By: Terrence Ott, U.S. Fish and Wildlife Service, La Crosse Fish Health Center, La Crosse, WI 608-781-6238
Warts or Lymphocystis, although rarely lethal, is a chronic infectious viral disease caused by an iridovirus that results in uniquely enlarged cells, appearing as warts, in the skin and fins of walleye and most centrarchids in North America.

The highly dermatotropic lymphocystis virus infects the cytoplasm of fibroblastic cells of the interstitial connective tissue. Infected skin and fin cells in the course of their cytological change do not multiply, but enlarge from their normal size by a factor of 50,000 to 100,000.

The name lymphocystis disease was coined by an investigator during the early 1900's on the presumption that the giant skin cells represented a protozoan parasite. The parasite said to be involved was Lymphocystis johnstonei the source of the modern accepted name of the disease.

The viral disease has been reported in at least 81 species of fishes from 33 taxonomic families and seven taxonomic orders. The most susceptible species are walleye, sauger, pike, and panfishes. The disease has not been reported in salmonids.

The viral pathogen is typically dermatotropic (has affinity for dermis) and superficial. It is considered to have low virulence, since the course of this disease is exclusively chronic and liver functions are hardly impaired. Numerous warts can be unsightly and the marketability of obviously infected walleye is limited. Mortalities due to lymphocystis disease are probably rare, but the disease can occur year round depending on water temperatures. The developmental period for the virus to produce warts in the walleye takes about two weeks at 12EC.

Infected walleye behave normally but extensive warty growths on the fins can slightly affect swimming speeds, and heavy infections on the head region may hinder vision and feeding movements. Any walleye living with lymphocystis lesions for a few months to a few years without being destroyed by the disease will spontaneously slough the warty growths and remain free of the disease. Contact transmission is the principle means by which the viral agent is spread. External surfaces, including the gills, are the chief portals of entry. The oral route seems not to be involved, and furthermore, no evidence exists for vertical transmission (via the gametes). Factors such as high population density, external parasites, and trauma enhance transmission of the disease. External trauma has been implicated in transmission of the disease during spawning from aggressive behavior over spawning and mates, as well as, from human activities such as netting, fin clipping, or tagging.

A prophylaxis treatment is only possible by placing walleye in quarantine for at least 2 months. The best method to eradicate the contagious viral agent from a walleye population is by destroying all infected fish from the population.