Gill rot disease is reported to be one of the main infectious diseases in China and affects especially grass carp (Ctenopharyngodonidella) fingerlings. The causative organism hasbeen described as Myxococcus piscicola. Whether it is the same as the bacterial gill disease described as affecting juvenile salmonids (Snieszko, 1958) is not clear. In juvenile salmonids, it can occur over a wide temperature range and is characterized by masses of myxobacteria on the gills and associated with hyperplasia of the gill epithelium. Several kinds of myxobacteria can be isolated from the gills, though not from the internal organs. It is suspected that the disease is secondary to some other predisposing stress factor and that the myxobacteria are only opportunists. Chinese workers have isolated Myxococcus piscicola from the affected gills. The disease symptoms are described as pale coloration of the gill filaments, then sloughing and accumulation of excess slime. The gill covers of seriously infected fish are inflamed and are eroded by the bacteria forming small transparent patches. The disease is reported to be prevalent in ponds overloaded with organic matter.
Farmers are advised to treat the infected ponds with bleaching powder (containing about 30 per cent available chlorine), at the rate of about 4 kg/ha. In serious cases, a treatment dose of 1 ppm is recommended. For salmonid gill disease, the addition of anti-bacterial chemicals to the water is suggested, as the infection is restricted to the gills. Hyamine 1622 at 2 ppm of commercial product or Diquat at 2 or 4 ppm of active ingredient can be used as one-hour baths daily for two to four days.
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