John K. Rosenberger
Professor and Department Chairperson
Department of Animal and Food Sciences
College of Agriculture and Natural Resources
University of Delaware
Newark, DE, U.S.A.
Immunosuppression is characterized by a diminished ability of the immune system to respond to viable and nonviable antigens. The immune responses affected may be cell or antibody mediated or likely, a combination of both. The magnitude and duration of immunosuppression is highly dependent on host-related factors such as age, gender, genetic predisposition, stress, and overall physiological health. The nature of the agents and/or conditions eliciting immunosuppression is also important in terms of both the type and severity of immune system dysfunction.
For commercially produced chickens, there are both general and specific indicators of damage to the bird's ability to respond immunologically. General indicators may include an increased incidence of common respiratory diseases with extensive secondary bacterial involvement and increased mortality and condemnations at slaughter that are not associated with the introduction of a "new" pathogen or inadequate management or vaccination programs. Other general indications suggestive of immune system disorders are, a failure to resolve vaccine reactions, a loss of medication efficacy, and an increased incidence of "unusual" infections such as, gangrenous dermatitis, aplastic anemia, or inclusion body hepatitis. Lower than expected serological (antibody) responses to vaccines or common pathogens may also be indicative of immunosuppression. It should be recognized, however, that it is possible to have enhanced antibody responses in birds with limited or partial immune system dysfunction. This may be because pathogens are allowed to become more invasive and persist longer under these conditions thus producing more antigens for a longer duration, which enhances the antibody response even though birds are somewhat immunocompromised.
More specific indicators of immune system damage that can be assessed in the commercial setting include: destruction of the bursa of Fabricius that occurs in chickens less than two weeks old, permanent or severe thymic atrophy in young birds, and general lymphoid cell depletion. Failure to respond to injected antigens as measured serologically or the inability to respond to fowl pox or laryngotracheitis vaccination is suggestive of either humoral or cell-mediated deficiency respectively.
The diagnosis of immune system damage can oftentimes be difficult or at least confusing because there are situations that may mimic or appear similar to immunological disorders. The introduction of a new infectious bronchitis virus serotype, for example, could render available vaccines ineffective. The result would be an increased incidence of respiratory disease that could be complicated by secondary Escherichia coli infection. This obviously is not an immune system disorder but would appear, in a production setting, very similar to what might be expected in immunosuppressed chickens. Development of resistance to frequently used antibiotics, poor husbandry, or lack of sanitation and breed changes can also increase disease losses but again are not related to immune system damage per se.
Typically, immunosuppression is linked to the destruction or modification of cells and tissues essential to the ontogeny and functionality of the immune system. The organs most frequently identified, as being important immunologically are the bursa of Fabricius and the thymus. However, other tissues and organs including the liver, spleen, bone marrow, lymphoid cell aggregates, and circulating lymphocytes all have an important role in the maintenance and integrity of the immune response. Accordingly, alterations in functionality or depletion of these tissues may modify the chicken's ability to resist disease or respond poorly to vaccinations.
There are numerous factors or conditions that can depress immune system function. Infectious agents that have been documented as being immunosuppressive include; infectious bursal disease virus, chicken infectious anemia virus, Marek's disease virus, selected reoviruses, reticuloendotheliosis virus, and interactions between and among those agents. Subgroup J avian leukosis virus has also been suggested as being immunosuppressive because affected birds often appear to be more susceptible to bacterial infections, Marek's disease, and perhaps coccidiosis.
Noninfectious conditions that can affect the relative ability of the chicken to respond include; age, gender, breed, nutritional status, and environment. Several mycotoxins are also documented immunosuppressives in chickens as well as other species. Generally younger birds (< 3 weeks) tend to be more susceptible to infectious disease because of the relatively immature immune system. Females are usually more susceptible to uncomplicated Marek's disease and particular genetic lines of meat-type birds appear to be less capable of resisting respiratory virus challenge and secondary bacterial infections. Non-specific stressors such as cold and heat stress, a lack of drinking water or feed, crowding, and poor ventilation can, via different mechanisms, reduce the ability to mount an effective immune response. All of these factors, both infectious and noninfectious, can interact exacerbating the loss of immune competence.
There are several approaches that can be taken to control or minimize the impact of immune system disorders. The first, and most important, is the accurate diagnosis of immunosuppression and recognition of potential etiologies. This would include the ability to recognize and control pathogens like infectious bursal disease virus, Marek's disease virus, chicken infectious anemia virus and selected reoviruses. It is essential to appreciate the potential for interaction between and among these pathogens and utilize this understanding to minimize their collective impact under modern intensive poultry rearing conditions.
Producers may not be aware of or overlook the very important role that growing conditions or environment can have on the ability of birds to respond immunologically. Poorly managed birds will generally experience higher morbidity and mortality rates, poorer feed conversions, and an increased susceptibility to a number of infectious disease conditions. The mechanisms involved vary, but many are immune system related.
Commercial poultry producers, biologics manufacturers, veterinarians, and researchers agree that immunosuppression is a very important economic consideration that can affect virtually all aspects of both health and productivity. Accordingly, it is essential that the causes and effects of immune system dysfunction be understood and recognized and appropriate control strategies be developed and implemented.
From Proceedings of the 38th National Meeting on Poultry Health and Processing, Ocean City, Maryland.
