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Classical form
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The characteristic finding is the enlargement of one or more peripheral nerves. Those most commonly affected and easily seen at post-mortem are the brachial and sciatic plexuses, coeliac plexus, abdominal vagus and intercostal nerves. Affected nerves are often two or three times their normal thickness, the normal cross-striated and glistening appearance is absent, and the nerve may appear greyish or yellowish, and sometimes oedematous. Lymphomas are sometimes present in the classical form of MD, most frequently as small, soft, grey tumours in the ovary, and sometimes also in the lungs, kidneys, heart, liver and other tissues. ‘Grey eye’ caused by an iridocyclitis that renders the bird unable to accommodate the iris in response to light and causes a distorted pupil is common in older (16–18 week) birds
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Visceral form
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Greyish-white tumours are found in the ovaries, liver, spleen, kidney, heart and other organs. Sometimes the liver and spleen are swollen without distinct tumours being present. lymphomas also arise in the skin around the feather follicles and in the skeletal muscles. Affected birds usually have enlarged peripheral nerves, as in the classical form. In younger birds, liver enlargement is usually moderate in extent, but in adult birds the liver may be greatly enlarged and the gross appearance identical to that seen in lymphoid leucosis. In both the classical and acute forms of MD, the disease starts as a proliferation of lymphoid cells, which is progressive in some cases and regressive in others. The peripheral nerves may be affected by proliferative, inflammatory or minor infiltrative changes, which are termed type A, B, and C lesions, respectively.
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The A-type lesions consist of infiltration by proliferating lymphoblasts, large, medium and small lymphocytes, and macrophages, and appear to be neoplastic in nature.
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The B-type lesion is characterised by interneuritic oedema, infiltration by mainly small lymphocytes and plasma cells, and Schwann cell proliferation, and appears to be inflammatory.
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The C-type lesion consists of a light scattering of mainly small lymphocytes, and is often seen in birds that show no gross lesions or clinical signs. It is a regressive, inflammatory lesion. Demyelination frequently occurs in nerves affected by the A- and B-type lesions, and is responsible for the clinical paralysis.
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