Viral encephalitis: the shared pattern
- Lesion
Viruses that infect the brain leave a common footprint of lymphocytes around vessels and microglial clusters, plus clues to each virus.
What it is
Viral encephalitis is infection of the brain parenchyma, usually with meningitis. Most viruses share the same histology: lymphocytes cuffing blood vessels, microglial nodules and neuronophagia. Particular viruses add tropism for certain cells or regions and their own inclusions. Mosquito-borne arboviruses cause outbreaks; measles can persist for years and cause subacute sclerosing panencephalitis, a fatal late complication.
Structure
- Arboviral encephalitis: widespread perivascular lymphocytes with neutrophils early, microglial nodules and focal necrosis, mostly in grey matter.
- SSPE: grey and white matter gliosis, neuronal loss, nuclear and cytoplasmic measles inclusions, neurofibrillary tangles in long cases.
- Some infections trigger an immune reaction rather than direct infection, as in post-infectious encephalomyelitis and autoimmune encephalitis.
Step by step
- A virus enters by mosquito bite, the gut, the respiratory tract or a nerve ending.
- It reaches the brain by the blood or by travelling along nerves.
- It infects neurons or glia according to its receptors and tropism.
- T cells and microglia gather around infected cells and vessels.
- Infected neurons are destroyed (neuronophagia); edema and necrosis follow in severe cases.
Viral encephalitis clues
| Virus | Favoured target | Distinctive finding |
|---|---|---|
| Herpes simplex virus type 1 | Temporal and orbitofrontal lobes | Hemorrhagic necrosis, Cowdry A inclusions |
| Poliovirus | Anterior horn motor neurons | Flaccid asymmetric paralysis |
| Rabies | Brainstem, limbic system, Purkinje cells | Negri bodies |
| Cytomegalovirus | Ependyma and subependymal region | Owl-eye inclusions, periventricular necrosis |
| JC virus | Oligodendrocytes | Demyelination (PML) |
| Measles (persistent) | Neurons and oligodendrocytes | SSPE, years after infection |
| West Nile and other arboviruses | Grey matter, including brainstem and anterior horns | Outbreaks, flaccid weakness possible |
Clinical notes
- Fever, headache, confusion, seizures and focal signs; MRI, CSF PCR and serology identify the virus.
- Autoimmune encephalitis (for example anti-NMDA receptor) can follow HSV encephalitis and mimics viral disease.
Test yourself
Think of the answer, then tap the question to see it.
What are the three shared histologic features of viral encephalitis?
Perivascular lymphocytic cuffs, microglial nodules and neuronophagia.
What is subacute sclerosing panencephalitis?
A progressive fatal encephalitis years after childhood measles, caused by persistent defective measles virus in the brain.
Which virus targets oligodendrocytes and causes demyelination?
JC polyomavirus, the cause of progressive multifocal leukoencephalopathy.
Connections
Educational content on the mechanisms of disease; not a diagnostic or treatment guide and no substitute for a health professional's advice. No doses are given.
Sources: Encephalitis | MedlinePlus (MedlinePlus, US National Library of Medicine, 2026).
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Representative model, for educational purposes only.