Pathology of ruminal acidosis in cattle.

Rumen acidosis (RA) is one of the most prevalent alimentary diseases of ruminants and is caused by excessive fermentation of feedstuff in the reticulorumen. This retrospective study aimed to characterize the microscopic lesions of acute and subacute cases of RA in cattle older than 6 months. Fifty-two RA cases with rumen fluid pH ≤5.5 at the time of necropsy were selected. Cases were classified as acute or subacute RA based on their clinical histories; 7 animals were included as controls. In rumen histologic sections, the number, length, and width of ruminal papillae and the thickness of the mucosal epithelium and stratum corneum were measured. Univariable linear mixed models were used to compare groups. Eleven of 28 acute cases (39%) had small (<150 µm in diameter) pustules within the ruminal epithelium. Histomorphometry of acute cases differed from control cases only by shorter ruminal papillae, highlighting the scant morphologic changes in this group. In cases of subacute RA (n = 24), the rumen papillae were shorter and wider than in control animals, and there was epithelial hyperplasia, parakeratotic hyperkeratosis, and, in 19/24 cases (79%), there were numerous intraepithelial pustules (150-500 µm in diameter). There was a higher number of ruminal papillae in subacute RA cases when compared with acute RA cases. Subacute RA cases had wider rumen papillae and thicker epithelium than acute RA cases. Our study highlights the distinctive lesions and histomorphometry of the rumen in cattle with acute and subacute RA.