Various bacterial and viral pathogens have been identified in cases of interstitial pneumonia in foals within the first month of life. A potential association has been suggested between equid gammaherpesvirus 5 (EqGHV5, equine herpesvirus 5; family Orthoherpesviridae, taxon species Percavirus equidgamma5) and interstitial pneumonia in foals; however, direct histologic evidence has been limited. We examined 2 neonates—12-d- and 24-d-old—with acute respiratory distress and poor response to treatment. Postmortem examination revealed interstitial pneumonia with diffuse alveolar damage and intranuclear inclusion bodies in alveolar macrophages, suggesting viral infection. EqGHV5 was identified in lung tissues by PCR and visualized in alveolar macrophages using in situ hybridization (ISH). No other bacterial or viral pathogens were detected in these cases. The consistent histologic lesions, inclusion bodies, and PCR and ISH data support a potential causal role of EqGHV5 in foal interstitial pneumonia. Additionally, we retrospectively investigated 23 cases of interstitial pneumonia (2–89-d-old foals) diagnosed in Hokkaido, Japan, between 2015 and 2020. We did not detect EqGHV5 in these 23 cases by PCR or ISH. Further studies are required to investigate the relationship between EqGHV5 and interstitial pneumonia in foals.
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