Chickens, turkeys, and other poultry in a production environment can be exposed to stressors and infectious diseases that impair innate and acquired immunity, erode general health and welfare, and diminish genetic and nutritional potential for efficient production. Innate immunity can be affected by stressful physiologic events related to hatching and to environmental factors during the first week of life. Exposure to environmental ammonia, foodborne mycotoxins, and suboptimal nutrition can diminish innate immunity. Infectious bursal disease (IBD), chicken infectious anemia (CIA), and Marek's disease (MD) are major infectious diseases that increase susceptibility to viral, bacterial, and parasitic diseases and interfere with acquired vaccinal immunity. A shared feature is lymphocytolytic infection capable of suppressing both humoral and cell-mediated immune functions. Enteric viral infections can be accompanied by atrophic and depleted lymphoid organs, but the immunosuppressive features are modestly characterized. Some reoviruses cause atrophy of lymphoid organs and replicate in blood monocytes. Enteric parvoviruses of chickens and turkeys merit further study for immunosuppression. Hemorrhagic enteritis of turkeys has immunosuppressive features similar to IBD. Other virulent fowl adenoviruses have immunosuppressive capabilities. Newcastle disease can damage lymphoid tissues and macrophages. Avian pneumovirus infections impair the mucociliary functions of the upper respiratory tract and augment deeper bacterial infections. Recognition of immunosuppression involves detection of specific diseases using diagnostic tests such as serology, etiologic agent detection, and pathology. Broader measurements of immunosuppression by combined noninfectious and infectious causes have not found general application. Microarray technology to detect genetic expression of immunologic mediators and receptors offers potential advances but is currently at the developmental state. Control methods for immunosuppressive diseases rely largely on minimizing stress, reducing exposure to infectious agents through biosecurity, and increasing host resistance to infectious immunosuppressive diseases by vaccination. A longer term approach involves genetic selection for resistance to immunosuppressive diseases, which has shown promising results for MD but equivocal results for IBD and CIA.

Abbreviations: BF = bursa of Fabricius; CAV = chicken anemia virus; CIA = chicken infectious anemia; FAV = fowl adenovirus; HE = hemorrhagic enteritis; HEV = hemorrhagic enteritis virus; HVT = turkey herpesvirus; IBD = infectious bursal disease; IBDV = infectious bursal disease virus; IBV = infectious bronchitis virus; MD = Marek's disease; MDV = Marek's disease virus; MHC = major histocompatibility complex; NDV = Newcastle disease virus; ORT = Ornithobacterium rhinotracheale; PEMS = poultry enteritis and mortality syndrome; REV = reticuloendotheliosis virus; RSS = runting stunting syndrome; SE = Salmonella enterica serovar Enteritidis; ST = Salmonella Typhimurium

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