Poultry immunosuppression refers to a group of diseases caused by various pathogens that damage poultry immune organs to some extent, significantly reducing cellular and humoral immune functions. This condition weakens the overall functioning of the poultry immune system, thereby increasing susceptibility to various diseases. Common characteristics include a relatively slow onset, considerable variation in incidence, and cases that are not concentrated within the flock but occur in rotation, with the period during which cases occur in rotation across the entire flock lasting a relatively long time. Apart from a small number of acute cases, most cases have a low mortality rate, and some can also lead to tumor development.
When immune function is normal, the body's ability to resist pathogens is enhanced. When immune function is abnormal, poultry may experience stress in response to vaccination, meaning that the body's immune response capacity decreases. In this situation, the expected immune protection will not develop; at best, immunization may fail, and at worst, it may instead trigger disease in the animal. Clinically, secondary or mixed infections involving immunosuppressive pathogens are often observed. If immune protection is inadequate, stress or pathogen contamination of the vaccine may occur, leading to transmission to the flock after the vaccine is used.
After a flock contracts an immunosuppressive disease, particularly when mixed or secondary infections occur clinically, the culling and mortality rate rises sharply, causing considerable harm and losses to the poultry industry. Therefore, immunosuppression and immunosuppressive diseases are currently receiving considerable research attention. Much of this research is based on the epidemiology of these immunosuppressive pathogens, as well as the pathology and symptoms of immunosuppressive diseases. Many research findings have been partly applied in production practice, and some studies have achieved relatively notable results.
I. What harm do immunosuppressive diseases cause to poultry?
Poultry affected by immunosuppressive diseases have reduced resistance to stress. In production practice, many stress factors affect immunity, including vaccination procedures, temperature changes, noise, and rodent infestations. If these stress factors are not eliminated, vaccination may not achieve the intended effect and may even lead to frequent disease outbreaks in poultry. Once immunosuppression occurs, the effects of vaccination or clinical treatment may be reduced or absent, resulting in vaccination failure. This not only wastes money but can also lead to more frequent disease outbreaks.
Clearly, after immunosuppression, poultry have a greatly reduced resistance to pathogens and become more susceptible to certain important diseases, such as avian influenza. Immunosuppression is a major risk in poultry farming and can cause a range of harm, including but not limited to stress responses, reduced resistance to pathogens, and an increased likelihood of disease occurrence or outbreaks. If disease occurs, its effects may be more severe than in normal cases, and susceptibility to certain pathogens may increase—for example, to pathogens commonly encountered in production, such as Escherichia coli and coccidia. Clinical signs include poor growth and development in broiler chickens, low egg production rates in laying hens, and short peak laying periods. Reduced production performance and more frequent disease outbreaks increase farming costs and reduce profits.
II. Causes of Immunosuppression in Poultry
Immunosuppression is generally divided into primary and secondary immunosuppression. Poultry raised on farms are prone to secondary immunosuppression early in life. The effects of this type of immunosuppression are not very significant on some small farms. However, on large-scale, intensive farms, secondary immunosuppression can cause greater harm and more serious consequences than primary immunosuppression.
During production, reovirus infection, reticuloendotheliosis, infectious bursal disease, Marek’s disease, J-subgroup leukosis, infectious anemia, and fungal infections are all important immunosuppressive diseases. Mycotoxins such as aflatoxin, produced by molds of the genus Aspergillus, mainly affect animals by causing tumors, immunosuppression, mutations, and teratogenic effects; the main target organ is the liver. Deficiencies in certain nutrients can directly affect the development and function of the immune system, such as deficiencies in trace elements (e.g., selenium and iron) and vitamins (e.g., vitamin A). Poor poultry-house management, such as inadequate ventilation, excessive stocking density, and poor housing conditions, may also negatively affect flock immunity.
1. Drug-induced immunosuppression
Adrenocortical hormones, alkylating agents (such as cyclophosphamide), antimetabolites, and some antibiotics commonly cause immunosuppression. Adrenocortical hormone drugs generally refer to glucocorticoids; common examples include dexamethasone injection, prednisone, and hydrocortisone. Alkylating agents can be divided into nitrogen mustards, ethyleneimines, sulfonate esters and polyols, nitrosoureas, triazene imidazoles, and hydrazines. Antimetabolites generally work by binding to enzymes in metabolic pathways, thereby blocking normal metabolic processes. In production, various antibiotics are widely used to prevent bacterial infections. Some antibiotics, such as penicillin and streptomycin, may also cause immunosuppression when fed over long periods or used excessively, and this warrants attention.
2. Immunosuppression caused by nutritional deficiencies
Malnutrition here may be caused by insufficient nutrients in food or feed, or may occur secondarily to other diseases that impair the body’s ability to absorb nutrients. It is easy to understand that when the body is malnourished, protein synthesis is insufficient and there are too few anti-stress factors, inevitably causing immunosuppression. Antibody levels, immune cell activity, complement activity, and cytokine production in the body are insufficient, making it impossible to establish an effective immune protection system.
3. Immunosuppression caused by mycotoxins
Mycotoxins are produced mainly when feed becomes moldy. If feed is not stored and used properly on a farm, mycotoxins can develop. During production, corn may become infected with mold while growing or during harvest, or may not be dried and stored properly after harvest. Even if it has not become mold-infected or has been stored properly, damp, rainy conditions during processing and use can cause corn or soybean meal to become moldy to varying degrees. Considerable evidence indicates that mycotoxins produced by moldy feed can cause degeneration, necrosis, or tumors in internal organs, especially detoxifying organs such as the liver. If this happens, the body’s organs may be damaged, and immunity may also be adversely affected, for example, through immunosuppression.
4. Immunosuppression caused by infectious agents
Generally, immunosuppression caused by pathogenic infectious agents does not easily result in major losses on ordinary small-scale farms. However, on large-scale farms, immunosuppression caused by infectious agents is one of the more serious forms of immunosuppression seen in production. Studies have shown that bacteria or viruses capable of causing disease in the host may also impair immune function when the host exhibits obvious symptoms, whether the infection is overt or subclinical. For example, in cases involving acute infection or death, this is often not readily apparent clinically. Some pathogens generally cause persistent subclinical infections, which are also difficult to detect. In recent years, production practice has identified an increasing number of viruses that can cause immunosuppression in flocks while remaining in a subclinical infection state, and these viruses have also become more widespread.
Three. Several Major Immunosuppressive Viruses in Poultry
1. Reticuloendotheliosis virus (REV)
This virus can naturally infect a variety of poultry and is naturally distributed among wild birds. Experimental studies have shown that when chicks are artificially infected with REV at 1 days of age, they exhibit varying degrees of poor response to the live attenuated Newcastle disease vaccine, or suppression of the antibody response produced by the host. REV is generally transmitted vertically. When infection occurs early after hatching, its effects are more apparent. Some studies suggest that many chickens infected vertically with REV do not produce antibodies to REV, but instead exhibit persistent or intermittent viremia, known as tolerant viremia. Chickens infected with REV have impaired immune responses and are more susceptible hosts for infection by other pathogens in the flock.
2. Marek's disease virus (MDV)
The virus is highly contagious. The target cells of MDV are lymphocytes. When the body lacks immune cells—lymphocytes—immunosuppression naturally occurs. One important characteristic of this virus is its “occupancy effect”: normal immune cells are damaged by the virus and cannot function, while the number of cells that can cause immunosuppression increases, naturally making this one of the causes of immunosuppression.
3. Avian leukosis virus (ALV)
This virus is a retrovirus and can be divided into different subtypes. Infection is most common in laying hens, and it manifests as lymphoid tumors. ALV damages the body's immune cells—lymphocytes—and naturally causes immunosuppression.
4. Chicken infectious anemia virus (CIAV)
This virus primarily attacks the hematopoietic tissue of the bone marrow and lymphoid tissues such as the thymus and bursa of Fabricius in chicks. It can be transmitted both vertically and horizontally. It is now widely recognized that CIAV infection causes immunosuppression. Many poultry farmers are extremely wary of this virus. In production, the disease is vaccinated against on schedule according to the vaccination program. However, note that once a flock is infected with the disease, subsequent vaccination is of little value; culling is preferable.
5. Reovirus (REOV)
The virus can cause infectious arthritis in chicks, but not all strains exhibit a clear immunosuppressive effect on the body; generally, they may only cause subclinical infection. Although subclinical infection has no obvious symptoms and does not have a major impact on the flock, once it occurs, this virus is taken very seriously because subclinical infection can cause varying degrees of immunosuppression.
6. Infectious bursal disease virus (IBDV)
This virus is quite well known in the poultry industry, where almost everyone involved in the sector has heard of it. IBDV infection primarily targets the bursa of Fabricius, a central immune organ in the body. Today, different IBDV strains are present both in poultry farm environments and in the surrounding areas outside farms. These strains vary greatly in their pathogenicity to chickens, with mortality rates ranging from zero to one hundred percent. However, strains with low mortality rates should not be underestimated: they can significantly suppress the body's immune organs and therefore also cause immunosuppression in chickens. Many studies have shown that after infectious bursal disease occurs on a chicken farm, the flock's resistance to various other infectious diseases is significantly reduced.
In summary, if a poultry farm is infected with an infectious disease that can cause immunosuppression, it can not only result in the deaths of large numbers of chickens, but also interfere with the immune responses to other vaccines, causing substantial production losses. Therefore, in clinical production practice, these diseases should be given serious attention, and effective measures should be taken to safeguard the healthy development of the poultry industry.
The copyright of this article belongs to China Poultry Industry Guide. The original article was published in China Poultry Industry Guide, issue 8 of 2025. If reproducing it, please cite the source.
Authors: 施网女, 宦震, and 范新民