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Fish Diseases

Questions and Answers Concerning Furunculosis (pdf, 26kb)

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. The site is routinely updated so check back for new postings.

Freshwater Fish:

Northern pike with blackspot lesions
Blackspot in Northern Pike
Click to enlarge
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
Click to enlarge
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
Click to enlarge
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
Click to enlarge
The nematode (roundworm) Philometra rubra in the visceral cavity of a striped bass.

Back to Fish Health in New Jersey

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Department of Environmental Protection
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Trenton, NJ 08625-0402

Last Updated: March 28, 2014