Interstitial pneumonias of undetermined etiology in foals in California, 1990–2020

Interstitial and bronchointerstitial pneumonias of undetermined etiology in young foals are relatively common in autopsy services with an equine focus. Unknown viruses, toxins, hyperthermia, surfactant or alveolar macrophage function deficiency, certain antibiotics, and aberrant responses to <i>Rhodococcus equi</i> or other bacteria have been proposed as causes. We performed a retrospective study of autopsies on foals with a diagnosis of interstitial or bronchointerstitial pneumonia with an unidentified etiology. Forty-one foals (median age: 3-mo-old) were included. Most were received in summer (<i>n</i> = 28) and spring (<i>n</i> = 10). The most frequently reported clinical signs were dyspnea and/or tachypnea (<i>n</i> = 28) and fever (<i>n</i> = 19). Antibiotic treatment was reported in 21 cases, and the most frequently used antibiotics were penicillin (<i>n</i> = 9) and gentamicin (<i>n</i> = 8). Grossly, most of the lungs were diffusely rubbery-to-firm (<i>n</i> = 35) and did not collapse (<i>n</i> = 22). Histologically, combinations of exudative (E; hyaline membranes), proliferative (P; type II pneumocyte hyperplasia), and fibrotic (F; fibroplasia) phases were common (E + P, <i>n</i> = 15; E + P + F, <i>n</i> = 13) in the interstitial component. Necrosis of the bronchiolar epithelium was rare (<i>n</i> = 4), concurrent suppurative bronchopneumonia was common (<i>n</i> = 22), and a few foals (<i>n</i> = 5) had pulmonary pyogranulomas. <i>Pneumocystis</i> spp. organisms were observed in 8 cases using Grocott-Gomori methenamine silver stain. Bacteria were recovered from the lungs in 22 cases, with <i>R. equi</i> (<i>n</i> = 7) and <i>E. coli</i> (<i>n</i> = 6) being the most common isolates. No unequivocal viral causes were identified during the regular diagnostic work-up and after using novel diagnostic approaches such as herpesvirus consensus PCR and viral metagenomics in a subset of the cases.