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Bovine Polioencephalomalacia Study Guide

📅 March 28, 2026 ⏱ 25 min read

Polioencephalomalacia (PEM), also known as cerebrocortical necrosis (CCN), is an important non-infectious neurological disease of ruminants characterized by necrosis of the cerebral cortex.

Overview and Clinical Importance

Polioencephalomalacia (PEM), also known as cerebrocortical necrosis (CCN), is an important non-infectious neurological disease of ruminants characterized by necrosis of the cerebral cortex. The term derives from Greek: polio (gray) + encephalo (brain) + malacia (softening), literally meaning softening of the gray matter of the brain.

PEM is a common neurological disorder affecting cattle, sheep, goats, deer, and camelids worldwide. In cattle, it most commonly affects calves aged 6-18 months, particularly those on high-concentrate diets or in feedlot settings. The condition represents a pathological end point resulting from several distinct etiologies, most notably thiamine (Vitamin B1) deficiency and sulfur toxicosis.

High-YieldOn the NAVLE, PEM questions frequently test your ability to recognize the classic clinical presentation (stargazing, cortical blindness, dorsomedial strabismus) and appropriate treatment with thiamine. Remember that PEM is NOT the same as polio in humans - it is not infectious!

Etiology	Mechanism	Risk Factors
Thiamine Deficiency	Reduced thiamine synthesis by rumen microbes or destruction by thiaminase enzymes	High-concentrate, low-fiber diets; ruminal acidosis; rapid feed changes
Sulfur Toxicosis	Excess ruminal hydrogen sulfide (H2S) production causing direct neurotoxicity	High-sulfur water (greater than 2000 ppm); distillers grains; brassica forages
Thiaminase-Containing Plants	Thiaminase I enzyme destroys thiamine and creates antagonistic analogs	Bracken fern (Pteridium aquilinum); horsetail; nardoo fern; kochia
Drug-Induced	Amprolium acts as thiamine antagonist; some anthelmintics act as thiaminase cofactors	Amprolium overdose; levamisole; thiabendazole treatment

Etiology and Pathophysiology

Primary Causes of PEM

Pathophysiology of Thiamine Deficiency

Thiamine pyrophosphate (TPP), the active form of thiamine, serves as an essential cofactor for three critical enzymes in glucose metabolism:

Pyruvate dehydrogenase (PDH): Converts pyruvate to acetyl-CoA, linking glycolysis to the Krebs cycle
Alpha-ketoglutarate dehydrogenase (?-KGDH): Functions within the Krebs cycle for ATP production
Transketolase: Operates in the pentose phosphate pathway, providing NADPH and ribose-5-phosphate

When thiamine is deficient, these enzymes cannot function properly, leading to impaired aerobic glucose metabolism. The brain, which relies almost exclusively on glucose for energy, is particularly vulnerable. Pyruvate accumulates and is converted to lactate, causing focal lactic acidosis within the brain parenchyma. This triggers a cascade of cellular damage including cytotoxic edema, neuronal necrosis, and ultimately the characteristic laminar cortical necrosis of PEM.

NAVLE TipRemember the brain's glucose dependency! The cerebral cortex is the first to suffer when thiamine-dependent glucose metabolism fails. This explains why PEM causes cortical blindness (occipital cortex necrosis) while sparing brainstem functions like pupillary light reflexes.

Sulfur-Induced PEM Pathophysiology

In sulfur toxicosis, rumen microbes reduce dietary sulfur to hydrogen sulfide (H2S), a potent neurotoxin. H2S is absorbed from the rumen and competes with oxygen for binding to hemoglobin, reducing oxygen delivery to tissues. Within the brain, H2S directly inhibits cytochrome c oxidase in mitochondria, blocking cellular respiration and causing histotoxic hypoxia.

Importantly, high dietary sulfur also increases metabolic demand for thiamine, potentially causing secondary thiamine deficiency. This explains why animals with sulfur-induced PEM may still respond to thiamine therapy.

Clinical Sign	Description and Clinical Significance
Stargazing (Opisthotonus)	Head and neck extended dorsally with muzzle pointed upward. Results from cerebral edema causing increased intracranial pressure. HIGHLY CHARACTERISTIC of PEM.
Cortical Blindness	Absent menace response with NORMAL pupillary light reflex. Indicates occipital cortex damage while CN II and brainstem pathways remain intact. Often first sign to appear, last to resolve.
Dorsomedial Strabismus	Eyes deviated dorsally and medially ("sunset eyes"). Results from trochlear nerve (CN IV) dysfunction due to increased intracranial pressure. Bilateral presentation.

Clinical Presentation

PEM can present in acute or subacute forms. Recognition of early signs is critical, as prompt treatment dramatically improves prognosis.

Early (Subacute) Signs

Separation from the herd, isolation behavior
Anorexia and depression
Mild diarrhea (occasionally)
Ear twitching and facial muscle tremors
Aimless wandering, hyperesthesia

Classic (Acute) Signs - Pathognomonic Triad

Progressive Signs

Head pressing against fixed objects
Bruxism (teeth grinding)
Nystagmus (horizontal or rotary)
Ataxia and incoordination
Recumbency with paddling movements
Tonic-clonic seizures
Coma and death (24-96 hours if untreated)

High-YieldThe key distinguishing feature of cortical blindness is the ABSENCE of menace response with PRESERVATION of pupillary light reflexes. This pattern indicates cortical (occipital lobe) damage while the optic nerve (CN II) and brainstem oculomotor pathways remain functional.

Physical Examination Findings

Temperature: Normal unless secondary hyperthermia from excessive muscle activity/seizures
Menace response: Absent bilaterally
Pupillary light reflex: Present and normal
Palpebral reflex: Present
Hypersalivation: May be present

Test	Normal Range	PEM Findings
Blood Thiamine	50-150 nmol/L	Less than 50 nmol/L (may be as low as 6-12 nmol/L)
CSF Analysis	Clear, low protein, no cells	Acute: Usually normal or mildly elevated protein. Chronic: Elevated protein, mild mononuclear pleocytosis
Ruminal H2S	Less than 500 ppm	Greater than 1000 ppm in sulfur toxicosis
Water Sulfate	Less than 500 ppm	Greater than 2000 ppm associated with PEM risk

Diagnosis

Diagnosis of PEM is typically presumptive based on clinical signs, history, and response to thiamine therapy. Definitive diagnosis requires postmortem examination with histopathology.

Antemortem Diagnosis

Clinical Criteria

Classic neurological signs (stargazing, cortical blindness, dorsomedial strabismus)
History of dietary risk factors (high-concentrate diet, sulfur exposure, rapid feed change)
Age group typically affected (6-18 months in cattle)
Response to thiamine therapy (often used as a diagnostic tool)

Laboratory Findings

Postmortem Diagnosis

Gross Pathology

Cerebral swelling: Flattening of gyri and widening of sulci
Yellow discoloration: Faint yellow color of affected cortical tissue
Cerebellar coning: Herniation of cerebellar vermis through foramen magnum in severe cases
Cortical cavitation: In chronic cases, visible softening and cavitation of gray matter

UV Fluorescence Test (Wood's Lamp)

A key diagnostic feature of PEM is autofluorescence of necrotic cortical tissue under UV light (365 nm). The affected cerebral cortex emits a characteristic apple-green fluorescence due to accumulation of ceroid-lipofuscin in lipophages as they engulf necrotic tissue.

High-YieldUV fluorescence may be ABSENT in very early (less than 24 hours) cases because insufficient lipophage accumulation has occurred. A negative UV test does NOT rule out PEM. Always submit brain tissue for histopathology.

Histopathology

Laminar cortical necrosis: Characteristic pattern affecting middle to deep cortical layers
Red neurons (eosinophilic neurons): Shrunken neurons with eosinophilic cytoplasm and pyknotic nuclei
Spongiosis: Vacuolation of neuropil due to cytotoxic edema
Gitter cells: Lipid-laden macrophages (lipophages) engulfing necrotic debris
Vascular changes: Endothelial hypertrophy and hyperplasia

Condition	Key Distinguishing Features	Diagnostic Test
Lead Poisoning	History of lead exposure (batteries, paint); may have GI signs; frenzy/bellowing; similar CNS signs to PEM	Blood lead greater than 0.35 ppm; tissue lead (liver, kidney) greater than 10 ppm
Listeriosis	UNILATERAL cranial nerve deficits (facial paralysis, head tilt); circling; silage-fed animals; winter-spring seasonality	CSF culture/PCR for Listeria monocytogenes; IHC of brainstem
Rabies	Behavioral changes (aggression or paralytic); progressive ascending paralysis; salivation; exposure to wildlife; ALWAYS FATAL	Fluorescent antibody test (FAT) on brain tissue (brainstem, cerebellum)
TEME (Histophilus somni)	Feedlot cattle 6-12 months; fever; sudden death; respiratory signs may precede; multisystemic disease	Culture or PCR; characteristic brain hemorrhages at necropsy
Salt Poisoning / Water Deprivation	History of water deprivation then access; eosinophilic perivascular cuffing on histopath (especially in pigs)	Brain sodium analysis; history of water restriction
Nervous Ketosis	Lactating dairy cows; periparturient period; acetone breath; pica; circling; hyperesthesia	Elevated blood ketones (BHB greater than 1.2 mmol/L)

Differential Diagnosis

Several neurological conditions in cattle present similarly to PEM. Accurate differentiation is critical for appropriate treatment and regulatory reporting (especially for rabies).

NAVLE TipThe key to differentiating PEM from listeriosis is SYMMETRY. PEM causes BILATERAL cortical blindness and dorsomedial strabismus, while listeriosis causes UNILATERAL cranial nerve deficits (facial paralysis, head tilt to one side). Listeriosis primarily affects the brainstem while PEM primarily affects the cerebral cortex.

Parameter	Recommendation	Notes
Dose	10-20 mg/kg body weight	Higher doses (up to 20 mg/kg) for severe cases
Route	IM preferred; slow IV possible	IV must be diluted and given slowly to prevent adverse reactions
Frequency	Every 6-8 hours (TID-QID)	Thiamine is water-soluble and rapidly excreted; repeated dosing essential
Duration	3-5 days or until clinical improvement	Continue treatment even after improvement begins
Expected Response	Improvement within hours to 24-48 hours if treated early	Blindness may take up to 1 week to resolve after other signs improve

Treatment

Early, aggressive treatment is essential for successful outcomes. Response to thiamine is often used as therapeutic confirmation of PEM.

Thiamine Hydrochloride - Primary Treatment

Adjunctive Therapies

High-YieldSulfur-induced PEM may have a poorer response to thiamine therapy than classic thiamine-deficiency PEM. However, because high sulfur intake can cause secondary thiamine deficiency through increased metabolic demand, thiamine treatment should still be attempted. Always address the underlying sulfur source simultaneously.

Prognosis

Excellent: Early cases (less than 24 hours) treated promptly with thiamine - full recovery expected
Good: Subacute cases with mild-moderate signs - recovery likely but blindness may persist temporarily
Poor: Recumbent animals or those with prolonged clinical signs (greater than 48 hours) - irreversible cortical damage likely
Grave: No response to thiamine within 24-48 hours - consider sulfur toxicity or alternate diagnosis

Medication	Dosage	Purpose
Dexamethasone	0.1-0.2 mg/kg IV once	Reduce cerebral edema (evidence limited)
Mannitol	1 g/kg IV (20% solution)	Osmotic diuretic to reduce intracranial pressure
Diazepam	0.1-0.5 mg/kg IV	Seizure control if convulsing

Prevention and Control

Dietary Management

Ensure adequate fiber (roughage) in the diet - minimum 15-20% of dry matter
Implement gradual dietary transitions (2-3 weeks) when changing from forage to concentrate diets
Monitor total dietary sulfur - maintain less than 0.4% sulfur in dry matter for high-forage diets, less than 0.3% for high-concentrate diets
Test water sources for sulfate - target less than 500 ppm sulfate
Limit use of high-sulfur by-products (distillers grains, beet pulp) in rations

Outbreak Management

Remove animals from suspected feed/water source
Provide additional roughage to affected group
Consider prophylactic thiamine supplementation for in-contact animals (10 mg/kg IM weekly)
Analyze feed and water for sulfur content
Thiamine may be added to feed at 3-10 mg/kg of diet during outbreaks

Memory Aids and Board Tips

PEM = "STAR" Mnemonic

Stargazing posture (opisthotonus)
Thiamine treatment is the answer
Absent menace, present PLR (cortical blindness)
Rumen dysfunction (high grain, high sulfur) is the cause

"Three Ts" for Thiamine

TPP (thiamine pyrophosphate) is the active form
TID-QID dosing (every 6-8 hours)
Ten to Twenty mg/kg is the dose (10-20 mg/kg)

Differentiating PEM from Listeriosis

"PEM is BILATERAL, Listeria is UNILATERAL"

PEM: Bilateral cortical blindness, bilateral dorsomedial strabismus, NORMAL temperature
Listeriosis: Unilateral facial paralysis, head tilt to one side, circling, may have FEVER

NAVLE TipWhen an NAVLE question describes a feedlot calf with sudden onset blindness, stargazing, and normal pupillary light reflexes - think PEM FIRST! If the same question mentions silage feeding and unilateral facial paralysis - think listeriosis. The symmetry of neurological deficits is your key differentiator.

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