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Polioencephalomalacia (PEM) literally means softening of the cerebrocortical grey matter distributed in a laminar (layered) pattern. It is also called Laminar cortical necrosis or Cortical necrosis. PEM is a sporadic disease of unknown cause occurring in cattle, sheep and goats. PEM is most commonly seen in cattle at 6-18 months of age when fed concentrate rations. Sheep are usually affected at 2-7 months of age. The lesion is associated with thiamine deficiency or a disturbance in thiamine metabolism. Ruminants are supplied with thiamine by synthetic activity of ruminal bacteria. PEM most commonly develops in cattle fed carbohydrate-rich and roughage-poor rations, which leads to subclinical lactic acidosis and hence an alteration in ruminal microflora. Other mechanisms for disturbances in thiamine deficiency include;- Destruction of thiamine within the gastrointestinal tract (for example by thiaminases in bracken fern)- Inactivation of thiamine by excess sulphates or sulphides or elemental sulphur- Production of inactive thiamine analogues- Decreased thiamine absorption- Increased faecal excretion of thiamineClinical signs of PEM include head pressing, dullness, central blindness, anorexia, muscle tremors, teeth grinding, salivation, convulsions, nystagmus, and recumbancy. Early administration of thiamine may be curative but if the lesion is more advances then surviving animals may remain partially blind and mentally dull.
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