Fungal infections, another important cause of disease in fish, are usually the result of immunosuppression associated with
poor water quality, stress, and other diseases. Prolonged treatment with antibiotics may also predispose fish to fungal infections.
Saprolegniasis is a catch-all term for white, fuzzy mold growth on the skin of fish. Saprolegnia is a genus of water mold that commonly infects fish and their eggs. Immunosuppression, resulting from, for example, a drop
in temperature or stress from overcrowding, can predispose fish to such fungal infections. Fish with fungal infections are
treated with antifungal agents given orally in food, by injections, or by the water-borne route.1,2,5,6
Fifty-six viruses have been reported in fish.7
Viral diseases are frequently associated with secondary bacterial, fungal, or parasitic infections that may lead to a misdiagnosis.
Figure 7: A wet mount of a lymphocystis lesion exhibiting the enlarged virus-infected dermal fibroblasts (100X).
Lymphocystis disease, one of the first viral diseases described in fish, is common to freshwater and marine fish. It is caused
by a DNA iridovirus, and infected cells (fibroblasts) increase up to 50,000 times normal size (Figure 7). Advanced lesions exhibit large wartlike tumorous growths on the skin and fins. The disease is transmitted by direct contact
and is typically self-limiting, unless growths around the mouth cause starvation. Regression of the lesions may take several
months, and affected fish should be isolated from other fish. Keep recovered fish isolated for at least one month to prevent
spread of the virus to other fish.
Carp pox, or cyprinid herpesvirus I or Herpesvirus cyprini, is typically a benign self-limiting disease that causes epidermal hyperplasia that appears as superficial, soft, white to
gray, waxy or mucoid lesions on the skin and fins of carp and koi. The lesions can persist for several months in affected
fish, especially in cooler water temperatures, but resolve when the water temperature warms in the spring. Carp pox can result
in systemic disease and high mortality in juvenile cyprinids (less than 2 months of age), such as carp and koi.8
Spring viremia of carp is a reportable viral disease caused by Rhabdovirus carpio that causes high morbidity and mortality in cultivated carp, especially young carp, and related fish such as koi and goldfish.
Clinical signs of this viral infection are nonspecific because multiple organs are involved; however, dropsy, darkened skin,
and exophthalmia are common.8 Necropsy reveals small hemorrhages throughout the body, ascites, peritonitis, enteritis, and edema of internal organs. Outbreaks
of the infection are commonly seen in the spring but can occur whenever the water temperature drops below 64 F (17.8 C).8 A definitive diagnosis is made by virus isolation, or indirect tests such as ELISA and virus neutralization are available
to detect the virus in a fish population. There is no treatment for this disease, but keeping the water temperature above
68 F (20 C) helps decrease the mortality. Depopulation of infected fish followed by disinfection (1 part bleach to 10 parts
water) of the habitat is recommended to control the disease.