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🧠 MRI — Brain & Spine

MRI of CNS Infectious Disease in Dogs & Cats

MRI is the gold standard for evaluating infectious diseases of the brain and spinal cord. It reveals meningeal enhancement, parenchymal granulomas, empyema, and patterns of inflammation that help differentiate infection from immune-mediated disease or neoplasia — a distinction that fundamentally changes treatment.

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MRI for CNS Infections at a Glance

Primary Modality
3T MRI with pre- and post-contrast sequences (T1W, T2W, FLAIR, DWI)
Key Advantage
Detects meningeal enhancement, intra-axial vs. extra-axial masses, and spinal cord involvement invisible on radiographs or CT
Complementary Tests
CSF analysis, serology, PCR, and culture guided by MRI localization
Species
Dogs and cats; FIP is feline-specific, Neospora is primarily canine
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Fungal CNS Infections

CNS Cryptococcosis

Cryptococcus neoformans / C. gattii species complex

Cryptococcosis is one of the most common systemic fungal infections in cats and is acquired through inhalation of fungal spores from bird droppings. The organism has a tropism for the nasal cavity and CNS, where it typically spreads via direct extension through the cribriform plate. Dogs and cats can present with nasal disease alone, CNS involvement alone, or both simultaneously. Immunocompromised animals (FIV+ or FeLV+ cats) are at higher risk for disseminated disease.

Clinical Signs

  • Nasal discharge and nasal deformity
  • Seizures and altered mentation
  • Vestibular dysfunction
  • Vision changes and uveitis
  • Paraparesis (with spinal involvement)
  • Absent patellar reflexes (low lumbar lesions)
  • Weight loss and depression

MRI Findings

  • Nasal cavity disease (may be the only finding)
  • Contrast-enhancing masses (intra-axial or extra-axial)
  • Diffuse or regional meningeal enhancement
  • Spinal cord meningeal ± parenchymal lesions
  • Extension through cribriform plate from nasal cavity
  • Temporal bone and olfactory bulb involvement
Neurological localization can be multifocal. MRI may show a normal brain with nasal disease as the only abnormality — post-contrast sequences are critical for detecting subtle meningeal involvement.

CNS Coccidioidomycosis (Valley Fever)

Coccidioides immitis / C. posadasii

Coccidioidomycosis is endemic to the desert southwest United States and is acquired through inhalation of spores from disturbed soil. While primarily a pulmonary and osseous disease, CNS involvement represents one of the most serious complications. Forebrain signs (seizures, head pressing, altered mentation) indicate intracranial granuloma formation. The organism produces striking MRI abnormalities with significant perilesional edema.

Clinical Signs

  • Seizures and head pressing
  • Altered mentation and pacing
  • Concurrent cough or lameness (systemic spread)
  • Fever and weight loss
  • Uveitis or blindness
  • Spinal pain (with myelitis)

MRI Findings

  • Extra-axial or intra-axial enhancing mass with meningeal disease
  • Meningoencephalitis pattern
  • Significant perilesional edema
  • Low T2 signal lesions (characteristic)
  • Intramedullary spinal cord granuloma
  • Variable appearance between patients
Serologic testing (Coccidioides IgG EIA, e.g., MiraVista) supports diagnosis. MRI response to antifungal therapy (fluconazole) can be dramatic, with pre- and post-treatment comparison showing substantial granuloma resolution over months.

CNS Blastomycosis

Blastomyces dermatitidis

Blastomycosis is found in sandy soil near water, primarily in the Ohio and Mississippi River valleys, Great Lakes region, and the Pacific Northwest. Spores are inhaled or enter through skin wounds. While primarily a pulmonary disease, CNS dissemination produces pyogranulomatous meningoencephalitis. Dogs are more commonly affected than cats. Concurrent uveitis and pulmonary disease are important clues to systemic blastomycosis.

Clinical Signs

  • Neurological signs (seizures, ataxia)
  • Concurrent respiratory disease and cough
  • Uveitis and blindness
  • Cutaneous draining tracts
  • Fever, depression, weight loss
  • Lymphadenopathy

MRI Findings

  • Intracranial granulomatous masses
  • Meningeal enhancement
  • Pyogranulomatous encephalitis pattern
  • Variable mass effect and edema
Diagnosis often confirmed by urine antigen testing, lymph node FNA cytology, or culture. Thoracic radiographs should be obtained concurrently to evaluate for pulmonary nodular disease.

CNS Aspergillosis

Aspergillus spp.

Aspergillosis typically begins as nasal or sinonasal disease and can extend intracranially through the cribriform plate. Disseminated aspergillosis is rare but devastating, particularly in immunocompromised patients or breeds with suspected genetic predisposition (German Shepherds). CNS involvement carries a guarded prognosis.

Clinical Signs

  • Chronic nasal discharge (often unilateral)
  • Facial pain or nasal depigmentation
  • Neurological signs if intracranial extension
  • Fever and systemic illness (disseminated form)

MRI Findings

  • Nasal cavity and frontal sinus destruction
  • Cribriform plate lysis with intracranial extension
  • Extra-axial mass effect at the olfactory bulbs
  • Contrast-enhancing granulomatous tissue
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Bacterial CNS Infections

Bacterial CNS infections can arise from hematogenous spread, foreign body migration, direct extension from adjacent structures (ear, sinus), or post-surgical contamination. They produce some of the most dramatic MRI findings and often present as acute emergencies.

Bacterial Meningoencephalitis

Serratia marcescens, Staphylococcus aureus, E. coli, Streptococcus, and others

Bacterial meningitis in dogs and cats is a life-threatening emergency that requires rapid diagnosis and aggressive antimicrobial therapy. Young animals, immunocompromised patients, and those with recent surgery or bacteremia are at highest risk. CSF culture and sensitivity testing, guided by MRI localization, directs targeted antibiotic therapy. Fever (sometimes extreme, exceeding 107°F) is a hallmark presentation.

Clinical Signs

  • Seizures (often the presenting sign)
  • High fever (often >105°F)
  • Neck pain and rigidity
  • Altered mentation and depression
  • Walking gingerly and generalized pain
  • Acute onset and rapid progression

MRI Findings

  • Diffuse or focal meningeal enhancement
  • Parenchymal edema and signal change
  • Empyema (subdural/epidural abscess collections)
  • Cerebral edema with mass effect
CSF culture should be performed when bacterial meningitis is suspected. Staphylococcus aureus and Serratia marcescens are among the most commonly identified organisms. Response to appropriate antibiotic therapy can be monitored with follow-up MRI.

Multi-System Bacterial Infection

Physitis, osteomyelitis, discospondylitis, empyema, steatitis, neuritis, myositis

Some bacterial infections produce widespread multi-system involvement visible on MRI. Hematogenous spread can cause simultaneous physitis, osteomyelitis, steatitis (fat inflammation), neuritis (nerve inflammation), myositis (muscle inflammation), meningitis, and epidural empyema. Young large-breed dogs are particularly susceptible. MRI of the spine reveals the full extent of involvement across multiple tissue compartments.

Clinical Signs

  • Acute pain, panting, difficulty standing
  • Progressive neurological decline (24–36 hrs)
  • High fever
  • Reluctance to move
  • Young age at presentation

MRI Findings

  • Vertebral physitis and endplate changes
  • Epidural empyema (fluid collections)
  • Paraspinal myositis and steatitis
  • Nerve root enhancement (neuritis)
  • Retroperitoneal inflammation
  • Meningeal enhancement

Discospondylitis (MRI Evaluation)

Brucella canis, Staphylococcus, Streptococcus, E. coli, fungal organisms

While discospondylitis is often first identified on radiographs, MRI provides critical additional information: it visualizes spinal cord compression, epidural empyema, and the extent of paravertebral soft tissue involvement. MRI is essential when neurological signs accompany spinal pain, as it determines whether surgical intervention is needed. Brucella canis is a particularly important cause in intact and rescue dogs and carries zoonotic significance.

Clinical Signs

  • Spinal pain (often severe and intermittent)
  • Reluctance to run, jump, or climb stairs
  • Fever and depression
  • Weight loss and anorexia
  • Neurological deficits if cord compression

MRI Findings

  • Disc space signal change and enhancement
  • Adjacent endplate destruction and edema
  • Epidural empyema with cord compression
  • Paravertebral soft tissue inflammation
  • Vertebral body osteomyelitis
For comprehensive radiographic evaluation of discospondylitis, including Brucella canis cases, see our Radiographic Evaluation guide.
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Protozoal CNS Infections

Neosporosis

Neospora caninum

Neospora caninum is a protozoal parasite that produces CNS disease primarily in dogs. It can cause devastating meningoencephalomyelitis with both brain and spinal cord involvement. Congenital infection through transplacental transmission is well documented. Young dogs may present with ascending paralysis, while adult dogs more commonly show intracranial signs. Long-term prognosis with treatment (clindamycin, sulfonamide/trimethoprim) can be surprisingly good.

Clinical Signs

  • Cervical pain
  • Hind limb weakness and ataxia
  • Generalized cerebellovestibular ataxia
  • Truncal sway at rest
  • Progressive neurological decline
  • Ascending paralysis (young dogs)

MRI Findings

  • Multifocal brain lesions with enhancement
  • Cerebellar and brainstem involvement
  • Spinal cord intramedullary lesions
  • Meningeal enhancement
  • May normalize completely after treatment
Follow-up MRI can show complete normalization of brain imaging years after treatment — a finding that distinguishes infectious meningoencephalitis from neoplasia, where lesions typically persist or progress.

Toxoplasmosis (CNS Form)

Toxoplasma gondii

Toxoplasma gondii is a single-celled parasite that can infect virtually all mammals and birds. Cats serve as the definitive host. CNS toxoplasmosis is most commonly seen in immunocompromised cats and occasionally in dogs. It produces meningoencephalitis that is difficult to distinguish from other infectious or immune-mediated CNS diseases on MRI alone. Serology and CSF PCR help confirm diagnosis.

Clinical Signs

  • Seizures and altered mentation
  • Ataxia and vestibular dysfunction
  • Fever and lethargy
  • Respiratory disease (pneumonia)
  • Uveitis (ocular form)
  • Weight loss

MRI Findings

  • Multifocal intra-axial signal changes
  • Meningeal and parenchymal enhancement
  • Periventricular lesions
  • Similar pattern to other infectious meningoencephalitides
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Viral CNS Infections

Feline Infectious Peritonitis (Neurological FIP)

Feline coronavirus (FCoV) mutation

FIP is caused by a mutation of feline enteric coronavirus and produces devastating pyogranulomatous disease. The neurological form affects the CNS with meningitis, ependymitis, and choroid plexitis. Young cats (typically under 2 years, often from shelters or multi-cat households) are most commonly affected. Recent advances in antiviral treatment (GS-441524 and related compounds) have dramatically improved prognosis for neurological FIP when diagnosed early.

Clinical Signs

  • Mentation changes and lethargy
  • Wobbliness and ataxia
  • Seizures and circling behavior
  • Inappropriate elimination
  • Walking low to the ground
  • Decreased appetite
  • Obtunded mentation

MRI Findings

  • Periventricular enhancement (hallmark)
  • Ependymal and choroid plexus enhancement
  • Obstructive hydrocephalus
  • Meningeal enhancement
  • Brainstem and cerebellar involvement
  • Signal changes in periventricular white matter
Periventricular and ependymal enhancement is relatively specific for FIP in young cats and helps differentiate it from lymphoma or other infectious meningoencephalitis. Early MRI diagnosis is critical for initiating antiviral therapy before irreversible CNS damage occurs.

Canine Distemper (CNS Form)

Canine distemper virus (CDV)

Canine distemper virus can produce demyelinating encephalomyelitis, particularly in unvaccinated or under-vaccinated dogs. The CNS form may appear weeks to months after the initial systemic illness (respiratory and GI signs), or occasionally without prior systemic signs. Myoclonus (rhythmic muscle twitching) is a characteristic finding. Prognosis for the neurological form is guarded to poor.

Clinical Signs

  • Seizures and myoclonus
  • Ataxia and circling
  • Prior or concurrent respiratory signs
  • Hyperkeratosis of nose and footpads
  • Enamel hypoplasia in young dogs

MRI Findings

  • White matter T2 hyperintensities (demyelination)
  • Cerebellar and brainstem signal changes
  • Variable enhancement pattern
  • May affect cerebral cortex and temporal lobes
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Parasitic CNS Infections

Feline Ischemic Encephalopathy (Cuterebra)

Cuterebra larval migration

Feline ischemic encephalopathy is caused by aberrant migration of Cuterebra (bot fly) larvae through the brain of outdoor cats. The bot fly lays eggs near rabbit or rodent den openings; cats investigate these dens and inhale larvae, which migrate from the nasal cavity through the cribriform plate to the olfactory bulb and middle cerebral artery territory. This occurs primarily in summer months in the northeastern United States. The resulting vascular insult produces ischemic infarction in the MCA territory.

Clinical Signs

  • Acute-onset seizures
  • Circling behavior
  • Altered mentation and obtundation
  • Outdoor access history
  • Summer and early fall seasonality
  • Young to middle-aged cats

MRI Findings

  • Middle cerebral artery territory infarction
  • Unilateral cortical signal changes
  • Olfactory bulb involvement
  • DWI restriction in acute phase
  • Cerebral atrophy in chronic cases
Geographic distribution is limited to areas where Cuterebra species are endemic (primarily northeastern U.S.). Not reported in New Zealand or Australia. Outdoor cats with access to rodent dens are at highest risk during summer months.
Clinical Decision-Making

When to Choose MRI for Suspected CNS Infection

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Seizures in a young to middle-aged patient — especially with concurrent fever, suggests infectious meningoencephalitis rather than idiopathic epilepsy. MRI + CSF analysis is the diagnostic standard.
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Multifocal neurological signs — when lesions localize to multiple CNS regions (brain + spine, forebrain + brainstem), infection and immune-mediated disease rise to the top of the differential list.
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Acute severe spinal pain with fever — suggests bacterial meningomyelitis, discospondylitis with epidural extension, or multi-compartment infection. MRI reveals the full extent of involvement.
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Young cat with neurological signs — FIP should be high on the differential. MRI periventricular enhancement can provide rapid diagnostic support for initiating antiviral therapy.
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Known systemic fungal infection with new neurological signs — patients with diagnosed coccidiomycosis, cryptococcosis, or blastomycosis who develop seizures or ataxia need MRI to evaluate for CNS dissemination.

Need an MRI for Suspected CNS Infection?

Our 3T MRI and board-certified radiologists provide rapid, definitive evaluation of infectious meningoencephalitis, granulomas, and spinal cord disease.

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Round Rock, Texas
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Sandy, Utah
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Frequently Asked Questions

MRI & CNS Infections FAQ

MRI provides important clues — certain patterns (e.g., periventricular enhancement in FIP, nasal disease with intracranial extension in cryptococcosis) are more suggestive of specific infections. However, many infectious and immune-mediated diseases (GME, NME, MUE) produce overlapping MRI patterns. CSF analysis, serology, PCR testing, and clinical context are essential complements to MRI for definitive differentiation. The distinction matters enormously because treatment is opposite: immunosuppression for immune-mediated disease, antimicrobials for infection.
Yes, general anesthesia is required for veterinary brain and spinal MRI to ensure the patient remains completely still during image acquisition. At Sage Veterinary Imaging, board-certified or residency-trained anesthesiologists oversee each procedure. The anesthesia protocol is tailored to the patient's condition — particularly important for patients with active seizures or elevated intracranial pressure. Most patients recover quickly and go home the same day.
A typical brain MRI takes approximately 30–45 minutes of scan time. If the spine is also being evaluated (common for multifocal infectious disease), the total scan time may extend to 60–90 minutes. Our 3T MRI provides faster acquisition and higher resolution than 1.5T systems, which can reduce total anesthesia time while improving diagnostic quality.
If your pet lives in or has traveled to the desert southwest (Arizona, parts of Texas, California, New Mexico, Utah) and presents with neurological signs, both serology and MRI are important. Serology (Coccidioides IgG EIA) provides evidence of exposure but doesn't confirm CNS involvement. MRI is needed to determine whether the brain or spinal cord is affected and to guide treatment decisions. Many veterinary neurologists will run serology and schedule MRI concurrently to avoid diagnostic delay.