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Polioencephalomalacia
Polioencephalomalacia is a destruction of the surface layer of the brain, the one that controls conscious vision and behavior. The name is intimidating, but the mechanism is simple: the brain lacks vitamin B1, also called thiamine, which se...
What is it?
Polioencephalomalacia is a destruction of the surface layer of the brain, the one that controls conscious vision and behavior. The name is intimidating, but the mechanism is simple: the brain lacks vitamin B1, also called thiamine, which serves as fuel. In ruminants, this vitamin is not provided by food: it is produced by the microbes in the rumen. Anything that suddenly disrupts the rumen flora therefore cuts off the supply to the brain. A too rapid change in diet, a ration very rich in cereals, an excess of sulfur, certain plants, are enough to trigger the crisis. The animal becomes blind in a few hours, even though its eyes are perfectly normal: it is the brain which no longer processes the image. He starts pushing his head against the walls, wandering, turning, then convulses. One point makes this disease exciting and decisive for the breeder: treated early, it responds spectacularly. A blind lamb lying down in the morning can get up and regain its sight in a few hours after the right treatment. Treated late, it leaves permanent after-effects or kills. It is therefore a pure emergency: the prognosis is decided in the first hours, not the next day.
Key point
A sheep that goes blind overnight and pushes against walls is an early emergency. If treated well straight away, he can regain his sight; processed two days later, often longer.
Commonly observed signs
These signs are neither exhaustive nor specific. Only a veterinarian can establish a diagnosis.
Related signs — each one links to its page:
The animal stops ruminating, refuses to eat and isolates itself from the crowd.
Sudden blindness: he no longer sees, stumbles into obstacles, no longer reacts to the movement of the hand in front of his eyes. However, the eye is normal and the pupil still reacts to light, which always surprises the breeder.
Head pushed against a wall, barrier or post for several minutes.
Wandering, walking aimlessly, going around in circles, animal that gets stuck in a corner and never comes out.
Head thrown back, neck stiff, muzzle pointed upwards.
Tremors, contractions of facial muscles, eyelids fluttering.
Staggering gait, falls, then animal lying on its side, limbs outstretched, pedaling.
Convulsions in advanced forms.
Usually no marked fever, which helps differentiate it from brain infections.
Often several animals from the same batch affected a few days apart, especially lambs and fattening youngsters, which points to food.
When to consult without delay
This list does not replace veterinary advice. It gathers situations that justify a prompt consultation.
Causes and risk factors
A sudden change in diet, in particular a rapid switch to a ration rich in cereals or concentrate, disrupts the rumen flora and stops the production of vitamin B1.
An excess of sulfur in the ration or in the water is a major and often unknown cause: certain industrial by-products, certain cakes, certain drilling waters are rich in it. Excess sulfur is transformed into compounds that are toxic to the brain.
Some plants contain substances that destroy vitamin B1: bracken and horsetail are the classic examples. A pasture invaded by these plants, or a hay that contains them, exposes the animals.
Certain products used against coccidiosis can, if used inappropriately, interfere with vitamin B1.
Any disease that blocks rumen function, such as acidosis or indigestion, can lead to the same result.
A prolonged lack of water followed by sudden rehydration can also cause a similar nervous picture, which the veterinarian must distinguish.
Fattening lambs and youngsters receiving plenty of concentrate, particularly when batching or switching to a new ration.
Too rapid dietary transitions, change of feed supplier, new harvest, sudden transition from pasture to sheepfold or vice versa.
Sulfur-rich bore water, sulfur-rich foods and by-products.
Pastures overgrown with bracken or horsetail, hay harvested in these areas.
Rumen acidosis, recent digestive disorders, animals which have had accidental access to a stock of cereals.
Fast-growing animals, tight fattening batches.
Inappropriate use of certain anticoccidial products.
Diagnosis and care
Diagnosis is the veterinarian's responsibility, based on clinical examination and laboratory tests. Care is decided case by case.
The veterinarian relies on a very telling picture: sudden blindness with a normal eye, head pushed against the wall, absence of marked fever, several animals from the same batch affected.
He examines the eyes to confirm that they are healthy and that the blindness is indeed coming from the brain.
He rules out other causes of nervous signs in sheep, which are very similar and do not have the same behavior: listeriosis, which causes paralysis on one side of the face, coenurosis, which progresses slowly, enterotoxemia, lead or salt poisoning, hypocalcemia, gestational toxemia and brain abscesses.
He analyzes the ration actually distributed, the drinking water and the pasture: this is where the cause lies.
Examination of the brain of a dead animal confirms the diagnosis and guides prevention for the rest of the flock. Do not throw away an animal that has died of nervous signs without talking to the veterinarian.
The response to treatment is itself part of the diagnosis: rapid improvement after administration of vitamin B1 is very suggestive.
Treatment
It's an emergency: each hour of delay reduces the chances of recovery and increases the risk of permanent after-effects.
The treatment is based on the administration of vitamin B1 by the veterinarian, through a route that allows immediate action. The form, the way and the repetition are his prescription. A simple vitamin supplement given in food has no effect in a declared crisis.
The response is often spectacular when we intervene early: the animal gets up and regains its sight within a few hours to one or two days.
Supportive treatments are associated depending on the case: anti-inflammatory to limit swelling of the brain, rehydration, calm, thick bedding, animal placed in a place without obstacles so that it does not get injured.
A blind animal must be fed and watered within immediate reach: it will not find the trough or the water trough.
At the same time, the ration of the entire flock must be reviewed immediately, because other animals are exposed to the same conditions and other cases often follow.
Don't wait to see: a blind sheep pushing against a wall must be seen the same day.
Treatment depends on the cause and the animal's condition: it must be determined by a veterinarian. No self-medication.
Prevention
Make all dietary transitions gradually, over several days, particularly when switching to a concentrate-rich ration.
Respect a sufficient proportion of long fibers in the ration of fattening animals: the rumen needs fodder to function.
Have drinking water analyzed when it comes from a borehole, particularly for its sulfur content, and have industrial foods and by-products used checked.
Monitor pastures overgrown with bracken or horsetail and avoid putting young animals there; check the presence of these plants in purchased hay.
Prevent any accidental access to a stock of cereals: a silo door left open can destroy an entire batch.
Distribute concentrates in several meals rather than all at once, and ensure that all animals eat at the same time.
Ask your veterinarian for advice before using anticoccidial products.
React from the first case: review the ration of the batch without waiting for the next one.
Possible complications
Permanent blindness when treatment is too late.
Permanent nervous after-effects: abnormal gait, animal which remains dazed and no longer eats alone.
Death in one to three days in untreated forms.
Injuries, fractures and drownings in blind wandering animals.
New cases in the same batch if the ration is not corrected immediately.
Weight loss and downgrading of surviving animals.
When to see a veterinarian
A sheep suddenly goes blind and bumps into everything: call the same day, it's an emergency.
He pushes his head against a wall or fence and stays there.
He turns in circles, staggers or keeps his head thrown back.
He stops ruminating and refuses to eat without further explanation.
He convulses or lies down while pedaling.
Several animals from the same batch show nervous signs: review the ration with the veterinarian immediately.
These signs follow a change of food, batching or accidental access to cereals: tell the veterinarian, this is essential information.
Prognosis
Excellent when treatment is administered in the first hours: many animals recover completely, including vision.
Reserved after a day or two: sight may not return and nervous after-effects may persist.
Bad in an animal that has already been lying down and convulsing for a long time.
At the batch level, the prognosis depends entirely on the correction of the ration: without it, cases follow one another.
Frequently asked questions
My sheep went blind overnight, is there a problem with his eyes?
Often no: in this disease, the eye is perfectly normal and it is the brain which no longer processes the image. It is precisely this contrast, a blind animal with healthy eyes, which must call the veterinarian urgently, because early treatment can restore its sight.
Can we give a vitamin supplement in food to treat the crisis?
No. In a declared crisis, administration by the veterinarian is necessary, through a route that acts immediately. Supplement in the trough will not reach the brain in time, and the blind animal will not eat anyway.
Why are several of my lambs affected at the same time?
Because the cause is almost always food and therefore concerns the whole lot: too rapid transition to concentrate, excess sulfur in the water or food, toxic plant in pasture. Treating the patient is not enough; the group's ration must be reviewed without waiting for the next case.
Is it the same as listeriosis or whirling disease?
No, these are different diseases which give similar nervous signs. Listeriosis often affects only one side of the face, the dizziness progresses slowly over weeks, it strikes suddenly with blindness. As the treatments are very different and some must be started within the hour, only the veterinarian can make the difference.
References
Content reviewed by the TunisieVet editorial team · Updated 11 August 2026
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