by Robert Van Saun, DVM, MS, PhD Department of Veterinary Science Pennsylvania State University
Clinical Signs. Polioencephalomalacia (PEM) is a disease of ruminant animals characterized by derangement of the central nervous system due to necrosis of the cerebral cortex of the brain.
Young animals on high grain diets are affected more often. Older animals and pastured animals may be occasionally involved. The onset is often sudden with blindness and disorientation. The head may be elevated. Excitement may be seen but is usually replaced with dullness. The animal may go down on its side with its head thrown back. The legs may be rigidly extended. Convulsions may occur. If untreated, death usually occurs within a few days. Appetite is lost and the animal does not drink. Temperature and respiratory rate are usually normal, but heart rate may be depressed.
Cause. Specific pathogenesis of this disease is not understood. Thiamine is an essential B-vitamin needed for many metabolic functions of the body. In a normal feeding situation, rumen microbes generate sufficient thiamine to meet animal needs. In PEM, thiaminase, an enzyme that destroys thiamine, is thought to be produced by certain starch fermenting bacteria within the rumen and thiamine deficiency develops. A thiamine - analogue is also produced within the rumen, which may replace thiamine in important metabolic reactions in the brain. Selected necrosis of the brain then occurs, resulting in the clinical signs.
Another nutritional situation, excess sulfur intake from either feed, water or both can produce clinical signs identical to thiamine-dependent PEM.
Treatment. Administration of large doses of thiamine intravenously, intramuscularly or both methods early in the disease will usually produce a dramatic improvement within a few hours. In the later stages of the disease, the brain necrosis may be too severe for the animal to recover.
Prevention. Gradual adaptation of animals to initial grain feeding. Until further elucidation of the cause and development of the disease, little can be done to economically prevent the disease. If a case of PEM is diagnosed in a group of animals, it is advisable to inject the remaining animals with thiamine to prevent further cases.