Mosquito Bites & Encephalitis: Risks, Symptoms & Prevention

Mosquitoes transmit several neurotropic arboviruses causing encephalitis (brain inflammation). While most bites cause only local reactions, encephalitis is a rare but devastating complication.

Major Mosquito-Borne Encephalitis Viruses (US Focus)

Encephalitis Presentation

• Fever, headache, altered mental status (confusion, lethargy, coma)
• Seizures, focal neurologic deficits, movement disorders
• Meningeal signs (nuchal rigidity, photophobia)
• EEEV: rapid progression, high CSF WBC, characteristic basal ganglia/thalamic MRI lesions

Diagnosis

• CSF: lymphocytic pleocytosis, normal glucose, elevated protein
• IgM capture ELISA (serum/CSF) — specific for each virus
• PCR (CSF) — early, sensitive for some (WNV, EEEV)
• MRI: temporal lobes (HSV), basal ganglia/thalami (EEEV, JEV), diffuse (WNV)

Prevention (No Specific Antivirals)

• Personal protection: EPA-registered repellents (DEET 20–30%, Picaridin, IR3535, OLE/PMD); permethrin-treated clothing
• Environmental: Eliminate standing water (breeding sites); larvicides; screens; AC
• Behavioral: Avoid dawn/dusk (Culex); wear long sleeves/pants
• Vaccines: JEV (travelers/endemic); EEEV/WNV veterinary vaccines only

Risk Perspective

Neuroinvasive disease incidence: ~0.5–1 per 100,000 in endemic areas. Not zero, but low. Vigilance, not panic. Early supportive care (ICU, seizure control, ICP management) improves outcomes.

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Virus	Vector	Geography	Reservoir	Clinical Notes
West Nile (WNV)	Culex spp.	Nationwide (peak late summer)	Birds	80% asymptomatic; 20% febrile; <1% neuroinvasive (encephalitis, meningitis, AFP); older adults highest risk
St. Louis Encephalitis (SLEV)	Culex spp.	South, Central, West US	Birds	Similar to WNV; older adults severe
La Crosse (LACV)	Aedes triseriatus	Upper Midwest, Appalachia	Small mammals	Children most affected; seizures common
Eastern Equine (EEEV)	Culiseta melanura (bridge vectors)	Atlantic/Gulf Coast, Great Lakes	Birds	Rare but deadliest (30% mortality); severe neuroinvasive; survivors with severe sequelae
Western Equine (WEEV)	Culex tarsalis	Western US/Canada	Birds	Less severe than EEE; children & elderly
Japanese Encephalitis (JEV)	Culex tritaeniorhynchus	Asia/Western Pacific	Pigs, wading birds	Vaccine available; major cause viral encephalitis globally