MULTICENTRIC HISTIOCYTOSIS IN YOUNG CHICKENS - GROSS AND LIGHT-MICROSCOPIC PATHOLOGY

During 1991-94, tissue specimens from 262 young chicken carcasses cond emned at slaughter contained novel multicentric proliferations of hist iocytelike cells. These tissues had been submitted to the USDA FSIS Ea stern Laboratory because of grossly enlarged spleens, livers, or kidne ys. The spleens were two to three times normal diameter and contained miliary white or yellow 1-3-mm foci. Similar miliary foci were present throughout the enlarged livers and kidneys. Microscopic examination o f these tissues revealed discrete circular nodules expanding splenic p eriarteriolar lymphoid sheaths, hepatic periportal nodules, and discre te perivascular and more diffuse interstitial nodules replacing renal tubules. Nodules also were present in the pancreas, bone marrow, prove ntriculus, and lung, with more diffuse infiltrates in intestinal lamin a propria. The cells composing these nodules contained irregularly ova l, folded, or pleomorphic nuclei and relatively abundant eosinophilic cytoplasm. Mitotic figures and pyknotic nuclei were common. These cell s were interpreted to be histiocytes (tissue macrophages or dendritic cells) and did not. resemble lymphocytes. These proliferating cells al so did not resemble the cell population of commonly diagnosed lymphoid neoplasms of young chickens. No intralesional organisms were detected and polymerase chain reaction analysis failed to detect Marek's herpe svirus DNA or leukosis/sarcoma and reticuloendotheliosis proviral DNA.