Fungal infections

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

Viral infections

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.