The Current Epidemic Situation and Control Strategies of Liver Diseases in Poultry

The Current Epidemic Situation and Control Strategies of Liver Diseases in Poultry

In recent years, liver diseases characterized by liver rupture and hepatosplenomegaly have occurred frequently in laying hens, broiler breeders and broilers. Investigations have revealed that there are many causes of liver diseases, including breed and genetic factors, as well as nutritional, disease and vaccination factors. In addition, improper management and environmental stress have greatly increased the risk of liver diseases. The occurrence of liver diseases is caused by the accumulation of multiple stimulating factors. In laying hens, the problem first appears at 7 weeks of age and is more common during the early laying period, with a weekly mortality and culling rate of 0.2-3%. The risk gradually decreases after 35 weeks of age. The formation of liver rupture is not an immediate process, but usually goes through a series of stages from congestion to hemorrhage, hematoma and finally liver rupture. Clinically, it is characterized by simultaneous or individual enlargement of the liver and spleen, also known as hepatosplenomegaly syndrome or big liver and big spleen disease. The incidence and mortality rate of liver diseases in laying hens are relatively high. Throughout the entire rearing period, the mortality rate caused by liver diseases can reach up to 60%, and it can also cause a 10-20% drop in egg production.

I. Breed and Genetic Factors

Different breeds have different tolerances to liver diseases and stress. From the perspective of breed, white-feathered broiler breeders, egg breeders and laying hens are more prone to liver diseases, while yellow-feathered broiler hens are less affected. In terms of the period of onset, liver diseases are more common during the growing period and the early laying period of laying hens. From the perspective of eggshell color, white-shelled and pink-shelled laying hens are more prone to liver diseases than brown-shelled ones. In terms of gender, hens are more affected, while roosters are rarely affected. Additionally, broiler breeders have relatively weaker disease resistance and a higher incidence rate than commercial chickens.

II. Nutritional Factors

The main nutritional factor that induces liver diseases is fatty liver syndrome.

Fatty liver syndrome in poultry is a nutritional metabolic disease characterized by abnormal fat metabolism in the liver, excessive fat deposition and liver steatosis. Clinically, it is manifested by sporadic mortality, enlarged yellow and greasy liver, liver rupture, liver hemorrhage, blood clots, brittle and fragile liver, and blood in the abdominal cavity. This disease mainly occurs in overweight caged laying hens, but it can also occur in free-range broiler breeders and laying hens.

The main causes of fatty liver syndrome in chickens include nutritional, environmental, hormonal and disease factors:

1. Excessive energy in the feed and excessive feed intake by the chickens.

2. Insufficient bile acid secretion by the liver affects the emulsification of fat, leading to fatty liver.

3. Lack of nutrients such as choline (or choline chloride, betaine), methionine and vitamins in the feed.

5. The higher the laying performance of laying hens, the higher the level of estrogen in their blood, making them more sensitive to fatty liver syndrome physiologically. This is the fundamental cause of fatty liver syndrome in laying hens.

6. Aflatoxin is considered an important factor in causing fatty liver hemorrhagic syndrome in laying hens. Laying hens fed with aflatoxin-contaminated feed have excessive fat accumulation in the liver, low levels of very low-density lipoprotein and triglycerides in the plasma, resulting in yellowish liver. Excessive feed intake reduces the liver's ability to synthesize lipoproteins, thereby reducing the ability to transport fat out of the liver and causing fat deposition in the liver, eventually leading to fatty liver hemorrhagic syndrome.

7. Limited space and insufficient exercise.

The prevention and treatment of fatty liver syndrome should be approached from multiple aspects:

1. Fine management of the flock, with reasonable stocking density and group feeding. Chickens that are 5% overweight should be immediately restricted in feed intake, which can effectively reduce the incidence of fatty liver.
2. Ensure a balanced diet for the hens, especially the ratio of protein to energy, to avoid high energy and low protein situations. This ensures the hens' health and production needs are met and helps prevent fatty liver disease.

3. Antioxidants can delay or prevent the auto-oxidation of nutrients in the feed. Therefore, adding antioxidants to the feed is a good choice.

4. Control the quality of the feed and add high-quality mycotoxin binders, choline chloride, bile acids, etc.

III. Vaccination Factors

Good protection effects from immunization depend on the health of the hens first, followed by the quality of the vaccine, scientific immunization programs, and standardized and meticulous immunization operations. These three factors affect the occurrence of liver diseases to varying degrees.

1. Vaccine Quality

The endotoxin, impurity protein, formaldehyde content, adjuvant quality, emulsification particle size, and viscosity of inactivated vaccines can all cause varying degrees of stress to the hens. Among them, endotoxin plays a major role. Generally speaking, the endotoxin content in viral oil vaccines is relatively low, while the endotoxin content in bacterial oil vaccines, especially those for pasteurellosis and rhinotracheitis, is uneven, causing greater stress during immunization and increasing the risk of liver disease and mortality. Additionally, high levels of impurity protein and formaldehyde in the vaccine can also cause significant stress to the hens. Therefore, when choosing a vaccine, safety should be the top priority, followed by effectiveness (efficacy), and then the price. Generally, vaccines from large manufacturers are preferred as they have better quality and safety, cause less stress to the hens, and have a lower risk of liver disease.

2. Immunization Program

A reasonable immunization program should be designed based on the local epidemiological situation. For example, if there are no cases of rhinotracheitis in the surrounding area, immunization may not be necessary. Using multi-valent vaccines can reduce the frequency of immunization and avoid frequent immunization stress. Advanced production processes can highly concentrate the antigen content, reducing the injection dose (e.g., 0.15 ml per bird during the brooding period, 0.3 ml per bird for young and laying hens), which can also reduce immunization stress.

3. Immunization Operations

Attention should be paid to the details of immunization operations. From the injection site, subcutaneous injection in the neck causes the least stress. Additionally, inactivated vaccines should be pre-warmed. For bacterial vaccines such as rhinotracheitis vaccines, the pre-warming temperature should be controlled well. If the temperature is too high, it can cause the bacteria to break and release endotoxins, increasing the endotoxin content. Therefore, the pre-warming temperature for bacterial vaccines should be around 37°C.

IV. Management and Stress Factors

1. Water Quality: Ensure that the water source meets the standards for pH, microorganisms, heavy metals, etc. Add bleach powder for disinfection and testing regularly to ensure the safety of the hens' drinking water. Soak the water lines once a month using a compound acid agent, etc. Soak at night and flush thoroughly before turning on the lights to remove biofilms and mold from the water lines. Flush the water lines once a week for more than 10 minutes each (ensuring sufficient water pressure).

2. Feed: Strictly control the quality of feed ingredients and pay attention to their storage environment. Use unconventional raw materials with caution to avoid excessive mycotoxins. Follow the proper feed transition process and regularly clean the feed trucks, feed towers, and feed troughs. Clean the feed troughs twice a week and empty the feed towers once a month to prevent feed from becoming moldy, rancid, or clumpy, ensuring the hens have fresh feed.

3. Precise Feeding: Feed the appropriate amount based on the hens' weight and egg production to ensure uniform distribution of hens, feed, and nutrition, and control the uniformity of the flock.

4. Good Environmental Control and Ventilation: Ensure good air quality with CO2 concentration below 3000 ppm. Keep the temperature difference within 3°C and avoid sudden temperature changes to maintain a stable temperature. Actively manage heat stress in summer and cold stress in winter, and prevent drafts. Control dust and appropriately increase the humidity in the chicken house.

5. Implement a reasonable lighting program. Reducing the light intensity (e.g., less than 10 lux) can lower the hens' activity level, reducing pecking and stress responses.

6. Timely culling: During the vaccination period, strengthen the selection and culling process, and regularly eliminate low-value chickens. Remove listless, weak, low-laying, and non-laying hens promptly, and replace them with new ones to reduce the density.

7. Biosecurity: Liver diseases are closely related to various pathogenic microorganisms. Strictly implement hygiene, isolation, and disinfection measures. Take effective measures to control mosquitoes, flies, birds, and rodents. Conduct regular in-barn disinfection.

8. Intestinal health: The function of the liver is closely related to intestinal health. Add probiotics and plant essential oils to improve the intestinal flora environment.

9. Reduce stress: Stress is the last barrier to liver diseases. It is essential to avoid the accumulation of multiple stress factors. If not possible, stagger or postpone several tasks. During the critical periods before and after stress, add liver and kidney protective or anti-stress drugs, such as glucose, vitamin C, multivitamins, and cod liver oil.