Detection of Escherichia coli in the lungs of dogs with necrohemorrhagic pneumonia using immunohistochemistry and fluorescence in situ hybridization.

Hemorrhagic necrotizing pneumonia caused by cytotoxic necrotizing factor-1-positive extraintestinal pathogenic Escherichia coli (ExPEC) is a rapidly fatal disease in dogs. Lung tissues from 6 client-owned dogs with culture-confirmed ExPEC pneumonia were analyzed using immunohistochemistry (IHC) and fluorescence in situ hybridization (FISH). Both assays localized E. coli within pneumocytes, alveolar macrophages, and alveolar spaces. Notably, one case exhibited numerous filamentous E. coli, representing a stress-induced morphologic adaptation previously unreported in canine ExPEC pneumonia. Recognizing filamentous E. coli is diagnostically crucial to avoid misidentification as other Gram-negative bacteria. While FISH provided high specificity, IHC offered practical, rapid detection compatible with routine microscopy. Four dogs had recent exposures to high-density facilities (breeding, boarding, or training), suggesting possible environmental transmission. These findings expand the clinicopathologic spectrum of canine ExPEC pneumonia and demonstrate the value of integrating spatial techniques with traditional culture to improve diagnostic accuracy.