Ruminal parakeratosis is a disease of cattle and sheep characterized by hardening and enlargement of the papillae of the rumen. It is most common in animals fed a high-concentrate ration during the finishing period. It also occurs in cattle that are fed rations of heat-treated alfalfa pellets, as well as in calves with prolonged ruminal acidosis due to reticuloruminal milk accumulation ("ruminal drinking"). It can occur, too, in young developing calves before weaning, if they are fed finely ground grain-based concentrates without additional forage. Ruminal parakeratosis does not appear to be related to the feeding of antimicrobials or protein concentrates. Incidence in a group may be as high as 40%. The lesions are thought to be caused by the lowered pH and the increased concentration of volatile fatty acids (VFAs) in the ruminal fluid, and the lesions do not usually develop in cattle that are fed unprocessed whole grain (on which animals gain weight as readily). This may be related to the higher pH and higher concentration of acetic acid than those of the longer-chain VFAs in the ruminal contents.
Many of the papillae are enlarged and hardened, and several may adhere together to form bundles. The papillae of the cranial ventral sac are commonly affected. In cattle, the roof of the dorsal sac may show multiple foci (each 2–3 cm2) of parakeratosis. In sheep, abnormal papillae may be visible and palpable through the wall of the intact rumen. Affected papillae contain excessive layers of keratinized epithelial cells, particles of food, and bacteria. The rumens of affected cattle are difficult to clean in the preparation of tripe. The abnormal epithelium, by interfering with absorption, may decrease the efficiency of feed use and the rate of weight gain, although there is little evidence to support this theory.
Ruminal parakeratosis may be prevented by finishing animals on rations that contain unground ingredients in the proportion of 1 part roughage to 3 parts concentrate. The necessity and economics of prevention are not well defined.