Diagnosis: EQUINE MULTINODULAR PULMONARY FIBROSIS
Cause: EQUINE HERPESVIRUS-5

Clinically, horses with multinodular pulmonary fibrosis syndrome have a mean age of 13-14 years and there is no sex or breed predilection. Lymphopenia, characteristic of acute viral infection, is found in some EMPF horses. Grossly, there are two morphological forms of the disease. Most common are large, individual to coalescing nodules of fibrosis that typically leave little to no normal lung left. A less common gross presentation are individual, disseminated nodules of fibrosis separated by normal lung tissue; this presentation can be confused with a metastatic, neoplastic process, which is often a clinical differential. Histologically, there is marked interstitial fibrosis that can either retain the open alveolar architecture or efface it by less well organized bundles of fibrous connective tissue deposition. Retained airways are often filled with inflammatory cells. EMPF has been associated with EHV-5 infection. Intranuclear viral inclusions consistent with herpesvirus are seen primarily within macrophages located in the lesions. One remarkable histologic aspect of this disease is the lack of temporal heterogeneity of the lesions. There is no progression in maturity of the lesions and they all appear to have begun at the same time.

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