Volume 4, Issue 10
Lymphocystis Disease is a chronic and benign infection caused by an iridovirus that results in uniquely hypertrophied cells, typically on the skin and fins of more advanced orders of fishes. Lymphocystis literature spans more than 100 years and initially misdiagnosed the virus as the parasite: Lymphocystis vitrel. About 1925, fish pathologists concluded that Lymphocystis Disease was caused by an “ultra-visible virus.” Lymphocystis virus still remains the largest iridovirus ever discovered at 200 ± 50 nm.
Figure 1. Lymphocystis infected cells in fin of redbreast sunfish. Large hyaline (white circles) coated cells with a large macronucleus in center (lighter pink), nucleolus (c shaped white in center of bottom left cell) and large cordlike inclusion bodies (purple) around periphery of the cell’s cytoplasm.
The virus is spread by physical contact between fish. Factors such as high population density and external trauma enhance transmission. The virus has an affinity for connective tissues and consequently manifests its tumors many times in fins, gills, or skin. Incubation times and lesion durations vary with species, however, cold water species can carry visible tumors for several months up to a year; while warm water species may have the lesions for only a few weeks. At 25°C, sunfish can develop this lesion in 10 days.
The disease has been found in many species of fishes including: herrings, smelts, temperate basses, sunfishes, perch, flounder, sole, and many others. It has not been diagnosed in salmonids although smelt are in the same Order and have been infected. The geographic range of lymphocystis virus is probably global. Most accounts of the disease are from Europe, the British Isles, and North America.
Figure 2. Massive Lymphocystis tumor on second dorsal fin of redbreast sunfish. Image courtesy of Maine Atlantic Salmon Commission personnel.
Research on bluegills infected by Lymphocystis indicated that the fish could be repeatedly infected with the virus, but the tumors were smaller after the first infection. This is evidence that the fish developed some immunity to the virus.
Lymphocystis Disease seldom causes serious injury to the fish, however, these unsightly infections can lower the value of fish sold for human consumption or aquariums. There is currently no treatment for the disease, no vaccines to prevent the disease, and no reporting requirements. The best control measure is avoidance of infected fish. Infected fish should be culled from broodfish stocks. In the United States, some commercial producers of yellow perch and largemouth bass have had populations of fingerlings with infections that deterred prospective buyers.
Special points of interest:
Fish with Lymphocystis Disease cannot transmit the disease to people.
Lymphocystis Disease is caused by a virus.
Lymphocystis Disease infects many species of freshwater and marine fishes.
For more information read: K. Wolf. Fish Viruses and Viral Diseases 1988, 389-398.
Images in this document were made possible in part by a grant from the Maine Outdoor Heritage Fund.