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Saprolegniasis (Winter Fungus) and Branchiomycosis of Commercially Cultured Channel Catfish

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Saprolegniasis (Winter Fungus) and Branchiomycosis of Commercially Cultured Channel Catfish SRAC Publication No. 4700 VI August 2003 PR Saprolegniasis (Winter Fungus) and Branchiomycosis of Commercially Cultured Channel Catfish Robert M. Durborow1, David J. Wise2 and Jeffery S. Terhune3 Many fungi cause diseases that Winter saprolegniasis cially vulnerable to winter sapro- can infect and kill channel catfish (winter fungus or winter legniasis. For this reason, it is a eggs, fry, fingerlings and adults. disease of great economic impor- Most fungal infections are caused mortality) tance. Winter saprolegniasis is by water molds of the family The most common and economi- characterized by: Saprolegniaceae, so fungal dis- cally important fungal disease of I brownish patches of cottony eases in catfish are commonly cultured channel catfish is winter fungal growth on the skin, called saprolegniasis. Within the saprolegniasis. The species of including the gills, (Figs. 1 Saprolegniaceae family, Saprolegnia responsible for this and 2); Saprolegnia sp., Achyla sp. and disease has not yet been identi- Branchiomyces sp. are the genera I dry, depigmented skin (Fig. fied. Other terms used to describe 3); and that cause most disease in channel this disease are winter fungus, catfish. Although some fungal winter mortality and winter kill I endophthalmia (sunken species within this family are pri- syndrome. The term winter kill is eyes) (Figs. 3 and 4). Saprolegnia mary pathogens (e.g., also used to describe massive fish At first lesions are small, circular, parasitica S. declina ), most (e.g., mortalities that occur in an ice- depigmented areas, sometimes S. saprolytica and ) cause disease and snow-covered pond, usually with hemorrhagic margins. In only when there is preexisting ill- because of the depletion of dis- advanced cases lesions can ness, mechanical injury, or envi- solved oxygen under the ice. This become ulcerative, penetrating ronmental stress. should not be confused with the through the skin and into the fungal disease. Winter saprolegniasis usually occurs between October and March when water temperatures o o 1Cooperative Extension Program Facility, are below 15 C (59 F). Mortality Aquaculture Program, Kentucky State usually increases as temperatures University, Frankfort, KY rise in early spring. The disease 2Delta Research and Extension Center, has been reported as early as Mississippi State University, Stoneville, September and as late as April. MS The disease causes chronic losses 3 Auburn University, Auburn, AL and usually affects harvestable size fish (>1 pound); very high mortalities can occur. Fish greater Figure 1. Patches of fungus, sometimes than 2 pounds in crowded pro- circular, can cover the skin surface of cat- duction ponds appear to be espe- fish affected by winter saprolegniasis. muscle tissue, and the fish can be almost completely covered with thick fungal growth. Death is thought to be related to the inabil- ity of fish to regulate the salt bal- ance in the blood. Protozoan para- sites are frequently found on the gills and skin of fish suffering from winter saprolegniasis. The cause and pathogenesis of this disease are largely unknown; however, sudden decreases in temperature and a significant number of pathogenic Saprolegnia sp. zoospores in the water Figure 2. The skin of channel catfish with winter saprolegniasis (≥ 5 spores/ml in laboratory sometimes has a hemorrhagic, “beat-up” appearance. experiments) have been identified as risk factors for the disease. The primary risk factor is thought to be the inability of the fish to adapt to rapidly fluctuating water tem- peratures during the winter months. In experimental trials, rapid decreases in water tempera- ture (72 oF down to 54 oF, or 22 oC down to 12 oC, in 24 hours) have been shown to impair the fish’s immune system, cause a loss of mucus from the skin, and tem- porarily suppress mucus produc- tion by goblet cells in the dermal layers of the skin. Mucus provides a physical barrier that prevents fungal spores from contacting and infecting the skin of the fish. Mucus also contains antimicrobial components (including immunoglobulin or antibodies, Figure 3. This channel catfish affected by winter saprolegniasis has dry skin lacking in lysozyme, complement, C-reactive mucus, a hemorrhagic area on its lateral skin surface and severe endophthalmia (sunken protein and proteolytic enzymes) eyes). that can destroy invading zoospores. Without mucus, skin is unprotected and fungal spores begin developing masses of fun- gal hyphae that extend into the muscle tissue. If fungal spores are not present in sufficient numbers to establish infection, fish can adapt to a change in temperature and regain normal function of goblet cells and mucus production within 6 days, and can regain their immune cell function within 5 weeks, based on research results. However, once the infec- tion is established, fish do not appear to regain normal immune function, which makes the infec- tion more severe. Any condition that causes a loss of mucus or compromises the skin or immune system will likely pre- Figure 4. Close-up of a channel catfish with sunken eyes (endophthalmia) caused by dehy- dration from winter saprolegniasis. dispose fish to fungal infections. the disease fish are infected on the Branchiomycosis Physical injuries caused by sein- skin surface with little or no inva- ing, handling or crowding, or sion of the muscle and can, there- Branchiomycosis affects a wide lesions caused by infectious fore, be marketed safely. variety of cultured fish through- out the world but is a relatively pathogens create sites where fun- Another factor that can influence gal infection can occur. new disease in channel catfish the development of winter sapro- culture. Branchiomycosis has been legniasis is pond depth. In theory, Treatment and prevention reported only in fry and small fin- deeper ponds have more capacity gerlings stocked in nursery ponds Because of the expense and to resist changes in temperature. at warm temperatures (above undocumented efficacy of chemi- Therefore, maintaining ponds at 68 oF or 20 oC). Infections are cal treatment, control of winter their maximum depth can reduce usually self-limiting and occur in saprolegniasis presently focuses temperature fluctuation and help fish up to 2 months old. Infections on prevention and development fish acclimate to changes. are located primarily in the blood of production strategies that limit One prevention strategy being vessels (intravascular) of the gill the economic loss from the dis- investigated is using prophylactic and are confined to the gill arches ease. Optimizing water quality chemical treatments to reduce the and the base of the primary lamel- and reducing stress, especially in abundance of pathogenic lae. Fungal hyphae can be the late summer and fall, can zoospores. Laboratory trials have observed only with a microscope. decrease the effect of this disease. shown that formalin (25 ppm), Swelling of the gill tissues and Diseases such as columnaris copper sulfate (rate dependent on blockages formed by the fungal (SRAC publication 479) that total alkalinity of the water), and hyphae in the vessels of the gills occur in late summer or early fall diquat (0.125 ppm) can prevent decrease respiratory efficiency. may predispose fish to winter the development of Saprolegnia Infected fish are also subject to saprolegniasis, so it is important infections in aquaria by inhibiting secondary bacterial and viral to diagnose and treat those condi- fungal zoospores. These studies infections, which greatly increase tions promptly. were conducted using well water mortality. Maintaining sufficient oxygen where the chemicals have higher There is no known treatment for concentrations (4 to 5 ppm) may potency and persist longer. branchiomycosis. Although cop- also be important in avoiding Higher chemical rates will likely per sulfate and formalin are sug- winter saprolegniasis. In a field be required in production ponds. gested for treating other fish trial at a commercial catfish opera- Once a successful pond treatment species, no trials have been con- tion, repeated stress caused by is identified, a method will be ducted to evaluate their efficacy low dissolved oxygen during the needed to determine which ponds with catfish. Because the gills are summer and early fall was corre- should be prophylactically treat- the primary site of infection, lated with increased occurrence of ed. In particular, a technique is supportive therapy involves winter saprolegniasis. Unfortun- needed to differentiate between maintaining adequately aerated ately, optimal water quality is dif- the pathogenic spores of the water and increasing the chloride ficult to maintain in large, heavily Saprolegnia species responsible for concentration of the water. An stocked and fed ponds. One winter saprolegniasis and non- antibiotic may also be necessary aspect of water quality that is rela- pathogenic spores such as those of if secondary bacterial infection tively easy to manage is maintain- S. declina and S. saprolytica. develops. Unfortunately, the dis- ing adequate chloride concentra- Future research for controlling ease affects very young fish that tions to prevent nitrite toxicity winter saprolegniasis may include may not yet accept prepared diets, (see SRAC publication 462). the use of fungicides (such as making drug delivery difficult. Reducing the standing crop of hydrogen peroxide or bronopol) Antibiotics usually must be harvestable fish also will decrease or improved diets. Hydrogen per- administered in a finely crum- the potential loss from winter
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