Pathology of Human Metapneumovirus Infection: Insights into the Pathogenesis of a Newly Identified Respiratory Virus

Human metapneumovirus (hMPV) is a recently discovered human virus that causes significant respiratory infections. Pathologic features of hMPV infection have not been described. A total of 1257 pediatric respiratory samples submitted for routine clinical virologic testing were additionally tested for hMPV by reverse transcriptase polymerase chain reaction (PCR). Pathology specimens, available in 6 of 53 hMPV-positive patients, were examined by light and electron microscopy and included 6 bronchoalveolar lavage (BAL) and 3 lung biopsy specimens from 6 patients (3 girls and 3 boys) ranging in age from 1 to 16 years. BAL from three patients performed within 4 days of the positive hMPV assay showed epithelial degenerative changes and eosinophilic cytoplasmic inclusions within epithelial cells, multinucleate giant cells, and histiocytes. Inclusions were not seen in three patients with BAL performed = 1 month from the time of their positive assay. Lung biopsy, performed in three patients, all = 1 month from the time of their positive assay, showed chronic airway inflammation and intraalveolar foamy and hemosiderin-laden macrophages; all three patients had an underlying pulmonary/systemic disorder. Our findings delineate the clinicopathologic features in hMPV-infected patients undergoing anatomic sampling, which may provide diagnostic guidance to a practicing pathologist. Further, they contribute toward understanding the pathogenesis of hMPV infection.