Multisystemic, eosinophilic, epitheliotropic disease with intestinal lymphosarcoma in a horse

Multisystemic, eosinophilic, epitheliotropic disease and intestinal lymphos arcoma were diagnosed in a Paso Fino mare that presented with anorexia and weight loss. The stomach, ileum, cecum, colon, pancreas, and lungs were inf iltrated by large numbers of eosinophils forming prominent eosinophilic gra nulomas, as well as lymphocytes and plasma cells. Two jejunal masses compos ed of solid sheets of neoplastic lymphocytes were present. In contrast to t he regions of inflammation, the infiltrates in these masses did not contain plasma cells, eosinophils, and eosinophilic granulomas. Immunohistochemica lly, the neoplastic lymphocytes expressed CD3 but not CD20 or kappa and lam bda light chains, supporting a diagnosis of T-cell lymphosarcoma. Concurren t diagnoses of hypereosinophilic syndrome and lymphosarcoma in this horse a nd several humans suggest that the multisystemic eosinophilic and lymphopla smacytic infiltrates were caused by the clonal proliferation of T-lymphocyt es that secreted interleukin-5 triggering differentiation and activation of eosinophils.