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Modern poultry lines are characterised by high performance and specialisation. Advances in genetics, feeding, management, diagnosis, environment and breeding have significantly improved production parameters.
While today’s birds are more efficient and productive, they are also less hardy than in the past, making them more susceptible to disease.
Because of this increased susceptibility, modern production requires strict biosecurity measures and efficient vaccination schedules.
More and better vaccines are now available, however, the immune system of the birds needs to be in optimal working condition.
Due to the multifactorial aetiology for immunosuppression, diagnosis is not always straightforward, requiring a clinical history, necropsies and complementary laboratory tests if necessary.
It is very important for the field clinician to be aware of the macroscopic changes in the immune system that can be observed during necropsy. This guides the diagnosis and facilitates sampling.
In the following, the most relevant aspects of immunosuppression-oriented pathology of the immune system are discussed.
The avian immune system consists of primary and secondary lymphoid organs.
During the embryonic stage, undifferentiated cells migrate from the yolk sac to the bone marrow, thymus and bursa of Fabricius.
In these organs the cells differentiate into T- or B-lymphocytes where they express surface markers and by negative selection those lymphocytes that are not useful are eliminated.
They then migrate to secondary lymphoid organs such as the spleen, caecal tonsils, Harder’s gland, mucosa-associated lymphoid tissue, and germinal centres in connective tissue.
The interpretation of lesions in lymphoid organs requires taking into account the age of the birds and vaccination schedule, as primary lymphoid organs atrophy when birds reach sexual maturity and most routine vaccines cause changes in the lymphoid organs.
Bursa of Fabricius
It is present only in birds, located in the dorsal part of the cloaca, connected to the intestine by a duct.
The bursa of Fabricius is the site of differentiation of B lymphocytes and captures antigens when the bird defecates, as the smooth muscle layer of the intestine continues into the bursa of Fabricius so that the contraction of the muscle makes the bursa function as a suction knob.
Inside the bursa of Fabricius...
