Mycotic Diseases in Aquaculture
The “fungi” and “fungi-like organisms” (see Mycotic Diseases of Fish) can affect aquacultured species. A number of these pathogens, previously lumped together as “fungi” based on similarity in morphology and saprophytic lifestyle, are unrelated taxonomically. The more common “water molds,” including Saprolegnia and Aphanomyces, are not true fungi but members of the Oomycota (Oomycetes). This distinction is important, because a more precise understanding of the biology of each group should lead to more targeted, effective management and chemotherapy.
Another true pigmented fungus, Veronaea botryosa, has been identified and described as the cause of a phaeohyphomycosis in aquacultured white sturgeon and Siberian sturgeon. Affected fish had one or more of the following clinical signs: abnormal orientation, buoyancy control problems, coelomic distention, reddening of the skin, emaciation, and ulceration of the skin or eye. Internal pathology included hemorrhages throughout the coelom, the presence of serosanguineous fluid, and organomegaly with nodules or cysts in multiple organs. Wet mounts of affected tissues often revealed presence of fungal hyphae, and positive confirmation was based on culture characteristics and identification by PCR. The disease appears to be linked to environmental stressors that may include temperature.
In general, systemic fungal infections in aquacultured species do not respond well to treatments. External infections of Saprolegnia may require environmental/handling modifications in addition to chemotherapy. For food fish use, specific commercial formalin and hydrogen peroxide products are FDA approved for use against fungus on fish eggs. Use of these products on other life stages is extra-label and requires veterinary oversight.
See also Mycotic Diseases of Fish. As in other species, mycotoxin contamination of feeds is associated with disease in aquaculture species. Specifically, exposure to aflatoxin-contaminated feed causes serious disease in rainbow trout. Aflatoxin is produced by fungi in the genus Aspergillus and is commonly associated with use of oil-seed crops such as corn, cottonseed meal, and peanuts. Toxin production may be associated with improper storage, with high humidity (>14%) and high temperatures (>27°C) being significant risk factors. Affected trout may have a grossly distended abdomen, and necropsy findings reveal a hepatoma or hepatic carcinoma. Liver lesions have been reported after exposure to concentrations as little as 1 mg/kg. Disease has been induced experimentally in tilapia at higher concentrations and/or extended exposure periods. In one study, tilapia fed 10 mg/kg for 8 wk had liver pathology and when fed 100 mg/kg for 8 wk, hepatic necrosis and mortalities were seen. In another study, tilapia fed 0.245 mg/kg over the course of 20 wk demonstrated reduced growth and liver pathology. Presence of the toxin can be confirmed using high-performance liquid chromatography (HPLC) or ELISA. Commercial test kits are also available.