Primary fungal infections of fish are considered to be rather rare, however, we have made this diagnosis in fish on more than one occasion.  Nevertheless, by far, the most common fungal infections of freshwater fish are due to a group of fungi referred to as “water molds”.  More correctly, these are fungi belonging to the Class Oomycetes, and most are due to the Family Saprolegniaceae which is discussed below (Saprolegniasis).

             Saprolegniasis

            This is due to a large class of water molds, most commonly known as “Saprolegnia”.  They are ubiquitous saprophytes in soil and freshwater.  Most transmission is probably via the motile zoospores which allows dissemination to distant sites.  Most fish infections are probably acquired from inanimate sources (i.e., fungi sporulating on dead organic matter). 

            The classic case of saprolegniasis  presents as a relatively superficial, cottony growth on the skin or gills (photo).  Such lesions usually begin as small, focal infections that can spread rapidly over the surface of the body.  Newly formed lesions are white due to the presence of the pure growth of mycelia.  However, with time, the lesions often become discolored red, brown or green due the trapping of sediment, algae, or debris in the mycelial mat.  The greatest mortalities of this disease are observed in conjunction with infection of the gill tissue due to osmoregulation and respiratory dysfunction.  This fungus is rarely thought to cause primary infections, and in most cases, occurs as a pathogen secondary to viral or bacterial diseases in association with other stressors, such as poor water quality, increased stocking densities, ectoparasitism, etc (photo).  Diagnosis of this disease can be made by confirming the presence of the fungus by cytologic evaluation of the affected tissues. 

Visceral Mycosis due to Sporobolomyces Salmonicolor

             This is an uncommon disease of salmon, which we have reported from this lab.  This case involved high morbidity, low mortality with emaciation, cutaneous discoloration and ascites in Chinook salmon fry.  Lesions observed histologically included:  aerocystitis (inflammation of the swim bladder), myositis, peritonitis, and dermatitis.  The diagnosis was made by fungal culture of Sporobolomyces salmonicolor, (so named for its characteristic salmon color in culture) and identification of morphologically consistent fungal hyphae within tissue sections of affected fry.