Immunosuppression is an industry threat worldwide, resulting in significant economic losses. Foundational immune health means providing poultry with early protection against the primary immunosuppressive diseases – most commonly Marek’s disease (MD) and infectious bursal disease (IBD, also known as Gumboro disease) in chickens – in order to allow their immune systems to thrive and reach full potential. Numerous studies have shown that when chickens start life with a strong immune system, everything that follows is better: their overall health, resistance to infection, uptake of other vaccines, growth, and productivity.1-13
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The Impact of Diseases on Immune Health
The avian immune system has many components that help the bird fight infection: lymphoid tissues, the bursa of Fabricius, thymus, spleen, and cellular effectors, mainly B and T lymphocytes, as well as other white blood cells, like macrophages, heterophils, natural killer cells, or dendritic cells. The Marek’s disease and IBD viruses attack lymphocytes, subsequently suppressing aspects of the bird’s immune functions.
The bursa of Fabricius is a unique, specialized immune organ in birds that plays an essential role in developing B cells and antibodies in response to invading pathogens. The bursa is still under-developed when chicks hatch – it continues to develop and serve its role during the first 10-15 weeks of a bird’s life. The IBD virus attacks the bursa, destroying B cells and damaging bursal function.
The cells associated with the cellular immune response are the T cells, which are released from the thymus. These cells are critical in the response to a number of diseases including Marek’s disease.
Marek’s Disease
Marek’s disease is a herpesvirus-induced lymphoproliferative disease of chickens. In the 1960’s, Marek’s disease caused huge economic losses to the industry due to high numbers of leukosis condemnations at processing as well as widespread immunosuppressive effects leading to reduced performance. The virus causes transformation and proliferation of T cells, with clinical presentations including visceral or subcutaneous tumors, neuronal paralysis, and blindness due to lymphocyte invasion of the iris.
Clinical presentation of Marek’s disease varies greatly and may include peripheral paralysis, neurological signs, diarrhea, blindness, red legs, subcutaneous tumors, and visceral tumors. It is also a major cause of immunosuppression due to early viral replication occurring in both B and T lymphocytes. After initial viral replication and immunosuppression, the virus goes latent in T cells. Infectious particles can then be shed via viral replication in feather follicles and/or T cell transformation and proliferation, leading to invasion of the peripheral nervous system or tumor formation in any organ.
The virus falls into three serotypes depending on oncogenicity and natural hosts. Serotype 1 contains oncogenic field viruses and is further categorized by virulence including mildly virulent, virulent, very virulent, and very virulent+ categories. Serotype 2 includes non-oncogenic field viruses. Serotype 3 is the turkey herpesvirus (HVT), which is nonpathogenic in chickens. However, HVT is immune-stimulating and serves as the foundation of the Marek’s vaccine in short lived, meat type birds.
