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Fish Diseases

It is common to encounter infectious agents, such as parasites, bacteria and viruses, in fish; even healthy fish may harbor these organisms. Many of these infectious agents form a commensal relationship with the fish, in which the organism will benefit from the fish without affecting the fishes health.

In a healthy ecosystem a balance is maintained between the fish and these organisms. If an environment is compromised in a way that is stressful to the fish the balance may be disturbed and health problems can arise. The majority of infectious agents found in fish are not dangerous to humans, and fully cooking fish will kill them.

This page shows some common fish diseases and infectious agents that can be seen in the state. For detailed information on these or other fish diseases contact the fish health staff.

Freshwater Fish:

Furunculosis on trout
Furunculosis in trout
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Furunculosis is named for the raised muscle lesions resembling skin boils (furuncles), which occur in chronically infected fish (a & b). When these boils are dissected then muscular necrosis and hemorrhage is evident (c). In the laboratory we can test for furunculosis by isolating the bacteria in culture media; typical isolates of the bacterium form a diffusible brown pigment in areas of bacterial growth (d). More on furunculosis.
Cyprinid herpesvirus-2 causing HVHN
Cyprinid herpesvirus-2 causing HVHN
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In June 2013, Goldfish Herpesviral Hematopoietic Necrosis Disease (HVHN) was responsible for a massive fish kill of goldfish in Runnemede Lake (Camden County), likely initiated by increasing water temperatures combined with the high densities of goldfish in the lake. Goldfish are an established invasive species in North America and are found within many waterways throughout the state.

The virus Cyprinid herpesvirus-2 (CyHV-2) is responsible for HVHN. This disease can cause high mortality during outbreaks, which are most common when water temperatures are above 65F (18C). Disease signs in goldfish include pale gills (a), lethargy, anorexia, and swelling of the spleen and kidney.

Using histology we can look for evidence of HVHN within the internal organs. Histological signs of HVHN include branchitis and fusion of gill lamellae (b), focal cell necrosis in the kidney and spleen (c), and cellular nuclei throughout the lesions containing marginated chromatin (d). To confirm the presence of the virus we use molecular biology methods or transmission electron microscopy to directly visualize the virus (e).

This virus is specific to goldfish and will not infect other species.

Larval nematodes found encysted beneath skin of margined madtom
Larval nematodes in the skin of a margined madtom
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Larval nematodes in Ictalurids (catfishes, such as the Margined Madtom, Noturus insignis) form blister-like cysts beneath the skin (a & b). Several nematodes, as seen with a light microscope (c) may be present in each blister.

Similar larval nematodes have been reported in other catfish, such as the channel catfish, brown bullhead and white catfish, but the nematode species remains undescribed and the life cycle is unknown. Infected fish should be handled with care since the definitive host of this parasite is unknown.

Northern pike with blackspot lesions
Blackspot in Northern Pike
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Blackspot is caused by a digenean parasite that can enter and encyst in the skin of fish, notice the small raised black spots over the skin of the fish. The black spot is melanin pigment, which is a host response to the presence of the parasite.
Bluegill with Lymphocystis
Lymphocystis In Bluegill Sunfish and Largemouth Bass
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Lymphocystis disease virus is a DNA iridovirus that causes tumorous growths on the skin and fins of fish. In the image is a bluegill sunfish with lymphocystis predominantly on the fins and also on the skin (figs. a & b). In severe cases, like this sunfish, the virus can be found in the internal organs.

The largemouth bass (figs. c & d) has hemorrhaged tumorous growths on the pectoral and anal fins caused by this virus. Generally this is a self-limiting disease that occurs in the peripheral organs (skin and fins).

Digenean cysts in sunfish kidney
Digenean Posthodiplostomum Minimum in Sunfish
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Posthodiplostomum minimum is a very common and widespread digenean parasite in fish. Encysted parasites are observed in the kidney of a sunfish (arrows). Fish and snails are intermediate hosts for thesee parasites, and birds are the definitive host.

Marine Fish:

Roundworm in striped bass
Philometra rubra in Striped Bass
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The nematode (roundworm) Philometra rubra in the visceral cavity of a striped bass.
Anchor worms
Anchor Worms Lernaeenicus sp.
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Atlantic menhaden (a & b) and a black drum (c & d) with anchor worms (Lernaeenicus sp.) attached to the flesh of the fish. These parasites are copepods (crustaceans) that have free-living and parasitic life stages.

The large parasites attached to the fish skin as seen in the picture are adult females. These parasites can cause irritations and secondary infections in the skin and muscle where the parasite is attached, although mortality is not commonly associated with the presence of the parasites.

Back to Fish Health in New Jersey

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Trenton, NJ 08625-0402

Last Updated: May 5, 2014