Encephalitis

Encephalitis is an inflammatory condition of the brain parenchyma, often accompanied by meningitis (meningoencephalitis) and/or myelitis (meningoencephalomyelitis). It results from infectious agents (viral, bacterial, fungal, parasitic, rickettsial) or non-infectious immune-mediated processes. The inflammation disrupts the blood-brain barrier, leading to edema, neuronal damage, and neurological deficits. Clinical presentation varies from subtle behavioral changes to severe seizures, coma, and death. The disease can be classified by etiology (infectious vs. non-infectious), time course (acute, subacute, chronic), and neuroanatomical localization (cerebral, brainstem, cerebellar).

Infectious causes include: Viral: canine distemper virus (CDV), rabies virus, parvovirus (in puppies), feline infectious peritonitis virus (FIPV), feline immunodeficiency virus (FIV), feline leukemia virus (FeLV), West Nile virus, pseudorabies virus, tick-borne encephalitis virus. Bacterial: Staphylococcus, Streptococcus, Escherichia coli, Pasteurella, Bartonella, Listeria monocytogenes, Mycobacterium, and anaerobic bacteria (often secondary to otitis interna, sinusitis, or hematogenous spread). Fungal: Cryptococcus neoformans, Blastomyces dermatitidis, Histoplasma capsulatum, Coccidioides immitis, Aspergillus species. Parasitic: Toxoplasma gondii, Neospora caninum, Dirofilaria immitis (heartworm), Cuterebra larvae (migratory myiasis), Angiostrongylus cantonensis. Rickettsial: Ehrlichia canis, Anaplasma phagocytophilum, Rickettsia rickettsii. Protozoal: Babesia canis (rarely). Non-infectious causes: immune-mediated (e.g., granulomatous meningoencephalomyelitis (GME), necrotizing meningoencephalitis (NME) in Pugs, Maltese, Chihuahuas; necrotizing leukoencephalitis (NLE) in Yorkshire Terriers), steroid-responsive meningitis-arteritis (SRMA), and paraneoplastic syndromes. Toxic causes: lead poisoning, metronidazole toxicity, and certain plant toxins. Genetic/idiopathic: breed-specific necrotizing encephalitides.

Encephalitis occurs in both dogs and cats. Viral causes: CDV is more common in unvaccinated puppies and dogs, with a worldwide distribution. Rabies is endemic in many regions and is zoonotic. FIPV typically affects young cats (<2 years) from multi-cat environments. Bacterial encephalitis is often secondary to otitis media/interna or hematogenous spread, with no breed predilection. Fungal encephalitis is more common in endemic areas (e.g., Cryptococcus in Pacific Northwest, Blastomyces in Mississippi River Valley). Parasitic: Toxoplasma gondii is more common in cats and immunocompromised dogs; Neospora caninum primarily affects dogs, especially puppies. Immune-mediated encephalitides: GME is more common in small-breed dogs, particularly Poodles, Terriers, and Spaniels, with a mean age of 5 years. NME is seen in Pugs, Maltese, and Chihuahuas, often young adults. NLE is seen in Yorkshire Terriers. No sex predilection is consistent. Seasonal patterns may occur for tick-borne diseases (e.g., Ehrlichia, Rickettsia) and mosquito-borne viruses (West Nile).

The inflammatory response in encephalitis involves activation of microglia, astrocytes, and infiltration of peripheral immune cells (neutrophils, lymphocytes, macrophages) across the blood-brain barrier. Infectious agents may directly invade neural tissue (e.g., CDV, rabies) or trigger immune-mediated damage. In viral encephalitis, viral replication in neurons and glial cells leads to cytolysis, perivascular cuffing, and demyelination (especially in CDV). Bacterial encephalitis often results in suppurative inflammation, abscess formation, and vasogenic edema. Fungal and parasitic infections cause granulomatous inflammation. In immune-mediated encephalitis (e.g., GME), there is a T-cell-mediated hypersensitivity reaction, leading to perivascular accumulation of lymphocytes and macrophages, often forming multifocal granulomas. The inflammatory cascade releases cytokines (TNF-α, IL-1, IL-6), causing endothelial activation, increased vascular permeability, and cerebral edema. Elevated intracranial pressure (ICP) can lead to brain herniation. Neuronal damage may be irreversible, resulting in permanent neurological deficits.

Intrinsic factors: Age (young animals more susceptible to infectious causes; middle-aged for immune-mediated), breed (genetic predisposition for NME, NLE, GME), immune status (immunosuppression increases risk of opportunistic infections), and concurrent systemic disease (e.g., neoplasia, endocrine disorders). Extrinsic factors: Vaccination status (lack of vaccination for CDV, rabies), exposure to vectors (ticks, mosquitoes), environmental exposure to fungal spores (soil, bird droppings), travel to endemic areas, and poor sanitation. Management factors: overcrowding, stress, poor nutrition, and concurrent infections (e.g., otitis media) can predispose to bacterial spread. Use of immunosuppressive drugs may reactivate latent infections (e.g., Toxoplasma).

Clinical signs vary depending on the affected neuroanatomical region. Peracute/acute: seizures (focal or generalized), altered mentation (depression, stupor, coma), fever, head pressing, circling, ataxia, and behavioral changes (aggression, disorientation). Subacute/chronic: progressive paresis, cranial nerve deficits (e.g., facial nerve paralysis, vestibular signs), visual deficits, proprioceptive deficits, and cervical pain (if meningitis is present). Systemic signs may include anorexia, lethargy, and weight loss. In CDV, there may be concurrent respiratory and gastrointestinal signs, as well as hyperkeratosis of footpads. In rabies, there is a rapid progression from behavioral changes to paralysis and death. In FIP, ocular and renal signs may be present. In GME, signs are often multifocal and progressive, with a predilection for the brainstem and cerebellum.

1. Meningitis (bacterial, viral, immune-mediated) - presents with fever, neck pain, and leukocytosis; CSF analysis shows neutrophilic pleocytosis in bacterial, lymphocytic in viral/immune-mediated. 2. Brain neoplasia (primary or metastatic) - often progressive, focal signs; MRI shows mass effect, contrast enhancement; CSF may show atypical cells. 3. Cerebrovascular accident (stroke) - acute onset, non-progressive; MRI shows ischemic or hemorrhagic lesion. 4. Hepatic encephalopathy - history of liver disease, elevated bile acids, ammonia; clinical signs often wax and wane. 5. Toxicity (e.g., lead, metronidazole) - history of exposure, specific signs (e.g., blindness in lead toxicity), blood lead levels. 6. Idiopathic epilepsy - normal interictal neurological exam, no inflammatory CSF changes. 7. Hydrocephalus - congenital, often in toy breeds, imaging shows ventricular dilation. 8. Granulomatous meningoencephalomyelitis (GME) - CSF lymphocytic pleocytosis, MRI multifocal lesions, breed predisposition. 9. Necrotizing meningoencephalitis (NME) - breed-specific (Pug, Maltese), MRI shows multifocal necrotic lesions. 10. Steroid-responsive meningitis-arteritis (SRMA) - young large-breed dogs, fever, neck pain, CSF neutrophilic pleocytosis, responds to steroids.

1. Initial triage: Complete history, physical and neurological examination. Assess for fever, neck pain, and mentation changes. 2. Minimum database: CBC, serum biochemistry, urinalysis, and blood pressure measurement. 3. Infectious disease screening: Serology/PCR for CDV, Toxoplasma, Neospora, Ehrlichia, Anaplasma, Rickettsia, Cryptococcus antigen, FIV/FeLV (in cats), and rabies (if suspected, follow public health protocols). 4. Advanced imaging: MRI of the brain is the gold standard; CT may be used if MRI unavailable. Look for contrast-enhancing lesions, edema, and mass effect. 5. Cerebrospinal fluid (CSF) analysis: Collect from the cisterna magna or lumbar site. Perform cell count, protein, cytology, and possibly PCR for infectious agents. 6. Electroencephalography (EEG) may be used to assess seizure activity. 7. If a mass lesion is found, consider brain biopsy for histopathology. 8. In cases of suspected bacterial encephalitis, culture and sensitivity of CSF (though often negative). 9. Rule out extracranial causes (e.g., hepatic encephalopathy) with bile acids, ammonia, and abdominal ultrasound.

Hematology: May show leukocytosis with left shift in bacterial infections; lymphopenia in viral infections; eosinophilia in parasitic or fungal infections. Anemia may be present in chronic disease. Serum biochemistry: Elevated liver enzymes (ALT, ALP) may be seen in hepatic encephalopathy or CDV. Hyperglobulinemia may occur in FIP or GME. Electrolyte imbalances (hyponatremia) can occur with brain injury. Urinalysis: Usually unremarkable, but proteinuria may be present in systemic disease. Blood gas analysis: May show respiratory alkalosis due to hyperventilation. Specific biomarkers: C-reactive protein (CRP) may be elevated in inflammatory conditions. Serology/PCR: Positive titers or PCR for specific infectious agents (e.g., CDV, Toxoplasma, Neospora, Ehrlichia, Anaplasma, Rickettsia, Cryptococcus). CSF analysis: Typically shows increased protein (>25 mg/dL) and nucleated cell count (>5 cells/µL). In bacterial encephalitis, neutrophilic pleocytosis (>50% neutrophils) with degenerate neutrophils. In viral/immune-mediated, lymphocytic pleocytosis. In GME, mixed or lymphocytic pleocytosis with elevated protein. In fungal, eosinophilic or mixed pleocytosis. CSF culture may be positive in bacterial cases.

Radiography: Thoracic radiographs may reveal interstitial or alveolar patterns in CDV or fungal infections. Abdominal radiographs may show hepatomegaly in hepatic encephalopathy. Ultrasonography: Abdominal ultrasound may be useful for detecting portosystemic shunts or other extracranial causes. Computed Tomography (CT): May show contrast-enhancing lesions, edema, or mass effect, but is less sensitive than MRI for parenchymal lesions. Magnetic Resonance Imaging (MRI): The preferred imaging modality. Findings include: T2-weighted hyperintensity, T1-weighted hypointensity, contrast enhancement (meningeal, periventricular, or parenchymal), and mass effect. In GME, multifocal contrast-enhancing lesions. In NME, asymmetric lesions with necrosis. In CDV, diffuse white matter hyperintensity. In bacterial abscess, ring-enhancing lesion. In FIP, hydrocephalus and periventricular enhancement. Endoscopy: Not typically used for encephalitis. Fluoroscopy: Not used. Echocardiography: May be indicated if infective endocarditis is suspected as a source of septic emboli.

Fine Needle Aspirate (FNA): Not typically performed for brain lesions due to risk; if a mass is accessible, cytology may show inflammatory cells or infectious organisms. CSF analysis: Cytology may show neutrophils, lymphocytes, macrophages, eosinophils, or neoplastic cells. Histopathology: Brain biopsy (antemortem) or necropsy findings: In GME, perivascular accumulation of lymphocytes and macrophages forming granulomas. In NME, multifocal necrosis with lymphocytic infiltration. In bacterial encephalitis, suppurative inflammation with abscess formation. In viral encephalitis, perivascular cuffing, neuronal necrosis, and intranuclear or intracytoplasmic inclusion bodies (e.g., CDV). In fungal, granulomatous inflammation with fungal organisms (e.g., Cryptococcus with mucicarmine stain). Special stains: Gram stain for bacteria, GMS stain for fungi, Ziehl-Neelsen for acid-fast organisms.

Emergency stabilization: Manage seizures with diazepam (0.5-1 mg/kg IV) or levetiracetam (20-60 mg/kg IV). Control cerebral edema with mannitol (0.5-1 g/kg IV over 20 minutes) or hypertonic saline (3-5 mL/kg IV). Fluid resuscitation: Use isotonic crystalloids (e.g., Lactated Ringer's) at maintenance rates (60-100 mL/kg/day in dogs, 40-60 mL/kg/day in cats) with careful monitoring to avoid overhydration. Primary medical therapy: For infectious encephalitis, specific antimicrobials: Doxycycline (5-10 mg/kg PO q12h) for rickettsial; Clindamycin (10-20 mg/kg PO q12h) for Toxoplasma/Neospora; Fluconazole (5-10 mg/kg PO q12h) or Amphotericin B (0.5-1 mg/kg IV q48h) for fungal; for bacterial, broad-spectrum antibiotics such as Ampicillin (20-30 mg/kg IV q8h) and Enrofloxacin (5-10 mg/kg IV/PO q24h) or Cefotaxime (25-50 mg/kg IV q8h). For viral, supportive care and vaccination prevention. For immune-mediated, immunosuppressive doses of Prednisone (1-2 mg/kg PO q12h) with or without Cytarabine (50 mg/m² SC q12h for 2 days) or Cyclosporine (5-10 mg/kg PO q12h). Supportive care: Nutritional support (feeding tube if anorexic), nursing care (turning, bladder care), and physical rehabilitation. Surgical intervention: Indicated for brain abscess drainage or mass removal if accessible.

Prognosis varies widely by etiology. Viral encephalitis (CDV) has a guarded to poor prognosis, with high mortality and potential for permanent neurological deficits. Rabies is almost always fatal. Bacterial encephalitis has a guarded prognosis, with early treatment improving survival. Fungal encephalitis has a poor to guarded prognosis, with long-term antifungal therapy required. Parasitic encephalitis (Toxoplasma, Neospora) has a fair to good prognosis if treated early. Immune-mediated encephalitis (GME, NME) has a guarded prognosis; GME may respond to immunosuppression, but NME often progresses rapidly. Negative prognostic indicators include: severe neurological deficits, coma, refractory seizures, and lack of response to therapy within 48-72 hours. Mortality rates can be as high as 50-70% for severe cases.

Re-check examinations: Initially every 1-2 weeks, then monthly. Serial neurological assessments to monitor improvement or deterioration. Repeat CSF analysis at 4-6 weeks to assess response to therapy; normalization of CSF parameters is a good prognostic sign. Repeat MRI at 2-3 months to evaluate lesion resolution. Monitor serum drug levels for certain medications (e.g., cyclosporine). Adjust immunosuppressive doses gradually based on clinical response and side effects. Long-term management: For immune-mediated encephalitis, lifelong immunosuppression may be required. For infectious causes, complete the full course of antimicrobials and repeat serology/PCR to confirm clearance. Provide client education on recognizing signs of relapse and the importance of medication compliance.

Pearls: Always consider infectious causes in young, unvaccinated animals. MRI is essential for diagnosis and to rule out other causes. CSF analysis is crucial; perform before starting antibiotics or steroids if possible. In suspected GME, a trial of corticosteroids may be both diagnostic and therapeutic. Monitor for increased ICP; use mannitol if signs of herniation. Pitfalls: Do not delay treatment while waiting for test results. Avoid using corticosteroids in infectious encephalitis without appropriate antimicrobial coverage. Do not perform a cisternal tap if increased ICP is suspected due to risk of herniation. Do not rely solely on serology; PCR and CSF analysis are more specific. Avoid using metronidazole in animals with suspected encephalitis as it can cause neurological signs.

Anticonvulsants: Diazepam (0.5-1 mg/kg IV, repeat as needed) for acute seizures; Levetiracetam (20-60 mg/kg IV/PO q8h) for maintenance; Phenobarbital (2.5-5 mg/kg PO q12h) for long-term control. Anti-inflammatory/immunosuppressive: Prednisone (1-2 mg/kg PO q12h, taper over 3-6 months); Cytarabine (50 mg/m² SC q12h for 2 consecutive days, repeat every 3 weeks); Cyclosporine (5-10 mg/kg PO q12h, monitor trough levels). Antimicrobials: Doxycycline (5-10 mg/kg PO q12h for 14-28 days) for rickettsial; Clindamycin (10-20 mg/kg PO q12h for 4-6 weeks) for Toxoplasma/Neospora; Fluconazole (5-10 mg/kg PO q12h for 6-12 months) for fungal; Amphotericin B (0.5-1 mg/kg IV q48h, cumulative dose 4-8 mg/kg) for severe fungal; Ampicillin (20-30 mg/kg IV q8h) and Enrofloxacin (5-10 mg/kg IV/PO q24h) for bacterial. Supportive: Mannitol (0.5-1 g/kg IV over 20 min, repeat if needed) for cerebral edema; Omeprazole (1 mg/kg PO q12h) for GI prophylaxis if on steroids. Adjust dosages for renal/hepatic impairment: Reduce doxycycline in hepatic disease; reduce enrofloxacin in renal disease. Contraindications: Avoid corticosteroids in systemic fungal infections without antifungal therapy; avoid NSAIDs in animals with GI ulcers.

ACVIM consensus statements on the diagnosis and treatment of canine and feline encephalitis emphasize the importance of MRI and CSF analysis. Studies have shown that GME responds to immunosuppressive therapy with prednisone and cytarabine, with a median survival time of 12-24 months. For NME, prognosis is poor, with a median survival of <6 months despite treatment. In infectious encephalitis, early diagnosis and targeted antimicrobial therapy improve outcomes. A study by Lowrie et al. (2013) reported that dogs with GME treated with cytarabine and prednisone had a 1-year survival rate of 70%. Another study by Coates et al. (2007) found that dogs with NME had a median survival of 5 months. For CDV, a study by Amude et al. (2007) reported a mortality rate of 50% in dogs with neurological signs. The use of PCR on CSF has been shown to be highly sensitive for detecting Toxoplasma and Neospora. Current guidelines recommend a stepwise approach: MRI, CSF analysis, and infectious disease testing before initiating immunosuppressive therapy.