Bovine Listeriosis Study Guide

Overview and Clinical Importance

Listeriosis (also known as Circling Disease or Silage Sickness) is a sporadic but serious bacterial infection caused by Listeria monocytogenes, a gram-positive, facultatively anaerobic, intracellular coccobacillus. The disease primarily manifests as encephalitis or meningoencephalitis in adult cattle, though it can also cause abortion and septicemia. This condition is particularly important for the NAVLE because it requires differentiation from other neurological diseases including rabies, polioencephalomalacia, and thromboembolic meningoencephalitis.

Listeriosis is a zoonotic disease with significant public health implications. Veterinarians treating affected cattle must take appropriate precautions, as humans can become infected through contaminated milk or direct contact with infected animals and tissues. The disease is most commonly associated with feeding poorly preserved silage with a pH greater than 4.0 to 5.5.

Etiology

Causative Organism

Listeria monocytogenes is the primary causative agent. Key characteristics include:

• Gram-positive, non-spore-forming, motile coccobacillus
• Facultatively anaerobic and facultatively intracellular pathogen
• Catalase-positive and oxidase-negative with beta-hemolysis on blood agar
• Capable of growth at refrigeration temperatures (4°C to 44°C), which is diagnostically significant (cold enrichment technique)
• Characteristic tumbling motility when viewed microscopically at room temperature (20-25°C)
• Ubiquitous in the environment: soil, water, decaying vegetation, silage, and GI tracts of healthy animals

Epidemiology

Distribution and Occurrence

• Geographic distribution: Worldwide, but more common in temperate and colder climates
• Seasonality: Winter-spring disease (December to May in Northern Hemisphere), coinciding with silage feeding season
• Age predisposition: More common in younger animals (1-3 years old), associated with tooth eruption and emergence of permanent molars
• Herd impact: Usually sporadic (1-2 animals affected), but outbreaks can affect up to 10-20% of herd
• Carrier state: High incidence of asymptomatic intestinal carriers that shed bacteria in feces

Risk Factors

Pathogenesis

Route of Infection and Neural Invasion

The pathogenesis of encephalitic listeriosis is unique and involves ascending infection via the trigeminal nerve. This distinctive mechanism explains the characteristic unilateral clinical signs and brainstem localization.

• Ingestion: Cattle ingest contaminated silage or pasture material containing L. monocytogenes
• Mucosal entry: Bacteria enter through breaks in oral mucosa (tooth eruption sites, wounds, abrasions)
• Axonal invasion: L. monocytogenes invades sensory nerve endings of the trigeminal nerve (CN V)
• Centripetal migration: Bacteria travel within axons (intra-axonal transport) to the trigeminal nucleus in the pons
• Brainstem infection: Bacteria multiply in the pons and medulla oblongata, causing microabscessation and inflammation
• Cranial nerve involvement: Inflammation damages cranial nerve nuclei (CN V, VII, VIII primarily), producing characteristic unilateral deficits

Clinical Presentation

Forms of Listeriosis in Cattle

Clinical Signs of Encephalitic Listeriosis

Early Signs

• Depression and dullness (ascending reticular activating system involvement)
• Anorexia and decreased water intake
• Fever (variable; often present early but may be absent in advanced cases)
• Isolation from herd

Classic Unilateral Neurological Signs

Additional Neurological Signs

• Circling: Compulsive movement toward the affected side (hence "Circling Disease")
• Head pressing: Animal pushes head against fixed objects, walls, or corners
• Profuse salivation: Continuous drooling due to inability to swallow and facial paralysis
• Ipsilateral weakness: Paresis on the same side as other deficits (long tract involvement)
• Behavioral changes: Occasional excitement or aggression, disorientation

Terminal Signs

• Recumbency (animal unable to stand)
• Convulsions and opisthotonus
• Coma and death

Diagnosis

Clinical Diagnosis

Presumptive diagnosis is based on the classic triad of: history of silage feeding + unilateral cranial nerve deficits + depression. This is often sufficient to initiate treatment while awaiting confirmatory testing.

Cerebrospinal Fluid (CSF) Analysis

CSF analysis from lumbosacral collection supports the diagnosis but is not definitive:

Definitive Diagnosis

Definitive diagnosis requires postmortem examination:

• Gross pathology: Often minimal findings; may see mild meningeal congestion or softening (malacia) of brainstem
• Histopathology (gold standard): Microabscesses (foci of neutrophils and macrophages), perivascular cuffing (mononuclear cells), and gliosis localized to pons and medulla oblongata
• Gram stain: May reveal gram-positive coccobacilli, but often few organisms visible
• Immunohistochemistry (IHC): Superior to culture (84% positive by IHC vs 39% by culture); detects listeriolysin O antigen
• Bacterial culture: From brainstem tissue; may use cold enrichment technique (incubation at 4°C)
• PCR: Molecular detection from brain tissue increasingly used

Exam Focus: Histopathologic lesions in listeriosis are ASYMMETRIC and LOCALIZED to the BRAINSTEM (pons and medulla). Key features are: microabscesses, perivascular mononuclear cuffing, and malacia. The unilateral nature of lesions correlates with the unilateral clinical signs. Immunohistochemistry is superior to culture for confirmation.

Differential Diagnosis

Treatment

Antimicrobial Therapy

Treatment success depends on early intervention before extensive brainstem damage occurs. The intracellular nature of L. monocytogenes and blood-brain barrier penetration requirements necessitate high doses and prolonged treatment (2-4 weeks).

Supportive Care

• Fluid therapy: Correct dehydration and electrolyte imbalances from anorexia and excessive salivation
• Acid-base correction: Sodium bicarbonate IV if metabolic acidosis present (loss of saliva buffer)
• Thiamine (Vitamin B1): 5-10 mg/kg to support nervous tissue and rule out concurrent PEM
• NSAIDs: Flunixin meglumine (2.2 mg/kg IV) for anti-inflammatory effects
• Nutritional support: Rumen transfaunation; stomach tube feeding if dysphagia not severe
• Eye care: Lubrication for exposure keratitis if eyelid paralysis present
• Nursing care: Prevent secondary musculoskeletal injury; deep bedding for recumbent animals

Prognosis

Prevention and Control

Silage Quality Management

The cornerstone of listeriosis prevention is proper silage management to achieve adequate fermentation and prevent Listeria proliferation:

• Target silage pH less than 4.5: Listeria cannot survive at low pH; proper lactic acid fermentation is critical
• Minimize soil contamination: Raise cutting height, avoid muddy fields, maintain equipment
• Ensure adequate packing density: Proper compaction reduces oxygen and promotes anaerobic fermentation
• Proper sealing: Use quality plastic covers; prevent air ingress during storage
• Appropriate moisture content: Not too wet (favors clostridial growth) or too dry (poor fermentation)
• Use silage additives: Formic acid or bacterial inoculants (lactic acid bacteria) improve fermentation
• Discard spoiled silage: Do not feed moldy, aerobically deteriorated, or visibly spoiled material

Additional Prevention Measures

• Remove affected silage from ration if cases occur (may prevent additional cases)
• Feed from troughs rather than directly on ground (pasture-associated cases)
• Avoid feeding silage to young animals during tooth eruption period if possible
• No effective vaccine is commercially available for cattle
• Good hygiene practices; proper disposal of aborted material

Zoonotic Considerations

Listeriosis is a significant zoonotic disease. Veterinarians and farm workers are at risk when handling affected animals or contaminated materials:

• Human infection routes: Contaminated milk, soft cheeses, ready-to-eat foods; direct contact with infected tissues
• High-risk groups: Pregnant women (risk of abortion, neonatal infection), elderly, immunocompromised individuals
• Protective measures: PPE when handling affected animals; proper hygiene; avoid raw milk consumption
• Milk withholding: Extended withdrawal times due to high-dose antibiotic treatment; consult FARAD