Camelidae and Cervidae Polioencephalomalacia Study Guide

Overview and Clinical Importance

Polioencephalomalacia (PEM), also known as cerebrocortical necrosis (CCN), is a significant neurological disease affecting ruminants and pseudoruminants worldwide, including camelids (llamas, alpacas) and cervids (deer, elk, moose). The term derives from Greek: polio (gray), encephalo (brain), and malacia (softening), describing the pathological softening of the cerebral gray matter.

PEM is characterized by acute to subacute neurological dysfunction resulting from laminar necrosis of the cerebral cortex. The condition is most commonly associated with thiamine (Vitamin B1) deficiency or sulfur toxicosis, though multiple etiologies can produce similar clinical and pathological presentations.

Etiology and Pathophysiology

Thiamine Deficiency Mechanism

Thiamine (Vitamin B1) is an essential cofactor for key enzymes in carbohydrate metabolism, particularly transketolase in the pentose phosphate pathway and pyruvate dehydrogenase in the Krebs cycle. The brain is highly dependent on glucose metabolism, making it exceptionally vulnerable to thiamine deficiency.

In adult ruminants and pseudoruminants, thiamine is synthesized by forestomach microflora. PEM develops not from dietary deficiency but from:

• Thiaminase enzyme production: Certain rumen bacteria (Clostridium sporogenes, Bacillus thiaminolyticus) produce thiaminase enzymes that destroy thiamine
• Thiaminase-containing plants: Bracken fern (Pteridium aquilinum), horsetail (Equisetum), nardoo fern (Marsilea drummondii), rock fern (Cheilanthes sieberi)
• Thiamine analogs: Amprolium (used for coccidiosis treatment) competitively inhibits thiamine uptake
• Decreased production: Rumen acidosis from high-concentrate diets reduces thiamine-producing bacteria

Sulfur Toxicosis Mechanism

High dietary sulfur intake leads to excessive hydrogen sulfide (H2S) production in the rumen. H2S is absorbed into the bloodstream and exerts direct neurotoxic effects on the brain by:

• Competing with oxygen for hemoglobin binding
• Generating free sulfite radicals that damage neurons
• Increasing thiamine demand, causing secondary thiamine deficiency
• Binding magnesium required for thiamine conversion to active thiamine pyrophosphate (TPP)

Species-Specific Considerations

Clinical Signs and Presentation

Early/Acute Signs

Clinical signs progress rapidly and reflect forebrain dysfunction:

• Behavioral changes: Depression, anorexia, separation from herd, dullness
• Visual abnormalities: Cortical blindness (absent menace response with INTACT pupillary light reflex)
• Stargazing: Head held elevated and extended - CLASSIC sign
• Ataxia: Aimless wandering, circling, incoordination
• Head pressing: Against fixed objects due to increased intracranial pressure
• Dorsomedial strabismus: Due to trochlear nerve (CN IV) dysfunction
• Muscle fasciculations: Especially facial and ear twitching, bruxism (teeth grinding)

Progressive/Severe Signs

• Recumbency: Sternal progressing to lateral
• Opisthotonus: Severe dorsal arching of head and neck
• Seizures: Tonic-clonic convulsions
• Nystagmus: Involuntary eye movements
• Hypersalivation: Excessive drooling
• Coma and death: Without treatment, typically within 24-72 hours

Diagnosis

Clinical Diagnosis

Diagnosis is primarily based on clinical presentation, history of risk factors, and response to treatment. Key diagnostic considerations include:

Postmortem Diagnosis

Gross Pathology

• Brain swelling with gyral flattening and sulcal narrowing
• Cerebellar herniation through foramen magnum (cerebellar coning)
• Yellow discoloration of neocortical gyri
• Softened, friable cortical tissue
• Cavitation in advanced cases

UV Light Examination (365 nm)

Autofluorescence of affected cerebral cortex is a rapid diagnostic tool. The fluorescence results from ceroid-lipofuscin accumulation in lipophages engulfing necrotic lipid material.

Histopathology

• Laminar cortical necrosis: Pseudolaminar pattern affecting middle to deep cortical laminae
• Neuronal necrosis: Red (eosinophilic) neurons with shrunken cytoplasm, pyknotic nuclei
• Cortical spongiosis: Perineuronal vacuolation in early phases
• Vascular changes: Endothelial hypertrophy and hyperplasia
• Macrophage infiltration: In later stages with tissue cavitation

Differential Diagnosis

Treatment

Thiamine Administration - Primary Treatment

Thiamine hydrochloride is the treatment of choice regardless of etiology. Early treatment is critical - delays significantly worsen prognosis.

Adjunctive Treatments

• Cerebral edema management: Dexamethasone 0.1-0.2 mg/kg IV once; Mannitol 20% at 0.25-1 g/kg IV once
• Seizure control: Diazepam 0.1-0.5 mg/kg IV as needed
• Supportive care: Fluid therapy, nutritional support, protection from injury
• Remove inciting cause: Discontinue sulfur source, correct diet, stop amprolium
• Rumen transfaunation: Consider in refractory cases or hand-reared animals with persistent thiamine deficiency

Prognosis

• Early treatment: Good to excellent prognosis with full recovery possible
• Delayed treatment: Poor prognosis; permanent neurological deficits including blindness and mental dullness
• Recumbent with seizures: Guarded to grave prognosis
• Blindness: Often the first sign to appear and last to resolve; may persist permanently
• Mortality rate: 50-90% without treatment; sheep and goats generally have shorter course with fewer survivors than cattle

Prevention

Dietary Management

• Avoid sudden feed changes - transition over 7-14 days minimum
• Maintain adequate roughage in diet (minimum 50% forage)
• Limit concentrate feeding to prevent rumen acidosis
• Monitor and limit dietary sulfur to less than 0.4% DM
• Test water sulfur content - high sulfur water is a common source
• Avoid high-sulfur feedstuffs: distillers grains, brassicas (turnips, rape), corn processing by-products

Medication Considerations

• Amprolium (Corid) use: Limit duration to less than 5 days at 10 mg/kg; consider prophylactic thiamine if treatment extends longer
• Sulfa drugs in camelids: Use with extreme caution - sulfa-induced PEM in camelids is often NON-THIAMINE RESPONSIVE and frequently fatal
• Anthelmintics: Some (levamisole, thiabendazole) may predispose to PEM

Herd Management

• Prophylactic thiamine supplementation: 3-10 mg/kg feed during high-risk periods
• During outbreaks: treat in-contact animals with single IM thiamine injection (10 mg/kg)
• Avoid access to thiaminase-containing plants
• Keep injectable thiamine on hand - EVERY camelid farm should have this available

Exam Focus: CAMELID OWNERS should have injectable thiamine available at all times. Any neurologic camelid should receive thiamine empirically - it is safe and can be life-saving. "Thiamine is a safe and useful therapy any time we suspect neurological insult."

Memory Aids for Boards

PEM = "STAR-BLIND"

S - Stargazing posture

T - Thiamine treatment (10-20 mg/kg)

A - Absent menace response

R - Rapid response to treatment (hours)

B - Brain cortex affected (gray matter)

L - Light reflex (PLR) INTACT

I - Illumination with UV shows fluorescence

N - Neuronal necrosis (laminar pattern)

D - Diet changes/Distillers grains cause it

Alpaca Alert: "A-2"

Alpacas develop PEM in 2 days (vs weeks in cattle) - remember "A-2" for Alpaca-2 days

Sulfur PEM: "No Glow"

Sulfur-induced PEM typically does NOT fluoresce under UV light - "Sulfur = No Glow"