Subacute ruminal acidosis (SARA), endotoxins and health consequences

Several symptoms of subacute ruminal acidosis (SARA) in cattle, including inflammation, have been attributed to the translocation of free bacterial lipopolysaccharide endotoxin (LPS) from the rumen into the interior circulation. The objective of this review is to assess evidence for this effect of LPS. Induction of SARA using grain-based SARA challenges increases LPS in the rumen, caecum, and feces. However, SARA induced using a nutritional challenge based on feeding pellets of ground alfalfa (alfalfa-pellet SARA challenge) increases LPS in the rumen, but not in the large intestine. This difference between the two SARA challenges may be due to the increases in starch and fermentation in the large intestine that only occur during grain-based SARA challenges.Grain-based SARA challenges have repeatedly been associated with increases in acute phase proteins in peripheral blood, which shows that these challenges cause an innate immune response. This may be the result of translocation of LPS. However, as inflammation of the epithelium of the digestive tract also causes an acute phase response, the acute phase response could have occurred without this translocation. Despite similar responses to grain-based and alfalfa-pellet SARA challenges in terms of rumen pH depression and rumen LPS concentration, only the grain-based SARA challenge causes an acute phase response. This shows that the combination of low rumen pH and high rumen LPS alone do not cause translocation and an acute phase response.Although high luminal LPS can reduce the barrier function of the rumen epithelium, this is unlikely the major cause of LPS translocation. High acidity of digesta in the large intestine may reduce the barrier function of its epithelium, and thereby cause the translocation of LPS. Despite this, evidence for increases in the concentration of LPS in peripheral blood during SARA is limited. This could be the result of inadequate sensitivities of the tests used for LPS measurement in blood, and the fast clearance rate of LPS from portal and peripheral blood.The symptoms of SARA vary depending on the method used for the SARA challenge, even if these challenges result in similar rumen pH depressions. This may be explained by effects of these challenge on digesta other than the acidity, such as the starch content of digesta in the rumen and in the large intestine.