Methods for the Prevention and Treatment of Hepatobiliary Syndrome in Black Carp Aquaculture

Black carp is one of the four major Chinese carps and a traditional aquaculture fish in our country. Because of its fast growth rate, low feed conversion ratio, tender flesh and delicious taste, it has long been favored by people. In recent years, the farming volume of black carp in Sichuan has gradually increased, but along with the increase in output, the accompanying diseases have seriously hindered the development of the black carp aquaculture industry and dampened the enthusiasm of many black carp farmers, among which hepatobiliary syndrome is one of the most serious diseases harming black carp aquaculture. Once this disease occurs, treatment is difficult, the cure rate is low, and affected fish do not tolerate transport, thus causing huge economic losses to farmers. Here we provide a brief introduction to the incidence of hepatobiliary syndrome in black carp that we have encountered, as well as the diagnosis, prevention and treatment of the disease.

1. Incidence and Symptoms

Investigations found that the disease occurs mainly from May to August, with a relatively long duration. It generally occurs when water temperature is high, mostly at 22-28°C. It is prone to occur in both pond culture and cage culture, especially in black carp weighing more than 1 kg. Simple hepatobiliary syndrome alone does not cause a high mortality rate, mostly between 2% and 5%, but after secondary bacterial infection the mortality rate rises substantially, reaching 10%-30%. Because after onset the common manifestations are symptoms of bacterial disease, farmers generally treat it with antibiotics, antibacterial Chinese herbal medicines, etc., but the results are not ideal.

The lesions are mainly in the liver and gallbladder. On dissection, the liver is commonly enlarged, fragile and easily broken, and severely yellowed or partially yellowed; some livers show mottled striping, presenting a mottled liver appearance. The gallbladder is often enlarged, sometimes to 2-3 times normal, and in some cases the gallbladder wall is seen to be thinned, with bile color deepened or yellowed. Most affected fish develop secondary bacterial infection, showing loosened and shed scales on the body surface, with some skin exposed, and in severe cases a red-skin appearance; at the same time, gill filaments may appear swollen or rotten; the intestine is often congested and red, with the intestinal cavity filled with inflammatory exudate.

2. Analysis of Causes

As for the cause of this disease, there is currently no unified conclusion. Through comprehensive analysis of the incidence and epidemiology of multiple clinical cases, we found that the occurrence of the disease is relatively related to feed factors and environmental factors.

Feed factors: The black carp with hepatobiliary syndrome that we have encountered were all fed compound feed, and many black carp farmers directly use carp feed to raise black carp without using special black carp feed, while the protein content of most carp feed is around 34%, resulting in excessively high protein content, which causes black carp to grow too fast and the liver to be overwhelmed, leading to disease.

Environmental factors: Ponds prone to this disease often have relatively thick sludge at the bottom. In pursuit of output, farmers mostly increase stocking density, thereby correspondingly increasing the amount of feed applied. As the burden on the aquaculture water body increases and water quality regulation cannot keep up, the pond water environment deteriorates, and ammonia nitrogen and nitrite in the water generally exceed standards. The fish's own immunity weakens, making them more susceptible to pathogen infection and causing death.

3. Prevention

Because the disease is mainly related to nutritional factors, carp feed with 34% protein can be used for feeding in the early stage of black carp rearing, but after black carp reach 500 g/fish, the protein content in the feed should be gradually reduced. If there is no special black carp feed, grass carp feed with a protein content of 28% can be used. Attention should usually be paid to feeding management, and feeding should not be excessive. Because black carp have a strong ability to compete for food, as soon as farmers see black carp with such good feeding ability, they often increase the feeding amount, causing the black carp to overeat. Due to excessive feeding, the fish also grow very fast, which further increases the farmers' interest in feeding and further increases the feeding amount, entering a vicious cycle, and finally the fish's liver is overwhelmed and disease occurs. Therefore, the feeding amount must be controlled, and it is advisable for the fish to eat until 80% full, thereby controlling the growth rate of black carp and preventing liver burden caused by excessively fast growth.

In addition, attention should usually be paid to regulating water quality. Microbial preparations should be used more often to regulate water quality, decompose ammonia nitrogen and nitrite produced in the water body by excessive feeding and fish excrement, prevent ammonia nitrogen and nitrite from exceeding standards, keep the water quality clear and refreshing, give the cultured species a good living environment, and reduce the occurrence of disease.

4. Treatment

Based on the causes analyzed above, the following treatment principles are formulated: control feeding, protect the liver and gallbladder, control secondary infection, and regulate water quality. First, reduce the protein content of the feed and gradually use 28% grass carp feed for feeding, and control the feeding amount, with feeding to 70% full being more appropriate. Oral administration of liver- and gallbladder-protecting drugs: use commercially available liver-protecting drugs for oral administration, and at the same time orally administer Vc at 80-100 mg/kg fish body weight for 5-7 consecutive days. Control secondary infection: orally administer antibiotics such as florfenicol at 50 mg/kg fish body weight to kill the pathogens of secondary infection for 5-7 consecutive days, and at the same time externally use disinfectants such as chlorine dioxide for disinfection, once every 3 days for 2-3 consecutive times; generally after 7 days deaths decrease. To prevent recurrence, after 7 days Sanhuang powder, allicin and other Chinese medicines can be mixed into the feed and fed for 3-5 consecutive days, which is very good for consolidating the therapeutic effect. Afterward, microbial preparations such as photosynthetic bacteria preparations, EM bacteria, nitrifying bacteria, nitrosifying bacteria, and Bacillus can be used externally to improve the water, preventing deaths caused by water quality factors.

5. Summary

From the analysis of causes, the main causes of hepatobiliary syndrome in black carp are excessively high feed protein and excessively large feeding amount, causing excessively fast growth and increasing the burden on the liver. From our treatment experience, in the treatment of hepatobiliary syndrome in black carp, control of secondary infection is very important. Merely protecting the liver and gallbladder without controlling secondary infection gives poor efficacy. However, when it is often misdiagnosed as a bacterial disease and treated simply as a bacterial disease, it only treats the symptoms and not the root cause. Often with this approach, after 2 days of medication the number of deaths temporarily decreases, but after several days the mortality rate rises again, causing the disease to persist without being cured and prolonging the course of the disease. Therefore, in the process of black carp aquaculture, the prevention and treatment of hepatobiliary syndrome should adhere to the above prevention and treatment principles, so as to promote the healthy and rapid development of black carp aquaculture, reduce deaths, and lower economic losses.