Immunosuppression and Prevention in Chickens

Immunosuppression and Prevention in Chickens

Summary:

The immune system of chickens is an important component of the body's defense system. Through the collaboration of several types of immune forms, it can rapidly and specifically eliminate pathogens that invade the body. Immune responses can be broadly classified into two major parts: innate immune responses and acquired immune responses.

Innate immunity (non-specific immunity) is a series of natural defense functions gradually established by the organism during phylogenetic development and evolution. It is also the most fundamental defense system of the immune system and often serves as the first line of defense against foreign pathogens. It mainly consists of the barrier system, the chemical defense system, and the cellular defense system.

The acquired immune response is specific and is produced by the joint action of antigen-presenting cells, T-lymphocytes, B-lymphocytes, and various cytokines. The two major immune responses complement and cooperate with each other, constituting the complete immune function of the organism and jointly maintaining the health of the organism.

Immunosuppression refers to the situation where the immune system (including immune organs, immune cells and immune-related molecules) is damaged due to various factors, resulting in temporary or permanent impairment of the immune response function and increased susceptibility to diseases in livestock and poultry. The characteristics of immunosuppressive diseases in chickens are as follows:

1. Decreased production performance, poor feed conversion rate and poor uniformity;

2. Poor overall condition of the flock and poor individual health, making them prone to diseases;

3. Reduced protective response to vaccines, and even vaccine failure;

4. Increased incidence of secondary bacterial infections and poor response to antibiotic treatment;

5. Increased mortality and culling rate;

6. Increased rate of carcass rejection.

The factors causing immunosuppression and the reasons affecting the immune response are very complex. Any factor that can damage the immune system of livestock and poultry can be called an immunosuppressive factor. Current research reports on immunosuppressive diseases in chickens in China show that the pathogens of immunosuppressive diseases are widely prevalent in chicken flocks and often coexist.

Due to the existence of immunosuppression, various immunosuppressive diseases often occur in production practice, resulting in the death or culling of a large number of chickens and causing serious losses to the poultry industry.

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The main factors causing immunosuppression in chickens

1. Infectious immunosuppressive factors

Infectious immunosuppressive diseases are generally caused by viral infections, often presenting as subclinical infections and frequently in the form of mixed infections. The mixed infection of different immunosuppressive viruses can not only suppress the immune response, leading to a decline or failure of vaccine immunity, but also interact with each other to enhance their pathogenicity. Moreover, there is a significant synergistic effect in immunosuppression, which requires sufficient attention in clinical practice.

The infectious disease factors that cause immunosuppression in chickens include infectious bursal disease virus (IBDV), chicken anemia virus (CAV), Marek's disease virus (MDV), avian leukosis virus subgroup J (ALV-J), reticuloendotheliosis virus (REV), reovirus (RV), adenovirus, Cryptosporidium, and coccidiosis in chicken. Generally, the above-mentioned immunosuppressive factors often act in concert, leading to immunosuppression in chickens.

At present, infectious bursal disease, infectious anemia of chickens and leukemia are the three major immunosuppressive diseases of chickens.

① Marek's disease (MD) in chickens

MDV belongs to the Herpesviridae family and can cause lymphoid tissue hyperplasia in chickens, turkeys and other poultry, leading to malignant tumors. It is characterized by lymphocyte infiltration in peripheral nerves, gonads, various internal organs, iris, skin and muscles, etc. At the same time, the immune organs of the body are also affected. As MDV targets lymphocytes, it causes degeneration, necrosis, lysis and transformation of lymphocytes, thereby causing immunosuppression in infected chickens.

② Chicken Infectious Anemia (CIA)

CIA is caused by the chicken infectious anemia virus (CIAV) and is also known as "meat chicken hemorrhagic syndrome", "chicken anemia dermopathy", and "meat chicken blue wing disease". In 1979, Yuasa [26] first discovered the disease in Japan and isolated the pathogen. Subsequently, Yuasa and others conducted a series of research reports on CIAV, and the virus was also isolated in different countries successively. CIA has spread worldwide.

In China, since 1992, researchers have frequently isolated this virus from infected chicken flocks, and CIAV infection is relatively common in domestic chicken flocks.

The immunosuppression caused by CIAV is far more harmful than the clinical onset type. Due to the low mortality rate and inconspicuous clinical symptoms, farmers often overlook the risk of this disease. However, CIAV can be vertically transmitted to offspring, causing obvious clinical symptoms in the next generation of chickens.

CIAV belongs to the Circoviridae family and can cause immunosuppressive diseases characterized by aplastic anemia and atrophy of the lymphoid tissues throughout the body in chicken flocks. The main target cells of CIAV are hematopoietic cells in the bone marrow and cortical cells in the thymus. Studies have shown that thymocytes in chickens infected with CIAV undergo apoptosis. Chicks that are infected shortly after hatching suffer the most severe damage to their thymus and bone marrow. Immune organs such as the bursa of Fabricius, spleen, and cecal tonsils can also be infected, resulting in a significant reduction in the number of lymphocytes, leading to severe lymphopenia, bone marrow damage, and atrophy of lymphoid organs in the body. 3. Infectious Bursal Disease (IBD) in Chickens IBDV belongs to the Birnaviridae family and can cause illness and death in chicks under 3 weeks of age or older, and can also cause severe and long-term immunosuppression in young infected chickens. The main reason for the immunosuppression caused by IBDV is that it targets B cells. The target cells of IBDV are B cells with IgM membrane proteins, and they have a greater affinity for immature B cells or their precursor cells, causing them to quickly degenerate or die.

③ Infectious Bursal Disease (IBD) in Chickens

IBDV belongs to the Birnaviridae family and can cause illness and death in chicks under 3 weeks of age or older. It can also cause severe and long-term immunosuppression in young infected chickens. The main reason for IBDV causing immunosuppression is that it primarily targets B cells. The target cells of IBDV are B cells with IgM membrane proteins, and it has a greater affinity for immature B cells or their precursor cells, causing them to rapidly degenerate or die.

The target organ of IBDV is the lymphoid organ, especially the bursa of Fabricius (see the figure below). The virus replicates in the lymphoid organ, initially causing inflammation and subsequently leading to atrophy of the organ.

④ Avian Leukosis (AL)

AL is a general term for various avian tumor diseases caused by avian leukosis virus (ALV). ALV can cause avian leukosis, a contagious disease mainly occurring in broiler chickens, which mainly leads to myelocytoma in chickens. Studies have shown that ALV infection in the early stage can induce apoptosis of lymphocytes in the thymus and bursa of Fabricius, and this early lymphocyte apoptosis is one of the important reasons for the atrophy of the thymus and bursa of Fabricius. Dynamic pathological histological observations have shown that ALV mainly causes focal or diffuse proliferation of myeloid cells in the bone marrow. The lesion leads to impaired bone marrow function, resulting in a decline in the immune function of the body, which is the fundamental cause of immunosuppression. Luo Yujun et al. have shown that severe parenchymal atrophic lesions occurred in the immune organs of the virus-infected group. The occurrence of this lesion is not only related to the bone marrow lesion and lymphocyte apoptosis but also to the necrosis of lymphocytes in the middle and late stages, thereby leading to severe immunosuppression.

⑤ Reticuloendotheliosis (RE)

Reticuloendotheliosis (RE) is a series of pathological syndromes in chickens caused by the reticuloendotheliosis virus (REV). It includes acute reticulum cell tumor formation, dwarfism syndrome, and chronic tumor formation in lymphoid tissues and other tissues. It targets lymphocytes or reticuloendothelial cells. After infection with REV, it leads to tumor cell infiltration or atrophy and shrinkage of various immune organs in the body, severely damaging the immune function of the body and inducing significant cellular and humoral immune suppression. Especially when young chickens are infected, it can cause obvious immune suppression.

⑥ Coccidiosis in Chicken

Coccidiosis in chicken is a parasitic protozoal disease characterized by intestinal lesions and damage, which is caused by one or more species of Eimeria that parasitize the intestinal epithelial cells of chickens. The clinical symptoms can range from subclinical infection to diarrhea and even acute hemorrhagic enteritis with high mortality, depending on the type and quantity of coccidia. Chicken coccidia are intracellular parasites with a complex life cycle. During reproduction, they can cause damage to the intestinal mucosa and impair the immune system of chickens.

Since 2022, infectious bursal disease, infectious anemia and leukemia have been the three most common immunosuppressive diseases in chickens. The control of these diseases remains a huge challenge, mainly because they are subclinical in nature and are often discovered along with secondary diseases during the diagnostic process.

2. Non-infectious Immunosuppressive Factors

The main non-infectious factors include nutritional deficiencies, mycotoxins, bacterial endotoxins, management and stress (such as transportation of chicks, delayed first feeding time, ammonia and cold or heat stress, etc.).

① Adverse stress factors

Various stress factors such as excessive cold, high heat, overcrowding, poor ventilation in chicken houses, and long-distance transportation all cause significant immunosuppression.

The core harm of stress to animals lies in the fact that stress causes intestinal microecological disorder, leading to the massive proliferation of harmful bacteria and the consequent release of a large amount of bacterial endotoxins. This disrupts the intestinal mucosal barrier, allowing a large number of endotoxins to invade the bloodstream and thereby damaging the immune function of the organism.

② Mycotoxin Infection

The mycotoxins that have the greatest impact on the immune system of poultry are aflatoxin, ochratoxin, and trichothecenes (T2 toxin). Their mechanism of action is to damage immune tissues and organs (especially intestinal mucosal injury, atrophy of the bursa of Fabricius and thymus), inhibit the synthesis of antibody proteins, and thereby reduce the immune response.

The levels of mycotoxins that cause immunosuppression in poultry are far lower than those that lead to typical mycotoxin poisoning lesions. The widespread and long-term effects of mycotoxins on chickens, which severely damage their immune organs and greatly weaken their immune function, have become one of the important reasons for the frequent occurrence of diseases and the increasing complexity of epidemic situations.

③ Bacterial endotoxins

When the body is severely infected by bacteria, harmful bacteria in the intestinal cavity will multiply excessively. During the proliferation or lysis of bacteria, a large amount of metabolic products and toxins are produced, leading to increased vascular permeability, intestinal mucosal damage, and necrosis and shedding of intestinal epithelial cells, thereby weakening the intestinal barrier function and reducing the body's immunity.

 

④ The Impact of Nutritional Factors

Certain vitamins (such as complex B vitamins, vitamin C, etc.) and trace elements (such as copper, iron, zinc, selenium, etc.) are essential substances for the development of immune organs, the differentiation and proliferation of lymphocytes, the expression and activation of receptors, and the synthesis of antibodies and complements. If they are deficient, excessive, or improperly proportioned, they will inevitably induce secondary immune deficiency in the body.

⑤ The abuse of certain drugs

Such as neomycin, chloramphenicol drugs, tetracycline drugs, etc., even at therapeutic levels, long-term use can affect the immune system and cause immunosuppression.

Source from Huimu Power