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Main symptoms and pathological changes of 1.
　　
At the beginning of the disease, appetite decreased, and a small amount of silver carp was found dead. With the development of the disease, the sick fish swam slowly, unresponsive, appetite completely abandoned, and floated in the upper waters for a long time. As the course of the disease continues, the number of deaths rises sharply, the incidence rate can reach more than 30%, and the mortality rate can reach more than 50%. The appearance of the diseased fish shows that the body color is black, and the tissues and organs of the whole body are congested and bleeding to varying degrees. The opercular cover showed diffuse patchy bleeding. The mandible was severely filled with bleeding and the bleeding site was severely red. The body side and the base of each fin of the diseased fish had different degrees of filling and bleeding, and the jaw and abdomen below the whole fish body were the most serious. Sick fish gill filament swelling, part of the gill filament end decay, fall off, a slight gill rot phenomenon. The dorsal, pectoral and ventral fins show a slight rotten fin. After dissection, it can be seen that there is a small amount of light red to red ascites in the abdominal cavity. Fat bleeding redness; Intestinal mucosa congestion, bleeding, severe redness, intestinal wall thinning is transparent, there is no food in the intestine, there is light yellow mucoid substance; Liver and kidney enlargement, texture becomes brittle, with bleeding spots; Spleen congestion, bleeding, color becomes darker.
　　
2. diagnosis
　　
1. Water quality analysis Using ammonia nitrogen and nitrite rapid water quality detection kit to determine the ammonia nitrogen content and nitrite content in aquaculture water. The results showed that the content of ammonia nitrogen in the aquaculture water was 0.3 mg/L and the content of nitrite was 0.1 mg/L, which proved that the content of ammonia nitrogen and nitrite exceeded the standard, which had a certain promoting effect on the disease.
　　
2. Parasite detection Scraping the surface mucus of the diseased fish, and cutting a small amount of gill filaments, microscopic observation, body surface mucus and gills were no parasites.
　　
3. Isolation and identification of bacterial pathogens Under aseptic conditions, platinum ears were inoculated from the liver, spleen and kidney of diseased fish in TSA medium, and cultured at 28 ℃ for 24 hours to obtain a dominant bacterial group, whose colonies were round, with a colony diameter of 1~2mm, and were smooth, white and opaque colonies. The strain was identified as Aeromonas hydrophila by physiological and biochemical analysis.
　　
According to the results of epidemiology and etiology of the disease, the disease was diagnosed as silver carp bacterial septicemia.
　　
3. prevention and control
　　
The drug sensitivity of the pathogen was detected by disk diffusion method. The results showed that the bacteria were highly sensitive to florfenicol, azithromycin and enrofloxacin, moderately sensitive to meifaxane and norfloxacin, and not sensitive to doxycycline and neomycin. According to the experimental results of drug sensitivity test, florfenicol was used for mixing and feeding, the dosage of which was 50 mg/kg body weight for 5 days. At the same time, povidone iodine powder or chlorine dioxide [the second generation of bacteria poison clear or green oxygen source] was splashed in the whole pool to disinfect the water body once a day for two days. Through treatment, the condition was basically controlled, and the death was effectively controlled after two days of medication. On the third day, the appetite of the sick fish was significantly enhanced, the food intake increased, and the death rate further decreased, and it basically returned to normal on the fifth day.
　　
4. Summary
　　
Bacterial septicemia of common carp is a serious disease. It is easy to get sick when entering the high temperature season every year. Therefore, in the treatment process, it is necessary to combine internal and external medication. If conditions permit, it is best to conduct drug sensitivity screening to guide medication. The results of drug sensitivity showed that florfenicol had a significant antibacterial effect on the isolated Aeromonas hydrophila, so it played a significant role in preventing the development of the use in the treatment.
　　
In actual production, most farmers use fat and water to raise fish to raise silver carp. Generally, they do not feed, and sensitive drugs cannot be administered orally. This is a big problem for the treatment of silver carp diseases. In this case, the farmers in addition to the fertilizer and water farming method, but also fed powder farming silver carp, so we can easily take the drug orally. If you can't take it orally, you can only use external disinfectants to disinfect fish and water bodies directly.
　　
In this case, the content of ammonia nitrogen and nitrite in the water body exceeded the standard, which accelerated the development of the disease to a certain extent. Therefore, in the usual management process, the water quality should be adjusted in time to enhance the resistance of the fish and reduce the occurrence of diseases.