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 sulfates or sulfides or elemental sulfur

- Production of inactive thiamine analogues

- Decreased thiamine absorption

- Increased faecal excretion of thiamine

Clinical signs of PEM include head pressing, dullness, central blindness, anorexia, muscle tremors, teeth grinding, salivation, convulsions, nystagmus, and recumbency. [cite web | title = Polioencephalomalacia: Introduction | work = The Merck Veterinary Manual | date = 2006 | url = http://www.merckvetmanual.com/mvm/index.jsp?cfile=htm/bc/102000.htm&word=Polioencephalomalacia | accessdate = 2007-07-10 ] Early administration of thiamine may be curative but if the lesion is more advances then surviving animals may remain partially blind and mentally dull.


External links

* [http://www.tvsp.org/polio.html Polioencephalomalacia: Disease Caused by a Deficiency of Thiamine] at tvsp.org
* http://ag.ansc.purdue.edu/sheep/ansc442/Semprojs/2002/neurological/polio.htm
* http://www.merckvetmanual.com/mvm/index.jsp?cfile=htm/bc/102000.htm

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