STUDIES ON THE PATHOGENICITY AND HISTOPATHOLOGY OF FUSARIUM IN FISH

Infections caused by Fusarium have increased in recent years in freshwater fish. However, few reports to data have described the pathogenicity and histopathology concerning Fusarium-infection in fish. We have isolated a Fusarium species from Carasslus auratus and have identified it as Fusarium oxysporum. In this paper, four kinds of different infection methods had been used to artificially infect the model fish species-goldfish with Fusarium oxysporum strain FF08, the intention of which was to explore the Fusarium pathogenicity in fish under different infection modes. At the same time, paraffin section and HE staining were also applied to study the pathological mechanism in the artificial infected fish. The results showed that intraperitoneal injection, intramuscular injection and wound infection could cause high mortality in the goldfish and the mortality were up to 100%, 93% and 100% in 15d respectively. However, there were no fish die in bath infection group. These results suggested Fusarium infection in fish maybe associated with some primary pathogenic factor, for example wound or bad living environment. Fusarium could cause high pathogenicity only when they had the change to invade the epidermis. Infected fish have shown the common clinical symptoms as naturally. HE staining revealed that Fusarium FF08 is harmful to fish muscle tissue, liver, kidney and intestinal canal. Muscle fiber cells showed degeneration and necrosis, with a large number of inflammatory cell infiltration, and there also found invasive hyphae. Liver cell was edema. Congestion and vascular thrombosis were occurred. Conidiophores could intrude into blood vessel. In addition, hepatic tissue had a strong inflammatory response. Kidney tissue was hyperemia and part of it was necrosis collapse. The mesentery was desquamated. We speculated if broad hypha penetrated the mucous membrane and invade ground substance, blood vessel may be the important target organ. The focal necrosis would extend around the blood vessel. When their hypha had made incursions into the vessel, they could cause thrombus and result in infarction. These necrotic tissues were very appropriate to the mass propagation of the Fusarium, which accelerated the Fusarium infection. The infected fish maybe consequently died from dyspnoea.