Aquatic fungi often are considered secondary tissue invaders that follow traumatic injuries, infectious agents, or environmental insults such as poor water quality or low water temperatures. Because many fungi grow on decaying organic matter, they are especially common in the aquatic environment.
Saprolegnia infections are among the most common fungal infections of fish and fish eggs. Gross signs are grayish white, cotton-like growths on the skin, gills, eyes, or fins that may invade deeper tissues of the body. All freshwater fish should be considered susceptible. Microscopically, saprolegniasis can be recognized by making direct smears from the infected tissues and observing the nonseptate hyphal elements and mycelia. The sexual stages of the fungus can be seen only in cultures of the organism and are required for specific identification. Sabaroud's dextrose agar is acceptable for primary isolation of oomycetes, including the genus Saprolegnia. Preventive measures include removal of predisposing causes, eg, improper temperature, inadequate sanitation, excessive chemical treatment, or the presence of dead, infected fish and decaying organic material. If the environment is clean and skin pathogens have been eliminated, a single treatment with potassium permanganate, formalin, or hydrogen peroxide is often adequate to control external saprolegniasis. There are some effective over-the-counter products sold through the pet trade that contain malachite green. These dilute solutions should be effective in the home aquarium but should not be dispensed by the veterinarian and should never be used on food animals. Use in zoologic collections is also discouraged.
Epizootic ulcer disease is a reportable disease caused by the oomycete fungus, Aphanomycetes invadens. It has been reported in many species of freshwater and estuarine fish, including Atlantic menhaden and striped mullet, as well as ornamental species including gouramis and barbs. Tilapia seem to be resistant. The disease has been observed sporadically in the USA, especially in estuarine areas along the Atlantic seaboard. Lesions are characterized by deep ulcers that may extend into the visceral cavity. The center is typically red with a white margin. Invasive, nonseptate hyphae may be visible on a wet mount or histologically, often accompanied by a granulomatous tissue response. The organism can be cultured, but is difficult to identify because it is easily overgrown by more common fungal agents or by bacteria that may co-infect ulcerative lesions. If A invadens is suspected, both the local USDA area veterinarian in charge and the state veterinarian must be notified.
Fusarium solani is emerging as an important cause of disease in captive marine fish, particularly elasmobranchs. This organism is found in aquatic plants and soils in tropical and subtropical regions. Clinical disease has been reported in bonnethead and scalloped hammerhead sharks, as well as several species of marine fish including angelfish and parrotfish. Disease is associated with low water temperatures (<80°F or 27°C). Bonnethead sharks are particularly susceptible and develop erosions and granulomatous lesions along the head. Resolution of lesions requires warming the affected animals to a more appropriate temperature for the species.
Branchiomyces causes occasional fungal infections in gill tissue of warmwater fish. Two species have been implicated: B sanguinis intravascularly and B demigrans extravascularly. These may be two manifestations of the same agent. Clinical disease is associated with specific environmental conditions characterized by warm water (>68°F or 20°C), high concentrations of un-ionized ammonia, high organic load, crowded conditions, and algal blooms. Affected fish exhibit extreme respiratory distress and sudden increased mortality. Grossly, lesions are visible on gill tissue, typically at the base of the gill arch and primary lamellae. Wet mounts reveal branching, nonseptate fungal hyphae in blood vessels, which may be accompanied by a granulomatous tissue response.
Icthyophonus hoferi causes a systemic fungal infection of estuarine and marine fish, including ornamental species. It is worldwide in distribution, affecting captive and wild fish. It is rare in freshwater species; experimental infections have been demonstrated in goldfish and guppies. Signs are nonspecific and include weight loss, ascites, and abnormal swimming. On necropsy, primary lesions are granulomatous inflammation of muscle, heart, liver, and kidney with the primary differential diagnosis being Mycobacterium. Transmission is by ingestion of infective material. Wild fish serve as reservoirs of the fungus.
Many other less common mycotic infections have been reported in fish. Laboratory culture and complete clinical evaluations will further our understanding of these diseases.
Last full review/revision July 2011 by Ruth Francis-Floyd, DVM, MS, DACZM