Histopathologic and ultrastructural findings in oak ( spp.) toxicity on 2 beef cattle farms in Colorado.

Natural oak toxicity, a phenomenon sporadically reported in the United States, is due to consumption of any part of most oak trees ( spp.). Ruminants, mainly cattle, are disproportionately susceptible to oak toxicity. Toxicity is attributed to degradation of the oak plant hydrolysable tannins by rumen microbes and enzymes into absorbable low-molecular-weight metabolites, which are postulated to bind and damage endothelial cells by unknown mechanisms. The clinical manifestations of acute toxicosis are nonspecific or broadly suggestive of renal disease due to acute tubular injury. Here we document the clinical, gross, histopathologic, and novel ultrastructural features of natural acute oak nephrotoxicity in 3 beef calves on 2 farms in Colorado, USA. Gross postmortem findings included perirenal edema with renomegaly and hemorrhagic gastroenteritis. Histologically, renal tubular epithelial necrosis was severe, with hemorrhage and intratubular hyaline casts. Transmission electron microscopy revealed extensive involvement of proximal and distal convoluted tubules, with predominantly intact basement membranes, and glomerular and interstitial endothelial injury and necrosis. The ultrastructural details of toxic nephropathy and vasculopathy induced by oak metabolites in natural cases of bovine oak toxicosis have not been described previously, to our knowledge.