How to prevent and treat hemorrhagic ascites disease in bighead carp? A detailed guide to prevention and treatment strategies

As bighead carp has gradually become popular in the market, most ponds on the outskirts of our city have shifted from raising bighead carp as a secondary species to raising them as the main species. Bighead carp yields have grown from several dozen jin per mu when raised as a secondary species to over a thousand jin per mu now that they are the main species. Due to the increase in bighead carp farming density, the disease rate in bighead carp has also risen year by year, and mortality has climbed continuously, causing certain economic losses to farmers.

Hemorrhagic ascites disease in bighead carp is an explosive infectious disease, also known as bacterial septicemia, and is caused by bacteria. Before the diseased fish die, they mostly swim slowly around the pond with darkened backs. The oral cavity, jaw, operculum, and eyes of the diseased fish become congested first, after which the fins and both sides of the body show mild congestion symptoms, and there is relatively little food in the intestines. At the time of death, surface congestion intensifies, and muscle congestion or bleeding is severe. When the condition is relatively serious, perforation often occurs at the base of the pectoral fin, with yellow fluid flowing out, and this is mostly on the right pectoral fin; the eyeballs protrude noticeably, the abdomen becomes distended, the anus is red and swollen, and red mucus flows out when the abdomen is lightly pressed. The inner endothelium in the center of the operculum is corroded and shed, resembling an open "skylight," and the fish body is relatively thin and weak with the skull exposed. When the fish body is dissected, yellow or red ascites can be seen accumulated in the abdominal cavity. The liver, spleen, and kidney are enlarged, and the tissues are very fragile. The liver and kidney become pale in color and show an anemic state, the intestinal wall is congested and semi-transparent, the intestines contain gas and thin mucus, there is no food in the intestines, and the gill filaments are grayish-white with severe anemia and putrefaction at the ends.

I. Onset time and water temperature

The onset period of this disease is from early March to mid-November, and it occurs when the water temperature is 9-36°C, with the most severe onset especially at a water temperature of about 25-35°C. This disease can be divided into the acute type at high temperatures (28-35°C), which strikes fiercely and has an explosive nature, with 100 fish farms losing 600 kg in a single day at high temperatures, and the chronic type at low temperatures (9-16°C), in which during the peak mortality period a 16-mu pond loses 3-5 fish per day, though this should be distinguished from fish deaths during the recovery period.

II. Analysis of causes

An investigation of three fish farms where the disease occurred found that in 2003 the farming area was 950 mu, and cumulative fish deaths reached 16,800 kg.

1. Pathogenic bacteria: The pathogenic bacteria are mainly Aeromonas, and this bacterium has a suitable growth temperature of 20-30°C and very easily causes explosive epidemics.

2. This disease mostly occurs in old ponds with a long farming history. Domestic sewage in the suburbs of our district@#@233@#@and the use of livestock manure to fertilize fish ponds have a history of more than 20 years, so ponds with severe outbreaks have relatively large amounts of sludge, generally 10-20 cm, and some even reach 30-40 cm. Relatively deep sludge is an important factor in outbreaks of bacterial septicemia.

3. Excessive pond farming density, with more than 250 kg of bighead carp stocked per mu, is also a main cause of outbreaks of this disease. Therefore, under high-density farming conditions, it is recommended to be sure to add fresh water, turn on aeration equipment, and control water quality well.

4. Incomplete disinfection before stocking fingerlings leads to the introduction of pathogens and causes disease. The shared use of nets and tools, and the careless discarding of dead fish, are also main factors causing disease spread. Through investigation and analysis, if cross-infection does not occur between two adjacent fish farms, disease outbreaks generally will not occur.

5. During the high-temperature disease period under high-density farming, an increase in gulls in the farm area leads to mutual transmission.

III. Corresponding measures

1. Preventive measures

(1) If conditions permit, excess sludge should be removed from the pond, and the pond bottom should be exposed to sunlight. Use quicklime for dry pond clearing, using 50-70 kg per mu made into lime slurry and evenly splashed over the whole pond. For clearing the pond with water, use 125-150 kg of quicklime per mu at 1 m water depth and splash it over the whole pond. For ponds where the disease occurred in previous years, increase the amount of quicklime for pond clearing the following year by adding 100 kg per mu, and use bleaching powder (containing 30%-32% effective chlorine) at 20 g/cubic meter to splash over the whole pond for clearing.

(2) Do a good job of disinfection before stocking fingerlings. The stocking density of fingerlings should be reasonable and should be determined according to water quality conditions and the technical level of the contractor; generally, 50-100 kg per mu is suitable.

(3) Carry out regular drug prevention. In the first half of the month, splash quicklime once over the whole pond at 20-30 g/cubic meter; in the second half of the month, splash bleaching powder (30% effective chlorine) once over the whole pond at 1 g/cubic meter, or strong chlorine powder (84% effective chlorine) at 0.3 g/cubic meter.

(4) Never move nets in ponds where the disease has occurred; the next day after moving nets, large numbers of fish will die. In addition, when treating this disease, do not apply inorganic fertilizers such as ammonium bicarbonate to fertilize the water quality, otherwise it will affect the efficacy and fail to achieve the purpose of curing the disease.

2. Treatment measures

(1) Splash bleaching powder over the whole pond at 1 g/cubic meter, or splash strong chlorine powder over the whole pond at 0.3 g/cubic meter, or splash quicklime over the whole pond at 20-30 g/cubic meter; use quicklime, bleaching powder, or strong chlorine powder alternately once every half month.

(2) Use oral medicines or feed enrofloxacin (10-20 mg per kg of fish), or norfloxacin (20-50 mg per kg of fish), or rifampicin (10 mg per kg of fish) once a day, with 3 days as one course of treatment.

(3) For each mu at 1 m water depth, use 1.5 kg of Dryopteris crassirhizoma added to 5-7 kg of water, soak for 12 hours, then add 0.5 kg of alum and 30 kg of quicklime, dissolve in water, and splash over the whole pond. At the same time, grind 5 parts Scutellaria baicalensis, 2.5 parts Phellodendron amurense, and 2.5 parts Rheum officinale into powder, and mix 8-10 g into each kg of feed to make bait. For prevention, feed once every half month for 2 consecutive days; for treatment, feed once a day for 6 consecutive days.