Managing Parasites

Earthen ponds provide habitat for the tubifex worm, the intermediate host for the whirling disease organism, and exclusion of dirt-lined ponds for raising salmonids is an important strategy for managing and eliminating whirling disease at aquaculture facilities. However, salmonids destined for stocking in whirling disease-positive waters and cool and warm water species are still reared in earthen ponds throughout the state.

Unlike concrete or fiberglass raceways and tanks, earthen ponds play an important role in the life cycle of many parasites as they provide opportunities for presence of intermediate hosts, such as snails, clams, larval insects, and worms and are accessible by fish- eating birds and mammals which also play a role in the life cycle. Luckily, other than aesthetics, these parasites usually cause little harm to the fish, except in cases where the parasites become overwhelming, especially where fish are crowded for intensive culture or large numbers of intermediate hosts are present.

Many of the “usual” external parasites that can be found on fish raised in concrete raceways or fiber- glass tanks, such as Gyrodactylus, Costia, Trichodina, etc., are also found in earthen ponds. These parasites do not require an intermediate host species, and are easily passed from one fish to another.

One group of parasites observed commonly in fish raised in earthen ponds consists of several species of white, yellow or black “grubs” which are larval forms of digenean flukes. The intermediate hosts for these grubs include mollusks (snails and clams) and fish; the adult worms parasitize a fish-eating bird, fish or mammal. Larval flukes (cercariae) bore out of an infected mollusk and actively seek a host fish, entering through the gills or fins.

Advanced larvae (metacercariae) form cysts in internal organs such as the spleen, liver and surface of the heart. To the naked eye, these organs contain many small white or yellow spots, making the organ resemble pepperoni, rather than having the uniform dark red-brown color that is normal. If the infestation is heavy, it may cause reduced growth, organ inflammation or death of the fish. Since these organs are discarded by fishermen, few reports of infected fish are reported by the angling public.

Several species of larval flukes invade the skin or musculature of fish, and cause the skin to become unsightly or cause the fillets to be damaged. One commonly seen disease is “blackspot disease” caused by larvae in the skin, fins and musculature of fishes. The larva, or grub, is not black, but the inflammation of its presence in tissues causes host deposition of melanin, a black pigment responsible for the black speckles in the tissue, resembling ground pepper.

Another larval form of fluke Diplostomum spp. occurs in the eye of fish, where it can cause inflammation, “popeye”, cataracts, and starvation of blind fish. The life cycle of this parasite includes gulls and snails. This parasite has not been reported in any fish species raised in Colorado aquaculture facilities, but is probably present in free-ranging fish, with a historic report of it occurring in North Park in trout and suckers. Because the head is usually discarded by anglers, few if any reports have been received about this parasite.

Adult flukes living in the digestive tracts of fish also occur, but usually don’t cause harm to the fish unless present in large numbers. The adults are very small, about one-quarter to one-half inch in length, of ten disguised in the contents of the digestive tract, are discarded by anglers when the fish is cleaned, and usually go unnoticed. The required intermediate hosts for these parasites include mollusks and invertebrates such as may fly or dragonfly nymphs.

Another group of parasites seen in fish reared in earthen ponds are tapeworms. Adult tapeworms live in the intestinal tract of fishes, while the larval forms are found within the body cavity or muscle of fish. Although the head of the adult worm is firmly attached to the lining of the intestine, even heavy infestations rarely harm the fish. The life cycle of these parasites can be intricate, involving small crustaceans, fish, fish-eating birds and mammals, and sometimes segmented worms in the environment.

Of the tapeworms, the Asian tapeworm (Bothriocephalus acheilognathi), causes the most concern. The life cycle of this worm involves an aquatic invertebrate so it cannot be passed directly from fish to fish. The infestation can cause problems in intensively cultured fish, such as reduced growth, secondary bacterial infections, shortened life span, and inability to with-stand harvesting. In Colorado, the tapeworm has been identified in the San Luis Valley in fathead minnow and common carp, in cultured bonytail chub, and also in the lower Colorado River Valley in grass carp, Colorado River pikeminnow and fathead minnow.

Other tapeworms commonly encountered in the intestines of fish include Proteocephalus exiguous in salmonids in several earthen pond sites in Colorado and Proteocephalus ambloplitis in sun fish species at one public and several private aquaculture sites within the state. These species are fairly common in free ranging fish, as well.

Larger external parasites can infect pond-reared fish, including gill lice (Salmincola spp.) and anchor worm (Lernaea spp.). These parasites, which do not require an intermediate host, are on the skin, gills or fins of fish. Both of these parasites attach somewhat superficially and do not affect the flesh of the fish. If the infestations are heavy, especially with gill lice, the fish can succumb to warm water temperatures and low oxygen or from angling stress. Anchor worm causes local inflammation at the site of attachment and if the infestation is heavy, fish can have unsightly ulcers where the anchor worms once attached.

Although some of the described parasites are aesthetically unpleasing, they are not harmful to man, especially if fish heads and entrails are discarded and fillets are cooked thoroughly. One group of larval tapeworms found in the musculature of fish (Diphyllobothrium spp.) has been found to infect man if raw or inadequately cooked fillets are eaten, but causes few signs of disease. Fatigue, dizziness, intestinal symptoms and anemia can occur. The infection is diagnosed by analysis of a stool sample, and oral medication is effective. If it is desired to eat raw flesh of fish, freezing the fillets at 0°F for a minimum of 24 hours will kill the larvae. Salting the fish with a 10-20 per cent brine solution for two hours will also kill tapeworm larvae in fillets (Mitchum 1995).

Unfortunately, there are currently no FDA- approved drugs for removal of these flukes and tapeworms in food fish. Ponds can be treated by draining, dredging, and thoroughly drying to kill intermediate hosts. In some cases, application of lime or molluscides is required to kill intermediate hosts if the ground cannot be thoroughly dried or if eggs of the parasites, which can be quite resistant to drying, are present. Lining a pond can greatly decrease infestations of tapeworms and flukes. Gill lice can be treated with emamectin benzoate, a drug that must be used as an Investigational New Animal Drug and in compliance with all regulations of that program.