Lymphocystis disease is a unique, typically chronic, viral infection of wild or captive marine and freshwater fish.
The causal agent is an icosahedral DNA virus ~300 nm in diameter, of the Iridoviridae family. Infection may be manifest by benign cauliflower-like lesions typically located on fins. The disease affects a wide range of fish and is generally considered global. Within the aquarium trade, painted glass fish are commonly infected.
The host range of lymphocystivirus includes over 125 species of freshwater and marine fishes belonging to 34 families and 9 orders. These fishes are susceptible to lymphocystivirus infection regardless of the temperature or salinity of the water in which they live or their geographical location. Susceptible species include freshwater fishes such as killifishes, basses, sunfishes, perches, and crappie as well as brackish water and marine fishes such as herrings, smelts, batfishes, scorpion fishes, sea basses, snappers, drums, goatfishes, scats, butterfly fishes, cichlids, damselfishes, wrasses, gobies, rabbitfishes, flounders, and porcupine fishes.
Lymphocystis is transmitted by direct contact with infected individuals. In aquaria, resident fishes are infected most commonly via the introduction of new infected fish or exposure to fish products that are contaminated with the virus. Trauma to the skin via handling or netting, mating, parasitism, and aggressive behavior accelerates viral transmission among fish. Increased water temperature and stocking density also may facilitate the development of lymphocystis.
Viraemia last approximately four weeks (depending on species involved, water temperature, and other variables) and then the enlarged cells rupture or slough off and release the viral particles into the water. While infected, the fish may become slowed or weakened, or more visible, and thus be more prone to predation or attack. If there are mouth lesions, the fish may have difficulty in feeding or may not be able to feed. Mortality is associated with secondary bacterial or fungal infections. After lymphocystis lesions are lost, the host tissue heals up. Adhesions and scarring can occur during healing. If the gills are affected, the fish can have difficulty breathing, especially if gill surface areas are destroyed (no longer present), or adhesions or scarring occur and gill surfaces are thus reduced in surface area or functional quality for oxygen uptake.
Diagnosis is based on clinical signs. Diagnosis is confirmed histologically. Feulgen-positive cytoplasmic inclusions and a hypertrophied nucleus are pathognomonic.
Infection is rarely fatal and eventually (within 20 days to 4 months) regresses. Tissue response around the nodules is very restricted. Gradual regression of infection was found to coincide with evident degeneration of lymphocystis infected cells, while fractionation of the hyaline capsule was followed by subsequent invasion of the lumen by macrophages and lymphocytes. The regression of infection and absence of recurrent infection suggests acquired resistance.
Several other diseases may resemble lymphocystis. Cytology and histopathology are useful in the definitive diagnosis of each of these diseases.
Epitheliocystis (mucophilosis): Epitheliocystis is caused by a coccoid to coccobacillary organism. Lesions appear as small, 20 to 400 µm diameter, white nodules on the gills. Histologically, the hypertrophied chloride or goblet cells have granular, basophilic cytoplasm filled with coccoid or coccobacillary bodies. The nucleus, if present, is displaced to the periphery.
Ichthyophthirius multifiliis (Ich or white spot disease): This disease is commonly observed and is due to protozoal parasitic infection. Lesions appear as small white spots on the skin, fins, tail, and gills. Cytologically, the parasite is large and round with a ciliated plasma membrane and a distinct macronucleus.
Walleye dermal sarcoma: This is a neoplasm composed of spindle cells that can be diagnosed by histopathology.
Idiopathic epidermal hyperplasia: This is a rare but benign disease characterized by variably-sized, white to pink, raised, cutaneous lesions. Histopathology reveals an irregularly thickened epidermis.
Unfortunately, a specific treatment for lymphocystis is not available. Affected fish should be isolated and monitored for secondary bacterial or mycotic infections that should be treated with appropriate drugs. Lesions associated with lymphocystis eventually regress. In some individuals, tumorous lesions may require surgical debulking, particularly when they interfere with prehension.
Fish should remain quarantined for at least 1 month after recovery. It is unknown whether previous infection with lymphocystivirus is protective against reinfection; however, recrudescence of viral infection has been reported in stressed fishes. Therefore, stress and skin trauma should be minimized. In addition, decreased stocking density and water temperatures may aid in disease suppression by decreasing stress.
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- Merck Veterinary Manual
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