Pustular rumenitis associated with epithelial akeratosis, dyskeratosis, and parakeratosis in steers: report of two cases

Although occurrence of inflammatory lesions of the rumen due to a variety of factors is relatively common in the bovine species, there are a limited number of actually published reports on ruminal pustules. In order to deepen knowledge of the pathological process involving inflammatory rumen disorders in this species, histopathological investigations were performed on ruminal pustules in two 2-year-old steers which were slaughtered conventionally for food. On postmortem gross examination of the steers’ rumen, the mucosal surface of the ventral sac revealed focal areas of irregular arrangement, discoloration, or atrophy of the ruminal papillae. Histopathology demonstrated the presence of several intraepithelial pustules (<1.5 mm in diameter) in the ruminal papillae. Pustules which occurred separately or in contiguity with each other were characterized by well-circumscribed cavities filled with purulent exudate comprised of neutrophils, fibrin clots, serous fluid, or amorphous inflammatory products. Additional characteristic findings in both steers consisted of non-specific dystrophic changes—akeratosis, dyskeratosis, and parakeratosis—in the stratified squamous epithelial cells enclosing pustular lesions. The development of intraepithelial cyst-like cavities which were different from pustules and containing exfoliated parakeratotic keratinocytes was observed in one steer. Based on these findings, it is apparent that non-specific dystrophic changes in the ruminal epithelium might have been associated with the development of pustules. A possibility is that intraepithelial cyst-like cavities which were formed by focal degradation of parakeratotic keratinocytes played a role as a precursor lesion to the pustules.