A Detailed Guide to the Diagnosis and Treatment of Grass Carp Diseases
Grass carp is one of the four major Chinese carp species in freshwater aquaculture in China. It grows quickly and has a wide range of feed sources, making it a major freshwater fish farming species in China. If farming management is improper, it may become diseased. So how are grass carp diseases diagnosed and treated?
Grass carp diseases mainly include three types: bacterial complications, viral hemorrhagic disease, and hepatobiliary syndrome. After the onset of these three diseases, they can cause mass mortality of grass carp. Therefore, understanding their symptoms and taking timely prevention and control measures is of great significance.
1. Bacterial complications
Diagnosis: Fish suffering from this disease show symptoms combining three conditions: red skin disease, gill rot, and enteritis. The diseased fish has a dark body color, a dark head, swims alone away from the school, reacts slowly, has a swollen abdomen, and often has red patches on both sides, with obvious “fin erosion”. The anus is red and swollen and protrudes, becoming purplish red. When the abdomen is lightly pressed, yellow mucus and bloody pus flow out. When the abdomen is cut open, abdominal fluid can be seen, the intestinal wall is congested and inflamed, especially the hindgut, and the intestinal mucosal cells often ulcerate and slough off, mixing with blood to form bloody filth that fills the intestinal tract; the liver also often has red spots of congestion; the gill filaments of the diseased fish show pinpoint congestion, with rotting ends and exposed cartilage, causing the edges to turn white. The gill filaments carry mucus and mud. In severe cases, the opercular skin is congested, inflamed, and rotting, and the middle part is often corroded into a round transparent small window, commonly known as an “open skylight”; the body surface is locally inflamed, scales are loose and fall off, most obviously on both sides of the abdomen, the fin bases are congested, the ends rot, and water mold often parasitizes where scales have fallen off or fin rays have rotted.
Treatment: Thoroughly clear and disinfect the pond, and implement the “four disinfections” and “four fixings”; use 100 g of allicin per 40 kg of feed, mix with feed and feed continuously for 7 days; use 100 g of enrofloxacin per 40 kg of feed, mix with feed and feed continuously for 3 to 5 days, with the dosage doubled on the first day.
2. Viral hemorrhagic disease
Diagnosis: In the early stage of the disease, from external observation, the diseased fish shows mild congestion in the oral cavity, abdomen, operculum, eye sockets, fin rays, and both sides of the body. When the abdominal cavity is cut open, a small amount of food is still visible in the intestinal tract. As the disease progresses, the above-mentioned surface congestion intensifies, the flesh and bones show hemorrhagic symptoms, the area around the eye sockets is congested, the eyeballs protrude, and the abdomen is swollen and red. When the abdominal cavity is cut open, because the internal organs are damaged, yellow or red ascites accumulates in the abdominal cavity, the liver, spleen, and kidneys swell, and the intestinal wall is congested, inflated, and contains no food. The gills are grayish white, indicating anemia, sometimes purple and swollen, and in severe cases the ends of the gill filaments rot.
Treatment: Thoroughly clear and disinfect the pond, and implement the “four disinfections” and “four fixings”; orally administer 100 g of 10% enrofloxacin + 200 g of Sanhuang San mixed with 40 kg of feed, and feed continuously for 5 to 7 days.
3. Hepatobiliary syndrome
Diagnosis: In the early stage of the disease, the diseased fish drifts along the pond edge, reacts slowly, and reduces its food intake. No obvious symptoms are visible on the body surface by eye, and only a small number of fish have red eyeballs and eye sockets, and swollen gill filaments, with no parasites found under microscopic examination; dissection reveals no food in the intestinal tract, and a swollen liver that is white, green, or yellow. In severe cases, the liver is mushy and adheres to the intestinal mucosa and fat, filling the thoracic cavity. The gallbladder is sometimes enlarged and blackened, and sometimes absent, with severe ascites.
Treatment: Use 100 g of Gandanning + 100 g of allicin mixed with 40 kg of feed to make medicated bait, and feed continuously for 5 to 7 days.
In addition to treatment after grass carp become diseased, attention should also be paid to prevention in normal times, such as soaking and disinfecting fingerlings in 3% salt water before stocking, regularly splashing the whole pond with disinfectants such as 0.2 ppm chlorine dioxide or 0.3 ppm bromochlorohydantoin or 0.4 ppm strong chlorine, paying attention to reasonable stocking density, not feeding spoiled and deteriorated feed, not directly applying undecomposed animal manure, frequently adding new water or using biological agents to improve water quality, frequently removing feed residues, and regularly vaccinating.