Treat fish with formalin, hydrogen peroxide, or potassium permanganate once predisposing factors have been addressed for this type of infection, most often caused by Saprolegnia. Saprolegnia is a saprophytic fungus-like pathogen (technically, an oomycete water mold) that commonly causes disease in freshwater fish.
A direct skin scrape wet mount should reveal hyphae that appear as large, nonseptate filaments, often with long “cattail-like” tips known as zoosporangia, which house the motile infective stages known as zoospores.
Low water temperatures are a major predisposing factor for infection. Other possible factors include rough handling (which may strip protective mucus and/or some skin), poor sanitation, and the presence of decaying organic material (or dead fish which may act as a reservoir).
Products that contain malachite green are effective for aquarium (non-food) fish but are of concern because of potential carcinogenicity/teratogenicity. CANNOT be used on food fish.
A related oomycete, Aphanomyces invadans, causes epizootic ulcerative syndrome (EUS) in fish. EUS is a WOAH reportable disease, endemic to much of the U.S. in wild populations and is immediately notifiable in Canada. Saprolegnia and Aphanomyces can cause huge economic losses in farmed fish.
Crayfish have their own oomycete, A. astaci (“crayfish plague”). Though endemic in North American crayfish, it does not generally cause clinical disease. European and Australian crayfish are vulnerable to death and disease.
Images courtesy of Hans Lauterbach Furchenstein, Velela, and Roy Yanong, VMD.
