Detailed Explanation of Prevention and Treatment Methods for Bacterial Diseases in Sweetfish

I. Vibriosis

1. Symptoms Depending on the species of Vibrio involved, vibriosis can be roughly divided into three types: yellow liver type, intestinal tip type, and ulcerative type. Diseased fish show weak swimming, darkened body color, protruding eyeballs, and epidermal hemorrhage that in severe cases penetrates into the muscle layer. The inner surface of the operculum, fin bases, and snout tip show congestion followed by ulceration, the anus becomes red and swollen, and gill color becomes pale. The liver of diseased fish becomes enlarged, with some livers completely fading to a yellowish-earth color and others showing localized fading and congestion. Blister-like structures filled with bloody fluid appear in the liver of diseased fish, the gallbladder becomes enlarged and filled with orange-yellow or brown-green bile; the kidney loses elasticity, and some diseased fish show kidney enlargement or white block-like masses, sometimes with accumulation of clear ascites. Sweetfish suffering from vibriosis generally have mixed infections with other pathogens, but small fish with acute cases may die without showing any symptoms.

2. Prevalence and Harm Vibriosis generally occurs from January to September. The harm caused by vibriosis varies depending on drug resistance. In Shizuoka Prefecture, Japan, drug resistance tests were conducted on sweetfish Vibrio serotypes a and c using antibacterial drugs sulfamonomethoxine and oxolinic acid. The results showed that the drug sensitivity of Vibrio in sweetfish varied between different years, and serotype a had stronger drug resistance than serotype c, indicating that vibriosis in landlocked sweetfish is more difficult to control than in migratory sweetfish. The outbreak months of vibriosis also differ between serotypes: migratory sweetfish have a higher incidence from January to April but fewer cases after May, while landlocked sweetfish rarely develop the disease from January to April but have a high incidence after May. Vibriosis is the most harmful fish disease in sweetfish aquaculture and enhancement, with mortality rates exceeding 50% during outbreaks in culture, especially in juvenile fish. In recent years, losses have been reduced due to the promotion and application of sweetfish vaccines.

3. Prevention and Treatment (1) Prepare sweetfish Vibrio vaccine, dilute 10-fold, and immerse fry for 2 minutes; the immune protection period is 60-100 days. (2) Do not feed oxidized or spoiled feed; feed high-quality complete compound feed, pay attention to feeding frequency and method, and maintain clean water quality to prevent bacterial proliferation. (3) Periodically alternate the following drugs mixed into feed, which gives good results: a. Use 20 g of sulfamonomethoxine or other sulfonamide drugs per 100 kg of sweetfish, administer for 3 days, stop for one day, then administer for another 3 days; b. Use 4-6 g of tetracycline per 100 kg of sweetfish for 10 consecutive days.

II. Pseudomonas Disease

1. Symptoms Diseased sweetfish develop dotted white mucus on the lateral epidermis, which gradually expands. In severe cases, the body surface appears covered with a white film. Gill filaments lose blood with congestion at the tips, and the pectoral fin bases also show congestion. Internal organ symptoms are not prominent; the liver turns light brown, with varying degrees of kidney enlargement and ascites. A few diseased fish show cloudy, whitish eyeballs, or even eyeball detachment. Diseased fish swim slowly, have reduced appetite, become emaciated, have difficulty breathing, and sometimes swim erratically.

2. Prevalence and Harm Most susceptible in March and April. Affected fish are generally 10-20 g, with a daily mortality rate of 1%-2%, lasting about half a month, often accompanied by bacterial hemorrhage and septicemia symptoms. In Japan, the disease occurs mostly at water temperatures below 20°C; in Taiwan, China, the onset water temperature is around 22°C. Raising the water temperature above 26°C can stop the disease.

3. Prevention and Treatment (1) Prevent fish from being injured. Pathogen invasion is related to skin damage, so avoid injuring fish during transport, grading, and other operations. In addition, promptly kill parasites to prevent bacterial infection caused by parasite damage to tissues. Reduce stocking density and improve the water quality environment. (2) Mix any one of the following drug groups into feed for good efficacy: a. Use 10 g of compound sulfamethoxazole per 100 kg of sweetfish, halve the dose from the second to sixth day, and feed continuously for 6 days; b. Use 3-5 g of oxytetracycline per 100 kg of sweetfish, feed continuously for 5-7 days; c. Use rhubarb, Phellodendron bark, and Scutellaria root at a ratio of 5:3:2, and use 1 kg of the three-yellow powder per 100 kg of sweetfish for 3-5 consecutive days.

III. Aeromonas Disease

1. Symptoms Because Aeromonas species, biotypes, and the sites they attack in sweetfish differ, the signs shown by diseased fish also vary. It is usually related to hemorrhagic septicemia in fish under stress for some reason. The clinical and pathological manifestations are similar to Vibrio and Pseudomonas infections, causing systemic septic hemorrhagic disease, with darkened body color, large irregular hemorrhages on the epidermis and fin bases, anorexia, and death after gathering at the water outlet. Severe cases cause ulceration and fin/tail rot in diseased fish; bacterial isolation is required to confirm the cause. Intestinal-type Aeromonas punctata is an opportunistic pathogen that easily causes disease under adverse conditions such as deteriorated water quality, excessively low atmospheric pressure, and overfeeding, manifesting as slow movement, no appetite, darkened head, distended abdomen with red spots, red and swollen protruding anus, and blood-yellow mucus flowing out when the abdomen is gently pressed. There is abdominal dropsy, intestinal wall congestion and swelling, and the intestine generally contains no food but has much pale yellow mucus or pus. Other sites such as the liver also show congestion, appearing as red spotted congestion.

2. Prevalence and Harm This disease is more likely to occur during the alternating spring and summer seasons when the climate is changeable, often concurrent with Vibrio and Pseudomonas, and is very harmful.

3. Prevention and Treatment Maintain fresh water quality. Especially during seasons with changeable climate, carry out the "four fixed" feeding practices of fixed quantity, fixed location, fixed time, and fixed quality. When mild enteritis symptoms are found, mix allicin into feed at 0.3 g/kg feed or refer to the dosage described in the prevention and treatment of vibriosis.

IV. Streptococcosis (penicillin-sensitive)

1. Symptoms Diseased fish swim slowly, scatter in slow-flow areas, float at the water surface or hang with head up and tail down. Before death, diseased fish may swim erratically intermittently or turn belly up. Body color darkens, the body becomes emaciated, gill color fades, showing an anemic state; eyeballs become congested, protruding, and cloudy, or even detach. There is dotted hemorrhage on the ventral side, and the anus is red and swollen. Internal organ lesions and ascites are similar to the aforementioned Vibrio and Pseudomonas infections; diagnosis cannot be determined by external signs alone.

2. Prevalence and Harm Once infected with streptococcosis, losses are great. The bacterium has been isolated from wild fish and is relatively prevalent in Kagoshima, Miyazaki, and other areas of Japan. Its prevalence in China is still unclear. The suitable water temperature for onset is 25-28°C, so it is prevalent in hot summer seasons.

3. Prevention and Treatment (1) Avoid overcrowding, strengthen feeding management, feed high-quality feed, add vitamins in a timely manner, and pay attention to pond cleanliness and disinfection. Once the disease occurs, stopping feeding for 2-3 days can delay the condition. (2) Use 5 g of antibiotics (choose one of doxycycline, spiramycin, erythromycin, or oxytetracycline) + 1 g of compound vitamin B per 100 kg of sweetfish, mixed into feed for 5-10 consecutive days, with good results. (3) Use 5-20 g of sulfadimethoxine per 100 kg of sweetfish, double the dose on the first day, and administer continuously for 4-6 days for effective treatment.

V. Columnaris Disease

1. Symptoms Diseased fish are sluggish and have reduced appetite. It commonly occurs on the skin and gills of the head and back, but can also occur on other parts of the body. Initially, raised white patches appear surrounded by red congested areas, which soon develop into hemorrhagic ulcers covered with a matrix formed by bacterial cells and necrotic tissue. Gill damage is often more severe, with swelling of gill filaments, increased mucus, and decayed, missing, and whitened tips. The inner epidermis of the operculum is often congested, quickly causing death of diseased fish.

2. Prevalence and Harm More prevalent in spring, summer, and autumn, especially most severe in summer, often concurrent with enteritis and other diseases. Particularly in summer when rainwater makes water turbid, it often causes large-scale outbreaks. During outbreaks, it can cause mass mortality. In Japan, the harmfulness of this disease tends to increase, which may be related to water pollution and increased drug resistance of virulent strains.

3. Prevention and Treatment (1) Improve the environment, increase oxygen supply, control organic matter entering the water, and lower water temperature. Purification treatment of turbid water sources is quite effective as supportive therapy. (2) Splash the whole pond with 2-5 ppm gallnut decoction, or 0.3 ppm erythromycin, or 0.11 ppm clo, which has preventive and therapeutic effects. (3) When the condition is relatively severe, in addition to whole-pond medicated bathing, combine with oral administration by mixing drugs into feed. Specific dosages are: a. Use 5 g of erythromycin per 100 kg of sweetfish on the first day, then 2.5 g per day from the second to sixth day. b. Use 1 kg of rhubarb powder per 100 kg of sweetfish for 3-5 consecutive days.

VI. Cold Water Disease

1. Symptoms Macroscopic lesions are usually seen mostly in fins, skin, and muscle. Gill rot begins with varying degrees of gill filament anemia and swelling, followed by ulceration. Mouth rot begins with congestion and reddening of the lips followed by ulceration. Before fin rot and tail rot, the pectoral fin bases often redden first. In advanced stages, the bacterium can be isolated from internal organs. Because water temperature is low, inflammatory cell infiltration is generally not seen. When fin or tail ulceration is accompanied by poor water quality, the epidermis and dermis often show marked edema, and the lesion surface often already has abundant colony formation.

2. Prevalence and Harm In low water temperature aquaculture and enhancement facilities with very high stocking density and abundant organic debris, this disease has replaced vibriosis as the primary scourge of sweetfish aquaculture and enhancement in Japan. It has also been reported in Taiwan, China. Once sweetfish, especially juveniles, are infected, mass mortality can occur during outbreaks, with mortality rates exceeding 50%. There have been no reports of cold water disease in sweetfish in China. The disease epidemic season is generally at water temperatures below 16°C, presumably related to premature release of sweetfish and excessively low water temperature.

3. Prevention and Treatment Since cold water disease easily occurs in low water temperature and organic-rich waters, choosing enhancement and aquaculture sites with abundant water sources, good water quality, and suitable water temperature and improving environmental conditions are the fundamental ways to prevent this disease. It has been reported that formalin disinfection of the body surface is effective, and if diseased fish can feed, sulfonamide antibacterial drugs mixed into bait are also effective.