Hemorrhagic bowel syndrome (HBS) is a sporadic and fatal disease of predominantly lactating dairy cattle, characterized by segmental hemorrhage and luminal clot formation in the small intestine. Although, and have been associated with HBS, the pathogenesis and cause are currently unknown. In this study, 18 naturally occurring cases of HBS (7 necropsied immediately following euthanasia, 11 with 12-48 hour postmortem intervals) were investigated to characterize the pathology and the intestinal microbiome. Hemorrhagic bowel syndrome was characterized by a single small-intestinal, intramucosal hematoma with dissection of the lamina muscularis mucosae. In most cases necropsied immediately after euthanasia (4/7), the intestinal mucosa proximal to the hematoma contained 9 to 14, dispersed, solitary or clustered, erosions or lacerations measuring 4 to 45 mm. In 77% (37/48) of these mucosal lesions, microscopic splitting of the comparable to the hematoma was present. These findings suggest the intramucosal hematoma to originate from small mucosal erosions through dissecting hemorrhage within the lamina muscularis mucosae. No invasive fungal growth was observed in any tissue. Bacteriological cultivation and nanopore sequencing showed a polymicrobial population at the hematoma and unaffected intestine, with mostly mild presence of at selective culture. Gross and microscopic lesions, as well as the culture and sequencing results, were not in support of involvement of or in the pathogenesis of HBS.
