Pathological Features and Genomic Characterization of an Actinobacillus equuli subsp. equuli Bearing Unique Virulence-Associated Genes from an Adult Horse with Pleuropneumonia

Actinobacillus equuli subsp. equuli is the etiological agent of sleepy foal disease, an acute form of fatal septicemia in newborn foals. A. equuli is commonly found in the mucous membranes of healthy horses’ respiratory and alimentary tracts and rarely causes disease in adult horses. In this study, we report a case of a 22-year-old American Paint gelding presenting clinical signs associated with an atypical pattern of pleuropneumonia subjected to necropsy. The gross and histopathological examinations revealed a unilateral fibrinosuppurative and hemorrhagic pleuropneumonia with an infrequent parenchymal distribution and heavy isolation of A. equuli. The whole genome sequence analysis indicated that the isolate shared 95.9% homology with the only other complete genome of A. equuli subsp. equuli available in GenBank. Seven virulence-associated genes specific to the isolate were identified and categorized as iron acquisition proteins, lipopolysaccharides (LPS), and capsule polysaccharides. Moreover, four genes (glf, wbaP, glycosyltransferase family 2 protein, and apxIB ) shared higher amino acid similarity with the invasive Actinobacillus spp. than the reference A. equuli subsp. equuli genome. Availability of the whole genome sequence will allow a better characterization of virulence determinants of A. equuli subsp. equuli, which remain largely elusive. © 2023 by the authors.