Symptoms and Prevention and Treatment Methods of Fish Intestinal Fungal Disease

The pathogen is Basidiobolus sp., belonging to the class Zygomycetes, order Entomophthorales, family Basidiobolaceae.

Symptoms: The anterior abdomen and the area in front of the anus of diseased fish are noticeably swollen, forming two white protrusions. Because the anus is blocked, the diseased fish cannot defecate and soon dies. After death, the abdominal wall quickly rots through, exposing the viscous internal organs. Dissection of diseased fish reveals a white, sticky, fat-like substance coating the outside of the stomach and intestines; the intestines become thickened and white, with a thinned intestinal wall, and the interior is filled with hyphae and their conidia, especially densely packed at the end of the intestine. In a small number of diseased fish, hyphae are also distributed in the stomach, but no infection of the abdominal cavity or other organs was observed.

For prevention, oxytetracycline, nitrofuran antibacterial drugs, and antifungal drugs such as griseofulvin, clotrimazole, candicidin, and chlorhexidine were previously mixed into bait and fed, but none were effective. Through drug screening tests, three drugs - nystatin, rhubarb, and malachite green - were selected as having a clear inhibitory effect on Basidiobolus sp. In recent years, fish ponds have been regularly disinfected with malachite green during fry production, and the incidence rate has decreased significantly.