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Introduction to Polioencephalomalacia

Polioencephalomalacia (PEM), also referred to as cerebrocortical necrosis (CCN), is a neurological disease seen in ruminants that generally results from disrupted thiamine production in the body. Thiamine is a key chemical in glucose metabolism, and because the brain outcompetes all other organs in energy consumption, thiamine deficiency is most threatening to neurological activity. Cattles, sheep, goat, and other ruminants that are diagnosed with PEM or pre-PEM suffer opishotonus, cortical blindness, disoriented movement, and fatility, if left untreated. There is not a proven correlation between age and the onset of PEM, and current diagnosis records reveal infected ruminants of all ages.

Thiamine Regulation

Thiamine availability is regulated by the direct dietary consumption of thiamine and thiaminases, which are enzymes that readily cleave thiamine molecules and inhibit essential thiamine-regulated pathways such as the metabolism of glucose.

In the special case of animals with rumen, ruminants have working rumen microbes that synthetize thiamine molecules without pre-ingesting thiamine rich foods. However, feed concentrates given to livestock ruminants, specifically sheep and cattle, are often heavily stocked with thiaminases, which counter the production of thiamine. As a result thiaminases and rumen microbes work conjointly in a futile cycle, eventually reaching a state of thiamine deficiency. Thiaminase rich foods include different grains, fresh water fish, and plants, usually ferns, which are often processed together to make feed concentrate.

Since glucose metabolism is regulated by thiamine, the overconsumption of glucose can also result in thiamine inadequacy. By substituting food concentrates made up of complex fibers by simple carbohydrates, thiamine is equilibrated to break down excess carbohydrates until the rumen system is depleted of thiamine. In fact, PEM was first seen in the 19th century when ruminants were fed byproducts of sugar refining.

Sulfur Intake

In light of recent research, high concentrations of sulfur intake has also been deemed responsible for PEM. Sulfur is necessary for the synthesis of important sulfur-containing amino acids and their contribution to the synthesis of different hormones, enzymes, and structural proteins. The ruminant diet, especially that of cattle, can be overly concentrated with sulfur. In ruminants, the same rumen microbes that generate thiamine molecules reduce sulfur into toxic sulfides. Among the sulfide toxins is hydrogen sulfide, a gas compound that obstructs oxidation by competing with oxygen sites and eventually disrupting neural function.

Symptoms of Polioencephalomalcia

Clinical signs of PEM are variable depending on the area of the cerebral cortex affected and may include head pressing, dullness, opisthotonos, central blindness, anorexia, muscle tremors, teeth grinding, trismus, salivation, drooling, convulsions, nystagmus, clonic convulsions, and recumbency.^[2] Early administration of thiamine may be curative, but if the lesion is more advanced, then surviving animals may remain partially blind and mentally dull.

(words in bold are written by original author. Below is what the page had before, I am decided to incorporate only the symptoms section of the original)

Polioencephalomalacia (PEM) literally means softening of the cerebrocortical grey matter distributed in a laminar (layered) pattern. It is also called cerebrocortical necrosis (CCN),^[1] laminar cortical necrosis or cortical necrosis. PEM is a sporadic disease caused by the overconsumption of simple carbohydrates, and the underconsumption of fiber; it occurs in cattle, sheep and goats, and was first seen in the 19th century when ruminants were fed the byproducts of sugar refining. Today 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

Infection with Trypanosoma congolense can also cause focal PEM in the final stages of disease, likely due to ischemia from accumulation of parasites in the terminal capillaries of the brain.

Clinical signs of PEM are variable depending on the area of the cerebral cortex affected and may include head pressing, dullness, opisthotonos, central blindness, anorexia, muscle tremors, teeth grinding, trismus, salivation, drooling, convulsions, nystagmus, clonic convulsions, and recumbency.^[2] Early administration of thiamine may be curative, but if the lesion is more advanced, then surviving animals may remain partially blind and mentally dull.