Aphanomyces invadans: The Causative Agent of the Epizootic Ulcerative Syndrome in Albino Northern Snakeheads

Abstract

AbstractA suspected epizootic ulcerative syndrome with a 50–70% morbidity rate broke out in farmed albino‐type (AT) Northern Snakeheads Channa argus at Jinming farm, Chao‐Yang Town, southwest China. To investigate the pathogen, diseased fish were sampled and laboratory examinations were conducted. Three specific staining methods (Grocott’s hexamine silver, glycogen–periodic acid–Schiff, and modified Gram’s stain) were also applied for a clear observation of fungi in tissues. Histopathological changes were studied to reveal the pathogenic mechanism. Results showed that Aphanomyces invadans was the causative agent of the disease in AT fish, judging from the fungus‐like hyphae, fungal granulomas, and 99% similarity of PCR products to A. invadans (MN814020.1). Among the three fungal staining methods, Grocott’s hexamine silver was the best; this method dyed the fungal hyphae black, while the surrounding muscle was dyed brown. Histopathologically, the affected skin and muscle exhibited remarkable dermatitis, myositis, and fungal granulomas, whereas the other tissues (e.g., gills, liver, spleen, kidneys, and intestines) showed histopathological changes related to osmotic imbalance. In conclusion, the causative agent of disease in AT Northern Snakeheads was A. invadans, which invaded the skin and muscle, causing ulcers in the skin.