The pathology of subacute methylmerculialism in swine

Clinical signs of toxicosis, neurologic lesions, and elevated tissue residues of methylmercury (MM) were produced in 12 pigs by oral administration of 1.29, 0.86, 0.64, and 0.43 mg mercury/kg of body weight daily as methylmercuric hydroxide (MMH). Clinical signs which began on day 17 were ataxia, dysmetria, blindness, convulsions, paresis, and death. Time of onset of signs was inversely related to size of daily dose. Microscopic lesions were found in the cerebrum brain stem, and spinal cord, and correlated well with clinical signs. The cerebrum in which severity of lesions was directly related to length of exposure was the most severely affected region of the central nervous system (CNS). Lesions were neuronal necrosis, neuronophagia, cortical vacuolation, axon swelling, gliosis, leptomeningitis, and vascular fibrinoid necrosis. Neuronal necrosis was most extensive within mid and deep cerebrocortical laminae. Brain residues of MM were directly proportional to the size of daily dose, and statistically significant. Distribution of MM among different tissues was rather uniform with highest concentrations found in liver, followed by kidney, muscle, spleen, and brain.