With the increase of temperature and suitable conditions, some conditional pathogens that originally existed in water or fish began to multiply in large numbers, thus acting on the target organs of fish. For example, the pathogen causing bacterial gill rot in grass carp, Flavobacterium columniformis, only acts on the target organs of fish and causes gill rot symptoms in gills (of course, a small amount of Flavobacterium columniformis can also be detected in tissues and organs such as blood, but it is more concentrated in gills).

2. Feed infected by mold

This problem is very common. From the practice of many years, this problem is not only an important reason for the incurable bacterial infection of the second-year-old grass carp species for a long time, but also a long-term possibility that other cultured species may cause disease occurrence or recurrence under different breeding environment conditions.