Lymphocystis is a viral disease characterized by multiple creamy-white nodules on the skin and fins. It is probably the most common viral disease of ornamental fish. Caused by an iridovirus, the disease has been identified in over 100 species of fish. The disease is most common in recently purchased/shipped fish, and is more prevalent in brackish and marine species, as well as some New World cichlids.
Lymphocystitis is contagious from one fish to another, but all exposed fish are not affected by the disease. Viral particles can survive for at least several days in the water and sub-clinical carriers likely exist. The disease is frequently self-limiting (resolves on its own) although affected fish should be isolated.
The viral particles cause a marked enlargement of the dermal connective tissue cells resulting in cells up to one thousand times their normal size. Believe it or not, individual cells are frequently visible to the naked eye.
Affected fish have multiple raised epithelial (skin surface) nodules 1-2 mm in diameter ranging from pure white to pink in color. These nodules may be singular but more commonly appear in granular clusters of several dozen or more. Affected fish usually behave normally and mortality (death) is usually low or non-existent. The lesions are usually restricted to opercula (gill plates), skin, and fins and are rarely found on the surfaces of internal organs.
Other disease that resemble lymphocystis are "Ich" (Ichthyophthirius), "Marine Ich"(Cryptocaryon), cancer, Heteropolaria (a ciliated protozoan) and fungal skin disease.
Your veterinarian can diagnose this problem quickly, accurately, and inexpensively. A biopsy sample is examined using a standard laboratory microscope and "grape-like" clusters of hypertrophied fibroblasts (enlarged connective tissue cells) are seen in positive cases. Frequently, these cell clusters appear light orange or yellow in color.
Since the disease is usually self-limiting, the prognosis tends to be good. Severely infected individuals may succumb if the lesions are large enough to interfere with eating or respiration. In such severe cases, your veterinarian may elect to surgically remove these lesions.
Home Care and Prevention
If you notice lesions consistent with lymphocystis disease, you should isolate the affected fish for at least six weeks and transport the fish to a veterinarian for diagnosis.
All new fish should be quarantined for at least four weeks prior to being placed into any aquarium or pond. You should also carefully examine all fish in the aquarium at the pet store for signs of this diseases, especially if you are purchasing marine or brackish water fishes (including "painted glass fish"). By maintaining good water quality and not crowding your fish (in other words, keeping environmental stressors to a minimum) you can help prevent transfer of this disease from one fish to another.