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Control Methods for Bacterial Gill Rot Disease (Flexibacteriosis) in Koi

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I. Symptoms: The gills of diseased fish rot and carry some sludge. Sometimes the tissue at the tips of the gills rots, causing the gill edges to become incomplete and ragged; sometimes one or more spots on the gills rot, but not at the edges. The inner membrane skin of the operculum (gill cover) becomes congested, and sometimes breaks down and corrodes into a roughly circular transparent area, commonly known as "open skylight." Because the gill tissue is destroyed, the diseased fish has difficulty breathing and often swims near the water surface in a floating-head manner. In severely affected fish, the floating-head phenomenon persists even after changing to clean water. II. Pathogen: Cytophaga columnaris (columnar flexibacterium), synonym: Myxococcus piscicola.

I. Symptoms: The gills of diseased fish rot and carry some sludge. Sometimes the tissue at the tips of the gills rots, causing the gill edges to become incomplete and ragged; sometimes one or more spots on the gills rot, but not at the edges. The inner membrane skin of the operculum (gill cover) becomes congested, and sometimes breaks down and corrodes into a roughly circular transparent area, commonly known as "open skylight." Because the gill tissue is destroyed, the diseased fish has difficulty breathing and often swims near the water surface in a floating-head manner. In severely affected fish, the floating-head phenomenon persists even after changing to clean water.

II. Pathogen: Cytophaga columnaris, synonym: Myxococcus piscicola.

III. Harm and epidemic situation: This disease affects goldfish, koi, and red carp. It begins to spread when the water temperature is above 20°C. In the Yangtze River basin, late spring to autumn is the peak epidemic period. When the water temperature is below 15°C, diseased fish gradually decrease. This disease can cause mass mortality of fish in their first year, and large goldfish over one year old are also commonly affected; however, red carp and koi are less frequently affected. This disease is a common and frequently occurring disease of goldfish and is prevalent throughout the country.

IV. Prevention and control methods:

1. Young goldfish in their first year should be stocked at appropriately low density. Regularly feeding live food such as water fleas (Daphnia) and chironomid larvae has a clear effect on preventing this disease.

2. Soak in a 2% salt (sodium chloride) aqueous solution. At a water temperature below 32°C, soak for 5-10 minutes. This is effective for prevention and early treatment, especially for gill and body surface parasite infections.

3. Soak in 20 ppm nitrofurazone or furazolidone solution. Same as for treating skin inflammation and congestion disease.

4. Broadcast nitrofurazone or furazolidone over the entire pond. Same as for treating skin petechial congestion disease.

5. Broadcast bleaching powder over the entire pond at 1 ppm concentration. This method is suitable for large outdoor fish ponds.

6. Use the traditional Chinese medicine rhubarb (dahuang) at 2.5 ppm to 3.75 ppm concentration. For every 0.5 kg of rhubarb (dried product), soak in 10 kg of dilute ammonia water (0.3%) for 12 hours; the rhubarb dissolves, and then broadcast the medicinal liquid and residue together over the entire pond (regardless of the ammonia content of the ammonia water, it is always calculated as 100% pure ammonia concentration). This medicine is suitable for large outdoor ponds, especially for goldfish and koi farms that have used furan drugs for many years and have developed drug resistance; switching to rhubarb has significant therapeutic effects.

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