Polioencephalomalacia in Alpaca: Thiamine-Related Brain Disease and Blindness

Polioencephalomalacia, or PEM, is a serious neurologic condition where parts of the brain's outer gray matter become damaged. In alpacas, it often shows up as sudden behavior changes, poor coordination, cortical blindness, head pressing, or the classic "stargazing" posture. In severe cases, an alpaca may go down, have seizures, or become comatose.

Although many pet parents hear PEM described as a thiamine deficiency disease, the full picture is broader. Thiamine, also called vitamin B1, is essential for normal brain energy use. When thiamine is lacking, destroyed by microbes, blocked by certain drugs, or overwhelmed by high sulfur intake, the brain can no longer function normally. Merck notes that affected ruminants commonly develop blindness, ataxia, recumbency, seizures, and coma, and camelids can show a very similar pattern.

This is an emergency because the brain injury can progress quickly. The encouraging part is that some animals improve when treatment starts early, especially before prolonged recumbency or repeated seizures develop. That is why any alpaca with sudden blindness, abnormal head position, or unexplained neurologic signs needs prompt veterinary attention.

PEM in alpacas is most often discussed as a thiamine-related brain disease, but there are several ways thiamine function can be disrupted. Merck describes thiamine inadequacy developing when foregut microbial populations change, especially after high-concentrate feeding or a sudden diet shift. Those microbial changes may increase thiamine destruction or create compounds that interfere with thiamine's normal role in the body.

Another important cause is high sulfur intake. Excess sulfur can come from water, feed ingredients, forage, or by-product feeds. In ruminants, sulfur-associated PEM is increasingly recognized, and current evidence suggests high sulfur intake can increase the body's demand for thiamine, leading to a secondary deficiency state. This matters in alpacas too, especially when there has been a recent ration change, access to unusual feeds, or concern about sulfate-rich water.

Certain medications can also contribute. Amprolium, a coccidia treatment, is a thiamine analogue and can interfere with thiamine activity if used improperly or for too long. Plant thiaminases have been reported in other grazing species as a cause of PEM, though field cases are considered uncommon. In some alpacas, no single trigger is obvious, and your vet may need to look at the whole management picture, including feed, water, supplements, recent illness, and herd history.

Diagnosis is usually presumptive at first, based on the alpaca's neurologic signs, history, and exam findings. Your vet will look for patterns such as cortical blindness, stargazing, head pressing, ataxia, and seizures. A normal pupillary light reflex with absent menace response can support a brain-based cause of blindness rather than an eye problem.

Your vet will also ask about recent diet changes, grain access, by-product feeds, sulfur exposure, water source, coccidia treatment, and any medications or supplements. Bloodwork may help assess dehydration, inflammation, metabolic problems, or toxic exposure, but there is no single easy in-clinic test that confirms PEM. Merck notes that thiamine testing is difficult and not routinely available in many labs, so results must be interpreted carefully.

Because several dangerous conditions can look similar, diagnosis also means ruling out other causes. Differentials may include lead poisoning, listeriosis, salt toxicity or water deprivation, trauma, rabies, and other encephalopathies. In animals that die or are euthanized, necropsy with brain histopathology can confirm the diagnosis. In live patients, improvement after thiamine treatment may support PEM, but it does not prove the exact underlying cause on its own.

Prevention focuses on steady nutrition and careful management changes. Avoid abrupt shifts from forage-based feeding to higher-concentrate diets, and introduce any new ration gradually. If your alpacas receive by-product feeds or supplements, ask your vet or a livestock nutrition professional to review the full diet for sulfur load and overall balance.

Water matters too. In some regions, well or surface water can contain high sulfate levels, and sulfur intake can rise further during warm weather when animals drink more. If there has been a PEM case on your farm, testing the water source and reviewing feed tags can be very helpful. This is especially important if more than one alpaca seems dull, blind, or uncoordinated.

Use medications carefully and only under veterinary guidance. Because amprolium can interfere with thiamine, dosing errors or prolonged use can create problems. Good parasite and coccidia control, thoughtful feed storage, and prompt attention to any alpaca with appetite changes or odd neurologic behavior can all lower risk. If your herd has had a previous case, ask your vet whether ration changes, water testing, or preventive supplementation make sense for your situation.