, Richard R.

1954. Protozoology. Charles C.
Thonnas Publisher, Springfield, Illi-
nois, 966 p.

Trematodes

Bychowsky, B. E.

1957. Monogenetic trematodes; Their
systematics and phylogeny. Moscow -
Leningrad (in Russian), 509 p. (Eng-
lish translation by Pierre C. Ous-
tinoff, edited by William J. Hargis,
American Institute of Biological
Sciences, Washington, 627 p.)

Sproston, Nora G.

1946. A synopsis of the monogenetic
trematodes. Transactions Zoological
Society of London, vol. 25, No. 4,
p. 185-600.

16

Wolfgang, Robert W.

1954. Studies of the trematode
Stephana stomum baccatum (Nicoll, 1907).
II. Biology, with special reference
to the stages affecting the winter
flounder. Journal of the Fisheries
Research Board of Canada, vol. 11,
No. 6, p. 963-987.

Yamaguti, Satyu

1953. Systema Helminthum. Part I.
Digenetic trematodes of fishes. In-
terscience Publishers, Inc., New
York, 405 p.

Cestodes

Miller, Richard B.

1952. A review of the Triaenophorus
problem in Canadian lakes. Fisher-
ies Research Board of Canada, Bul-
letin No. 95. p. 1-42,

Wardle, Robert A.

1932a. The Cestoda of Canadianfishes.

I. The Pacific Coast region. Contri-
butions to Canadian Biology and Fish-
eries, vol. 7, p. 220-243.

1932b. The Cestoda of Canadianfishes.

II. The Hudson Bay drainage system.
Contributions to Canadian Biology
and Fisheries, vol. 7, p. 377-403.

Wardle, Robert A., and J. A. McLeod

1952. The zoology of tapeworms. Uni-
versity of Minnesota Press, Min-
neapolis, 780 p.

Yamaguti, Satyu

1959. Systema Helminthum, Vol. 2.
The cestodes of vertebrates. Inter-
science Publishers, Inc., New York,
860 p.

Nematodes

Yorke, Warrington, and P. A. Maplestone
1926. The nematode parasites of ver-
tebrates. J. & A. Churchill, London,
536 p.

Parasitic Copepods

Harding, J. P.

1950. On some species of Lemaea
(Crustacea, Copepods: parasites of
fresh-water fish). Bulletin British
Museum. (Natural History). Zoology,
vol. 1, p. 1-27.

Meehean, O. Lloyd

1940. A review of the parasitic Crus-
tacea of the genus Arffulus in the col-
lections of the United States National
Museum. Proceedings U. S. National
Museum, vol. 88, No. 3087, p. 459-522.

Pennak, Robert W.

1953. Fresh-water invertebrates of
the United States, Ronald Press Co,,
New York, 769 p.

Wilson, Mildred S.

1959. Branchiura and parasitic Cope-
poda (In Ward, Henry B., and G. C.
Whipple. Fresh-water biology, 2d
ed., John Wiley & Sons, New York,
p. 862-868),

Wilson, Charles B.

1901 - 1942. A series of monographs
on the parasitic copepods. Most of
these were published in the Pro-
ceedings of the U. S, National Mu-
seum (complete bibliography may be
found in "Index-catalogue of Medical
and Veterinary Zoology," Part 17,
p. 5542-5544).

Leeches

Meyer, Marvin C.

1940. A revision of the leeches (Pis-
cicolidae) living on fresh-water
fishes of North Annerica. Transac-
tions American Microscopical Soci-
ety, vol, 59, No. 3, p. 354-376.

1946. Further notes on the leeches
(Piscicolidae) living on fresh-water
fishes of North America, Transac-
tions of the American Microscopical
Society, vol. 65, No. 3, p. 237-249.

Moore, J. Percy

1959. Hirudinea, (In Ward, Henry B.,
and GeorgeC, Whipple, Fresh-water
biology. 2d ed,, John Wiley and
Sons, New York, p. 542-557).

Pennak, Robert W.

1953. Fresh-water invertebrates of
the United States, Ronald Press Co.,
New York, 769 p.

17

GPO 9 2a 6a I

MBL WHOI Ctbrnry Serial

SE 00229

The Department of the" Interior, created in 1849, is our Nation's De-
partment of Natural Resources, concerned with management, conservation,
and development of water, wildlife, fish, mineral, forest, and park and
recreational resources. It also has major responsibilities for Indian and
Territorial affairs.

As America's principal conservation agency, the Department works to
assure that nonrenewable resources are developed and used wisely, that
park and recreational resources are conserved for the future, and that re-
newable resources make their full contribution to the progress, prosperity,
and security of the United States, now and in the future.

UNITED STATES DEPARTMENT OF THE INTERIOR
Stewart L. Udall, Secretary-
Frank P. Briggs, Assistant Secretary for Fish and Wildlife
FISH AND WILDLIFE SERVICE
Clarence F. Pautzke, Commissioner
BUREAU OF SPORT FISHERIES AND WILDLIFE
Daniel H. Janzen, Director
BUREAU OF COMMERCIAL FISHERIES
Donald L. McKernan, Director