Dairy cow with polioencephalomalacia

What to look for

  •  Depression
  • A range of nervous signs - blindness, wandering aimlessly, excessive salivation and head pressing
  • A fine tremor of muscles may progress to convulsions
  • Death


Cause - Thiamine deficiency

Thiamine (vitamin B1) is usually produced in the rumen, but sometimes chemicals (known as thiaminases) break down the thiamine in the rumen. Thiaminases can arise from microorganisms that live in the rumen and it is also believed that ingestion of some plants may contribute to the problem. In some cases it is thought that an excessive amount of sulphur in the diet can cause changes in the rumen that limit thiamine production or availability.

Animals likely to be affected

Young animals between 6 - 18 months of age. Most cases involve recently weaned calves during the early summer months. Occasionally, adult animals are affected.

Other diseases with similar signs

Other diseases that cause nervous signs in animals include lead poisoning, salt poisoning, grass tetany, vitamin A deficiency and infection of the brain by microorganisms such as listeria.

Confirming the diagnosis

Diagnosis is made on the history of the outbreak, the clinical signs in affected animals and their response to thiamine. There are no routine laboratory tests for use in live animals but, if an animal dies, a post-mortem examination will reveal distinctive changes in the brain.


Animals suspected of having PEM should be promptly treated by thiamine injections into the vein. The effect of this treatment is relatively short-lived and it needs to be repeated every few hours and followed up by treatment over the next few days. Animals treated early in the course of the disease have a reasonable chance of recovery but treatment of seriously affected animals is unlikely to be successful.


PEM is not often seen and there is little that can be done to anticipate or prevent cases. If your farm is in an area where PEM occurs more often there may be some local information on plants that could be sources of thiaminases. If so, it may be worthwhile restricting access of susceptible animals to these plants.

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