NAVLE Rabbits

Rabbit Encephalitis Study Guide

📅 March 28, 2026 ⏱ 25 min read
Encephalitis in rabbits represents inflammation of the brain parenchyma and is a critical multisystemic disease with significant morbidity and mortality.

Overview and Clinical Importance

Encephalitis in rabbits represents inflammation of the brain parenchyma and is a critical multisystemic disease with significant morbidity and mortality. The condition frequently presents with neurologic signs and can be caused by infectious (parasitic, bacterial, viral) or non-infectious etiologies.

Encephalitozoon cuniculi, a microsporidial parasite, is the most common cause of encephalitis in pet rabbits. Understanding the differential diagnoses, diagnostic approach, and treatment protocols is essential for NAVLE success.

System Affected Clinical Signs
Central Nervous System (Most Common) Vestibular Signs: Head tilt (torticollis), nystagmus, ataxia, rolling, circling, loss of balance Other Neurologic Signs: Seizures, tremors, hemiparesis, posterior paresis, urinary incontinence, behavioral changes, mentation changes
Renal Polyuria, polydipsia (PU/PD), weight loss, anorexia, dehydration, chronic renal failure. May be subclinical with incidental findings on bloodwork.
Ocular Phacoclastic uveitis, cataracts (usually unilateral), white intraocular masses. Typically seen in young rabbits (6 months to 2 years) due to transplacental infection.

Encephalitozoon cuniculi (E. cuniculi)

Etiology and Pathophysiology

Organism: E. cuniculi is an obligate intracellular microsporidial parasite (phylum Microsporidia). It is a eukaryotic, unicellular, spore-forming organism measuring approximately 1.5 × 2.5-5 micrometers.

Genotypes: Four genotypes exist - Type I (rabbit strain), Type II (murine strain), Type III (canine strain), and Type IV (human strain). Type I is most commonly found in rabbits.

Transmission: Horizontal transmission occurs via ingestion of spores shed in urine of infected rabbits. Vertical (transplacental) transmission can occur in pregnant does. Environmental contamination is common as spores can survive for weeks in the environment.

Pathogenesis: After ingestion, spores germinate in the small intestine. The organism invades enterocytes and spreads hematogenously to target organs including brain, kidneys, eyes, heart, liver, and lungs. In the CNS, E. cuniculi causes granulomatous meningoencephalomyelitis with lymphoplasmacytic inflammation.

High-YieldUp to 50-80% of rabbits in the United States and Europe are seropositive for E. cuniculi, but most remain asymptomatic carriers. Only immunocompromised or stressed rabbits typically develop clinical disease.

Clinical Signs of E. cuniculi

NAVLE TipOn the NAVLE, remember that E. cuniculi classically presents with sudden onset of vestibular signs (head tilt, nystagmus, rolling) in a young to middle-aged rabbit following a stressful event. The key differential is Pasteurella multocida otitis interna/media.

Diagnosis of E. cuniculi

Definitive antemortem diagnosis is challenging. Most diagnoses are presumptive based on clinical signs combined with positive serology.

Diagnostic Tests

Exam Focus: A negative serologic test effectively rules out E. cuniculi, making it a useful tool. However, a positive test only confirms exposure, not active disease. Clinical diagnosis is often presumptive based on consistent signs plus positive serology.

Treatment of E. cuniculi

Remember: E=F×28 (Encephalitozoon = Fenbendazole times 28 days)

Prognosis

Variable and depends on severity of clinical signs and rapidity of treatment initiation. Rabbits with mild vestibular signs may improve with treatment, though head tilt may persist. Severe cases with seizures, posterior paresis, or coma carry a grave prognosis. Renal disease is typically chronic and progressive.

Test Interpretation Limitations
Serology (IgG, IgM) IgG positive: Exposure to organism (past or present). IgM positive: Recent or active infection. Negative serology: Rules out E. cuniculi. Positive titer does NOT correlate with clinical disease severity. Many healthy rabbits are seropositive. Antibodies persist lifelong. No protective immunity.
Urine PCR Detects spore shedding in urine. Best if collected over 3 consecutive days. Spores shed intermittently. False negatives common. Positive result confirms shedding but not necessarily clinical disease.
Histopathology Gold standard. Demonstrates granulomatous encephalitis, nephritis. Spores visible with Gram, Modified Trichrome Stain (MTS), or Ziehl-Neelsen stains. Requires necropsy. Lesion severity does NOT correlate with clinical signs.
Imaging (Skull Radiographs, CT) Helps differentiate from Pasteurella (look for tympanic bulla changes, middle ear disease) E. cuniculi lesions typically not visible on imaging. Useful for ruling out other causes.

Pasteurella multocida

Etiology and Pathophysiology

Organism: Pasteurella multocida is a Gram-negative, nonmotile coccobacillus that colonizes the upper respiratory tract of 30-90% of apparently healthy rabbits. Serogroups A and D are most commonly associated with disease in rabbits.

Transmission: Direct contact, aerosol, venereal transmission, or through wounds. Does transmit to kits immediately after birth.

Pathogenesis: Primary infection typically begins in nasal cavity. Extension to middle/inner ear via eustachian tube leads to otitis media/interna causing vestibular signs. Hematogenous spread can rarely cause CNS infection (encephalitis/meningoencephalitis), though this is less common than peripheral vestibular disease.

High-YieldIn dwarf rabbit breeds, E. cuniculi is more commonly the cause of head tilt, while in standard-sized breeds, Pasteurella multocida otitis media/interna is more frequently the culprit.

Clinical Signs

Respiratory System: Nasal discharge (serous to mucopurulent), sneezing, snuffles, dyspnea, matted fur on inside of front paws from wiping nose

Neurologic (Otitis Media/Interna): Head tilt, nystagmus (horizontal), circling, ataxia, rolling. Unlike E. cuniculi, ear discharge may be visible and rabbits may paw at affected ear.

Other Systems: Abscesses (subcutaneous, dental, retrobulbar), conjunctivitis, pneumonia, reproductive tract infections, septicemia

Diagnosis

Culture and Sensitivity: Nasal swabs, deep ear swabs, or abscess aspirates. However, positive nasal culture does NOT confirm disease (many healthy carriers).

Imaging: Skull radiographs or CT scan showing fluid density in tympanic bullae, bony lysis, or soft tissue opacity in middle ear

Serology: Limited value in adult rabbits due to cross-reactivity with other Gram-negative bacteria

PCR: More sensitive and specific than culture alone

NAVLE TipKey to differentiating Pasteurella from E. cuniculi: Look for respiratory signs, visible tympanic bulla changes on imaging, and response to antibiotics. E. cuniculi: sudden onset after stress, no respiratory signs, normal bullae on imaging, treated with fenbendazole.

Treatment

Antibiotics: Long-term therapy (4-6 weeks minimum), often requires months. Options include enrofloxacin (5-20 mg/kg PO/SC q12-24h), trimethoprim-sulfa (30 mg/kg PO q12h), azithromycin (15-30 mg/kg PO q24h), or chloramphenicol (50 mg/kg PO q12h).

Surgical intervention: Bulla osteotomy for chronic otitis media, abscess removal

Supportive care: NSAIDs, nebulization, nutritional support

Important: Treatment rarely eliminates infection; most rabbits become asymptomatic carriers. Recurrence is common.

Treatment Category Medication/Therapy Notes
Anti-parasitic Fenbendazole 20 mg/kg PO q24h × 28 days (Brand names: Panacur, Lapizole) Reduces spore replication and shedding. Does NOT eliminate infection or reverse existing damage. Most effective when started early.
Anti-inflammatory NSAIDs: Meloxicam 0.2-0.5 mg/kg PO q24h. Corticosteroids (controversial): Prednisolone 0.5-2 mg/kg PO q12-24h Reduces CNS inflammation. Steroids may worsen infection by immunosuppression - use cautiously.
Supportive Care Assisted feeding, fluid therapy, meclizine for vestibular signs, physical therapy, low-entry litter boxes Critical for recovery. Prevent aspiration in rabbits with severe head tilt. Ensure adequate nutrition and hydration.
Prophylaxis Fenbendazole 20 mg/kg PO q24h × 28 days for newly acquired rabbits or contact rabbits Can prevent clinical disease in exposed rabbits. Recommended for new additions to household or after diagnosis in cage-mate.

Other Causes of Encephalitis in Rabbits

Bacterial Causes

Listeria monocytogenes: Rare in rabbits (unlike ruminants). Causes septicemia, abortions in pregnant does. CNS involvement uncommon but can cause meningoencephalitis.

Staphylococcus aureus: Can cause abscesses, bacteremia with CNS seeding

Other bacteria: Escherichia coli, Pseudomonas aeruginosa (typically secondary to immunosuppression or trauma)

Parasitic Causes

Toxoplasma gondii: Rare but reported. Causes nonsuppurative encephalitis

Baylisascaris procyonis: Raccoon roundworm. Causes severe eosinophilic meningoencephalitis. Larval migration causes extensive CNS damage. Poor prognosis. Found in North America where raccoons are endemic.

Viral Causes

Rabies: Rare in pet rabbits but reportable. Progressive neurologic signs, aggression, paralysis

Herpesvirus (Human Herpesvirus 1): Rarely reported. Can cause fatal encephalitis

Non-Infectious Causes

Trauma: Head trauma from falls, kicks, predator attacks

Neoplasia: Lymphosarcoma (most common), pituitary tumors, other CNS neoplasia

Vascular: Cerebral infarcts, thromboembolic disease

Metabolic: Lead toxicity, hepatic encephalopathy, hypoglycemia

Congenital: Hydrocephalus

Feature E. cuniculi Pasteurella Trauma Neoplasia
Onset Acute, often after stress Gradual or acute Peracute, history of trauma Progressive, chronic
Respiratory Signs Absent Often present (snuffles) Absent Absent
Skull Radiographs Normal bullae Fluid in bullae, bony changes Skull fractures may be visible Mass effect, bone lysis
Serology/PCR E. cuniculi IgG/IgM positive Culture positive (nasal/ear) Not applicable Cytology/biopsy
Treatment Response Fenbendazole + supportive care Long-term antibiotics Supportive care, variable Poor prognosis

Differential Diagnosis of Vestibular Disease in Rabbits

Zoonotic and Public Health Considerations

E. cuniculi: Zoonotic potential exists, particularly for immunocompromised individuals (HIV/AIDS, transplant recipients, CD4+ T-cell deficiency). Transmission via contaminated food, water, or direct contact with infected rabbit urine. Proper hygiene and disinfection (1-10% bleach for 30 seconds) essential.

Pasteurella multocida: Can cause serious infections in humans through scratches, bites, or licks. Immunocompromised individuals and those with pulmonary disorders at higher risk.

Summary

Encephalitis in rabbits is a multisystemic disease most commonly caused by Encephalitozoon cuniculi, presenting with acute vestibular signs following stress. Diagnosis is presumptive based on clinical signs and positive serology, with definitive diagnosis requiring histopathology. Treatment consists of fenbendazole for 28 days plus supportive care. The primary differential diagnosis is Pasteurella multocida otitis media/interna, which can be distinguished by the presence of respiratory signs and tympanic bulla changes on imaging. Understanding these key differences and the zoonotic potential of E. cuniculi is essential for NAVLE success.

Practice NAVLE Questions

Test your knowledge with 10,000+ exam-style questions, detailed explanations, and timed exams.

Start Your Free Trial →

Related Articles

NAVLE

Rabbit Housing and Husbandry Study Guide

 NAVLE

Rabbit Fractured Lumbar Spine Study Guide

 NAVLE

Rabbit Coccidiosis Study Guide