Polioencephalomalacia in Cows: PEM and Blindness or Seizures in Cattle

Polioencephalomalacia, usually shortened to PEM, is a serious brain disease of ruminants. In cattle, it affects the outer layers of the brain and can cause sudden neurologic signs such as blindness, staggering, head pressing, star-gazing, seizures, and collapse. Merck Veterinary Manual describes PEM as a pathologic diagnosis and a common end point of several different problems rather than one single disease process.

In practical terms, PEM means the brain is not functioning normally and the cow needs urgent veterinary attention. Some cattle show a short period of dullness or separation from the herd first. Others are found suddenly blind, down, or actively seizuring. Younger cattle are affected more often, but adults can develop PEM too, especially when diet or water sulfur is high.

PEM is sometimes linked with thiamine, also called vitamin B1, because thiamine is important for normal brain metabolism. However, modern veterinary sources also emphasize sulfur-associated PEM, especially in cattle eating high-concentrate rations or feeds containing certain by-products, or drinking high-sulfur water. That is why your vet usually looks at the whole picture: the cow, the ration, the water source, and whether more than one animal is affected.

PEM has more than one possible cause. The two most discussed in cattle are altered thiamine status and high sulfur intake. Merck Veterinary Manual notes that PEM has historically been associated with thiamine problems, while more recent work highlights sulfur exposure from feed, water, or both. High-concentrate diets increase risk, and sulfur can be elevated in some by-product feeds such as distillers grains, corn gluten feed, beet-processing products, molasses, or water sources with high sulfate levels.

Diet changes matter too. Rapid shifts toward high-starch, low-roughage feeding can change rumen pH and rumen microbes. That can favor thiaminase-producing bacteria, which interfere with thiamine availability. University of Georgia Extension also notes that cattle on rapidly fermentable, high-concentrate diets are more likely to encounter the conditions that set PEM up.

PEM is also a syndrome, not a single-cause diagnosis. Merck lists other conditions that can produce similar brain lesions or similar signs, including lead poisoning and salt toxicosis from excess salt intake or water deprivation. In the field, that means your vet may treat presumptively for PEM while also investigating toxins, water quality, feed formulation, and other neurologic diseases.

When several cattle are affected at once, herd-level causes move higher on the list. Your vet may want feed tags, ration sheets, water test results, and a timeline of any recent changes in supplements, coproducts, pasture access, or mineral programs. That history can be just as important as the neurologic exam.

PEM is often diagnosed presumptively from the pattern of signs, the herd history, and the cow’s response to treatment. Merck Veterinary Manual states that presumptive diagnosis is based on clinical signs and response to thiamine administration, while definitive diagnosis is made at necropsy with characteristic brain lesions. Texas A&M Veterinary Medical Diagnostic Laboratory also notes that there is no simple ante-mortem blood test that confirms PEM in the field.

Your vet will usually start with a neurologic exam and a careful review of diet, water, and recent management changes. Blindness with a normal-looking eye, head pressing, seizures, and star-gazing can strongly raise suspicion. Feed and water may be tested for sulfur, especially if cattle are on concentrate-heavy rations or by-product feeds. Merck advises estimating total sulfur intake on a dry-matter basis, because both feed and water contribute.

Differentials are important because several emergencies can look similar. Merck lists lead poisoning, salt toxicosis, Histophilus meningoencephalitis, rabies, and nervous-system coccidiosis among the cattle differentials. In some cases, your vet may recommend necropsy on a deceased animal to confirm the diagnosis and help protect the rest of the herd.

A practical point for pet parents and producers: improvement after thiamine supports treatment decisions, but it does not prove the exact cause. Your vet may still recommend further testing so the herd problem is not missed and the risk of additional cases can be lowered.

Prevention focuses on ration management, water quality, and slow transitions. Merck Veterinary Manual recommends analyzing all possible sulfur sources, including water, and estimating total sulfur intake on a dry-matter basis when sulfur-associated PEM is suspected. Merck lists maximal tolerable sulfur concentrations of about 0.3% of diet dry matter for diets with more than 85% concentrate and 0.5% for diets with at least 45% forage, while also noting these are not absolute cutoffs for every situation.

In day-to-day herd management, avoid abrupt jumps into high-concentrate feeding. Keep enough effective roughage in the ration to support normal rumen function. If you use coproduct feeds, molasses, gypsum, ammonium sulfate, or other sulfur-contributing ingredients, ask your vet or nutritionist to review the full sulfur load rather than looking at one ingredient alone.

Water matters more than many people expect. A water source with high sulfate can push the total sulfur intake into a risky range even when the feed looks acceptable on paper. Testing wells or other suspect sources can be especially helpful if multiple cattle show neurologic signs or if PEM appears after a management change.

During or after a herd problem, your vet may discuss temporary thiamine support, ration reformulation, and closer monitoring of newly introduced feed ingredients. The goal is not one universal plan. It is matching prevention to your herd’s forage base, concentrate level, water source, and production stage.