Blackened, reddened or rotting fish gills are not to be taken lightly - beware of infecting other fish

Columnaris disease is a common and frequently occurring disease caused by Flavobacterium columnare. This disease is highly harmful to goldfish and koi. Foreign reports indicate that dozens of fish species can be affected and die from this disease, including Salmonidae, Cyprinidae, Catostomidae, Ictaluridae, Centrarchidae and Carangidae. Fish from fry to adult can be affected, with adult fish suffering the greatest harm. When the water temperature reaches above 15°C, the disease begins to occur; within the water temperature range of 15-30°C, the higher the water temperature, the more likely it is to break out and spread, and the shorter the time to death. The logarithm of the time to death has a linear relationship with water temperature. This disease can occur throughout the year, but is more common in late spring and early summer and in late summer and early autumn (April to October), and is most severe from July to September, often causing mass mortality of fish.

I

Causes

Domestic reports on the pathogens of bacterial gill rot in fish include Myrococcus piscina, Fleibacter columnaris, Cytophaga columnaris and others. At present, Myrococcus, Fleibacter columnaris and Cytophaga columnaris are collectively called Flavobacterium columnare, and the optimal growth temperature for this bacterium is 28°C.

Flavobacterium columnare is present in large quantities in carrier fish, contaminated water and pond mud. Diseased fish can continuously shed Flavobacterium columnare into the water, and the survival time of Flavobacterium columnare in water and pond mud is related to water temperature and water quality, but this has not yet been fully determined. According to reports, the higher the water hardness, the longer the survival period of Flavobacterium columnare in water. Flavobacterium columnare infects fish through direct contact, and infection is particularly likely when the gills are damaged (such as by parasite infestation, mechanical injury or harmful substances).

II

Main symptoms and pathological changes

The body color of diseased fish turns black, especially the head, commonly known as "black head disease". Diseased fish swim slowly, respond sluggishly to external stimuli, have difficulty breathing and reduced appetite; when the condition is severe, they leave the school and swim alone at the water surface, refuse to eat and lose response to external stimuli. In cases with slow onset and long course, the fish body becomes emaciated. When diseased fish are caught, the skin on the inner surface of the gill cover can often be seen to be congested and inflamed. The gill filaments of diseased fish appear pink or pale, then the tissue is destroyed, mucus increases and carries mud. In severe cases, the inner epidermis of the operculum becomes congested, and the middle part of the epidermis is also eroded into a roughly circular transparent area, commonly known as "open skylight". As the condition further develops, the secondary lamellae necrose and fall off, the tips of the gill filaments are damaged and appear "broom-like", with debris attached, and the gill filament cartilage is exposed; around the affected gill filaments, necrotic and shed cells, mucus, pathogens and various debris in the water often adhere. When these attachments are mainly mucus and pathogens, they appear pale yellow. The edges of the fins are often paler in color, appearing "edged". Because the gill filament tissue is destroyed, the diseased fish have difficulty breathing and often swim near the water surface in a gasping state. Severely affected fish still gasp at the surface even after being moved to clean water. Diseased fish often die of suffocation due to obstructed breathing, with a high mortality rate.

III

Key points for diagnosis

1. On-site diagnosis by naked eye observation can reveal typical symptoms of the disease such as blackened body, swollen gill filaments, increased mucus, decayed and damaged gill filament tips and exposed cartilage. Combined with the epidemic pattern, a preliminary diagnosis can be made.

IV

Prevention and control measures

Preventive measures

1

Remove excess mud at the bottom of the pond to reduce oxygen consumption by the mud during the hot season.

Feed reasonably and avoid deterioration of water quality in the pond/tank caused by excessive stocking density.

Feed nutritionally balanced extruded pellets to improve feed utilization and reduce water pollution caused by feeding.

Supplement vitamins appropriately to improve the immunity and disease resistance of the fish.

For fish ponds, the use of unfermented organic fertilizer is strictly prohibited in spring, and it is best not to apply organic fertilizer during the hot season.

During disease-prone seasons, promptly feed drugs for preventing and treating gill rot and drugs that boost immunity to prevent the occurrence of bacterial gill rot.

Apply Bacillus subtilis and photosynthetic bacteria to the tank/pond to promote the decomposition and utilization of organic matter in the water and reduce the formation of harmful water quality.

Promptly replace part of the old water in the tank/pond to ensure the water is fertile, lively, tender and fresh.

Ensure sufficient dissolved oxygen in the water and maintain the stability of the water environment in the tank/pond to reduce the decline in fish immunity caused by stress.

Regularly test the health status of the fish and the water quality, and take timely measures when problems are found.

Promptly kill parasites on the gills. Although there are not necessarily many parasites on the gills and parasitic infestation does not directly cause fish death, parasite infestation damages the gill tissue of the fish, which opens the door for the invasion of Flavobacterium columnare.

Treatment

2

In the early stage of the disease, external spraying of disinfectants alone can cure it; when the disease is severe, combining disinfectants with medicated feed is necessary to achieve good therapeutic results.

(1) For external use, choose one of the following and spray externally 1-3 times

Bleaching powder (containing 30% available chlorine), apply 1-1.2 grams per cubic meter of water. First dissolve the bleaching powder in water, filter out the residue, then sprinkle evenly over the entire pond.

Bleaching powder concentrate (containing 60% available chlorine), apply 0.5-0.6 grams per cubic meter of water.

Sodium dichloroisocyanurate (containing 60% available chlorine), apply 0.5-0.6 grams per cubic meter of water.

Trichloroisocyanuric acid (containing 85% available chlorine), apply 0.4-0.5 grams per cubic meter of water.

Dissolve penicillin or gentamicin in the pond (tank). The dosage is 800,000-1,200,000 international units of penicillin or 160,000 international units of gentamicin, dissolved in 50 kg of water, and sprinkled over the entire pond.

Apply 0.4 grams of povidone-iodine per cubic meter of water, and sprinkle over the entire pond at dusk for disinfection.

Rhubarb, apply 2.5-3.7 grams per cubic meter of water. First soak the rhubarb in 20 times its weight of 0.3% ammonia water to enhance its efficacy, then sprinkle the water and residue evenly over the entire pond; or use 1.5-3 grams of Chinese gallnut decoction per cubic meter of water and sprinkle over the entire pond.

(2) Oral medication

Add 2-3 grams of compound sulfamethoxazole per kilogram of feed, mix well, make into pellet medicated feed with good stability in water and feed. Feed continuously for 3-5 days, once each in the morning and afternoon.

Add 2-3 grams of sulfamonomethoxine per kilogram of feed, mix well, make into pellet medicated feed with good stability in water and feed. Feed continuously for 4-6 days, with the dosage doubled on the first day. Feed once daily.

V

Notes on diagnosis and treatment

1. Differential diagnosis: the causes of gill rot are mainly three types;

First, bacteria causing bacterial gill rot;

Second, fungi - branchiomycosis caused by gill fungi;

Third, various gill diseases caused by parasites, including various gill diseases caused by myxosporeans, Dactylogyrus and Sinergasilus. Attention should be paid to differential diagnosis.

(1) Look at the location of the lesions

In bacterial gill rot, the gill filaments rot, and in severe cases the gill cartilage is exposed and often carries mud. The inner surface of the gill cover is congested, and the central epidermis is often eroded into a circular transparent small hole, commonly known as "open skylight".

In branchiomycosis caused by fungi, the gills of diseased fish appear pale white, sometimes with pinpoint congestion or hemorrhage. This disease often appears as explosive sharp mortality, and microscopic examination will reveal gill fungal hyphae.

Gill diseases caused by parasites have pathogens including protozoa, myxosporeans, Dactylogyrus and Sinergasilus, among which:

1. Protozoa: their massive reproduction and irritation cause the gills of fish to produce large amounts of mucus, seriously affecting the fish's breathing. Therefore, gasping at the surface lasts longer, and in severe cases the body color turns black, the fish leave the school and swim alone, floating at the water surface;

2. Gill disease caused by myxosporeans generally shows many white dot-like or block-like cysts in the epidermal tissue of the gills, which are easy to see with the naked eye;

3. Gill disease caused by Dactylogyrus shows significant swelling, the operculum is slightly open, mucus increases, and the gill filaments are dark gray;

The above prevention and control measures are applicable to both fish tanks and fish ponds. I hope the above content can help everyone, and if there are any shortcomings, I hope the seniors will offer guidance.