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Reasons for Performing Study: The first outbreak of equine influenza virus (EIV) infection was confirmed in Australia in 2007. Some EIV-positive young foals died with bronchointerstitial pneumonia, an rare disease process in this age group that is often postulated to be caused by viral infection., Objectives: The aim of this study was to describe post mortem lesions in EIV-infected foals., Methods: Post mortem examinations were conducted on 11 young foals (age 2-12 days) submitted to the Scone Veterinary Hospital, NSW over a 2-month period in 2007. The foals had presented with or developed fatal pneumonia, and were known or suspected to be EIV-positive. Equine influenza virus nucleic acid was detected in tissue specimens using an Influenza A group reactive real-time reverse transcriptase PCR assay., Results: Grossly there was diffuse or extensive pulmonary consolidation. Histological changes included: bronchiolar and alveolar necrosis; neutrophilic infiltration; hyaline membrane formation; and hyperplasia and squamous metaplasia of airway epithelium. Tissues for 10 foals were EIV-positive, with a positive nasal swab from the remaining animal., Conclusions: This is the first detailed pathological description of bronchointerstitial pneumonia associated with EIV infection in young foals. It is also the first series of such cases in which a causative agent has consistently been detected., Potential Relevance: Given the findings in this outbreak, and a previous outbreak in the UK in 1965 involving a similarly naive population, veterinary clinicians and pathologists should be aware that EIV can cause fatal bronchointerstitial pneumonia in young foals that do not have maternal immunity. The lesions did not differ from those previously reported in foals of various ages with bronchointerstitial pneumonia of other or undefined causes, indicating that this is most likely a stereotypical response to a variety of insults. Therefore, tissue specimens should be obtained from cases of pneumonia in young foals for virological and bacteriological testing.