Acute interstitial pneumonia in foals: A severe, multifactorial syndrome with lung tissue recovery in surviving foals

BACKGROUND: Acute interstitial pneumonia in foals has been sparsely described in literature, and the individual authors disagree on the underlying aetiology. Histopathological follow‐up from surviving foals is not available. OBJECTIVES: Description of clinical and histopathological findings in the course of acute interstitial pneumonia and in recovery. Investigating the aetiology and possible triggering factors of acute interstitial pneumonia. STUDY DESIGN: Case series. METHODS: Post‐mortem examination of nine affected foals; seven died during the acute phase, and two had recovered from acute interstitial pneumonia. Data from clinical examinations on the day of death were recorded for all foals. Complete necropsy, special histological staining, virological and microbiological examinations were performed. RESULTS: Seven foals died during the acute phase with severe respiratory distress, fever and increased numbers of comet tail artefacts in lung ultrasound. In post‐mortem examination, a wide variety of possible triggering factors was identified. Microbiology revealed Escherichia coli, Rhodococcus equi and Klebsiella pneumoniae as the most common bacterial pathogens. Equine herpesvirus 2 was detected in all foals by PCR. Those with high viral loads also displayed histopathological changes suggestive of viral infections. Pneumocystis carinii was detected in all acutely affected foals. Histopathological changes in lung parenchyma clearly differed between the foals that had recovered from acute interstitial pneumonia and those dying in the acute phase. While lungs in acute phase featured marked parenchymal collapse and necrosis, the recovered foals revealed nearly normal ventilated lung parenchyma and alveolar structure. MAIN LIMITATIONS: Small number of cases. Because all foals are from the same breeding farm with endemic occurrence of pathogens, it is not certain whether the results of this study can be transferred to other foals without restrictions. CONCLUSIONS: Acute interstitial pneumonia seems to be based on a multifactorial aetiology. Lungs from foals that have survived acute interstitial pneumonia appear to be able to regenerate completely, leaving no permanent changes.