Raymond F. Sis1, DVM, PhD; Brett William2s, BS; Scott Johnson1, BS
A marine biology research project, conducted by Texas A&M at Galveston in May 1993, turned up a number of young fish (approximately 2-5% of the catch) with white vegetative growths all along the body. The fish affected were croaker, star drum and menhaden. These fish were presented to the College of Veterinary Medicine at Texas A&M University, College Station, Texas, for further evaluation. Upon gross examination of the growths, we made a tentative diagnosis of Lymphocystis. Grossly it was characterized by white wartlike masses covering the skin and/or fins. Microscopic examination revealed clusters of hypertrophied connective tissue cells. These cells averaged 100 microns in diameter, contained an enlarged nucleus and nucleolus, and distinct basophilic cytoplasmic inclusion bodies. The inclusion bodies were of two types: a massive, cordlike, discontinuous array or a netlike array throughout the entire cell. Each hypertrophied cell was surrounded by a hyaline capsule. The microscopic examination confirmed our tentative diagnosis of lymphocystis.
Lymphocystis is the oldest known virus of fish.1 It may easily be mistaken for a tumor upon gross inspection. This disorder is caused by an iridovirus that enters the fish through the gills or small openings in the skin.2 Things such as trauma and stress may predispose an animal to infection.3 Also, younger fish seem to have a higher infection rate than adults. Normally, infected fish show no changes in behavior and seem to live normally, unless the large tumorous masses involve the mouthparts, which interfere with feeding. Infected fish are the source of infection to other fish upon rupture of the infected cells or the fish s death. For this reason some sort of treatment or removal of the diseased fish is needed. Treatment is limited and most fish will heal naturally upon removal of the predisposing factors. If needed, surgical removal can be performed. One way to deal with this, especially in aquariums and breeder stocks, is to cull the infected fish.
1. Gratzek, John B.: Aquariology: The Science of Fish Health Management. Titra Press, Morris Plains, NJ, pp 294-296.
2. Stoskopf, Michael K.: Fish Medicine. W.B. Saunders Co. Philadelphia, PA, pp 287-288.
3. Wolf, Ken: Fish Viruses and Fish Viral Diseases, Cornell University Press, New York, NY, 1988, pp 268-291.