Eastern Equine Encephalitis

Eastern equine encephalitis is a rare but devastating mosquito-borne alphavirus infection with the highest case fatality rate (~41%) of any arboviral encephalitis in the US and >50% of survivors suffering long-term neurologic sequelae. [1-2] There is no specific antiviral treatment and no licensed human vaccine; management is entirely supportive. [1]

• Exposure history: Outdoor activities (camping, gardening, agricultural work) in endemic forested wetland areas (Atlantic Coast, Gulf Coast, Great Lakes, New England) during June–October, peaking in late August [1]
• Incubation period: 4–10 days after mosquito bite [1]
• Prodrome: Fevers, chills, malaise, arthralgias, myalgias lasting days before neurologic onset (median 5 days of prodrome before CNS symptoms in older children/adults; abrupt onset in infants) [1]
• Neurologic symptoms: Headache, confusion, altered mental status, focal deficits (hemiparesis, dysarthria, facial droop), seizures, rapid progression to stupor or coma [1][3]
• Ask about: recent travel to endemic areas, mosquito exposure, outdoor occupation/hobbies, immunosuppressive medications or transplant history

• Rapid neurologic deterioration — progression from prodrome to coma can occur within hours to days [1][4]
• Seizures or status epilepticus (clinical or nonconvulsive) [3]
• Inability to protect airway [1]
• Signs of elevated intracranial pressure (posturing, pupillary changes, Cushing triad) [1]
• Severe hyponatremia (Na ≤130 mmol/L) — associated with poor outcome [5-6]
• High CSF WBC count (>500/mm³) — predictor of poor prognosis [5]

• No approved antiviral therapy exists [1]
• Empiric coverage: Start IV acyclovir (for HSV encephalitis) and broad-spectrum antibiotics (for bacterial meningitis) until EEE is confirmed [3-4]
• Anticonvulsants: Levetiracetam commonly used; other standard AEDs as needed for seizure management [3]
• IVIG: Limited data suggest earlier administration may reduce long-term disability (average 5 days post-admission in one series, N=17); results remain inconclusive [1][7]
• High-dose IV methylprednisolone: Used in some cases for immunomodulation, but a meta-analysis of steroids for viral encephalitides showed no survival benefit [1][7]
• Avoid: Medications that lower seizure threshold; use caution with hypertonic solutions given risk of hyponatremia

• NPO if altered mental status or airway compromise anticipated
• Aggressive IV fluid resuscitation for dehydration from febrile illness, nausea, and vomiting [1]
• Monitor sodium closely — hyponatremia is common and prognostically significant; avoid hypotonic fluids [5-6]
• Enteral nutrition via NG/OG tube for prolonged ICU stays

• Neurologic: Headache, confusion, personality changes, focal weakness, speech difficulty, vision changes, seizures
• Constitutional: Fever, chills, rigors, malaise, fatigue
• Musculoskeletal: Arthralgias, myalgias
• GI: Nausea, vomiting (common with meningeal irritation)
• Respiratory: Assess for aspiration risk, respiratory failure requiring intubation (50% in one series) [3]
• Ophthalmologic: Photophobia

• Collateral from family/witnesses regarding timeline of symptom onset, baseline mental status, and rapidity of decline
• Confirm geographic exposure — residence near or travel to deciduous forested wetlands, swamps, or marshes in endemic regions [1]
• Occupational exposure (agricultural workers, forestry, outdoor recreation)
• Immunosuppression history: Organ transplant recipients, patients on B-cell inhibiting monoclonal antibodies (e.g., rituximab) at higher risk for neuroinvasive disease [1]
• Family history is not a significant contributor; EEE is not hereditary
• One documented case of solid organ transplant transmission involving 3 recipients [1]

• Age bimodal distribution: <5 years and >60 years at highest risk for neuroinvasive disease; case fatality highest in age ≥70 [1][8]
• Male sex: Predominance in reported cases (59–100% male in case series) [7][9]
• Geographic: Atlantic Coast, Gulf Coast, Great Lakes, and increasingly New England states (17/19 cases in 2024 from New England) [1]
• Seasonal: June–October, peak late August [1]
• Immunosuppression: Organ transplant, B-cell depleting therapies [1]
• Outdoor exposure: Living near or visiting forested wetlands/swamps; camping, gardening, agricultural work [1]
• Increased precipitation in preceding year may amplify mosquito vector populations [10]

• Herpes simplex encephalitis (HSV) — temporal lobe predominance on MRI (vs. basal ganglia/thalami in EEE); treat empirically with acyclovir until ruled out [6]
• West Nile virus encephalitis — similar arboviral presentation but lower mortality (~10%); may also involve basal ganglia but often shows anterior horn cell involvement [1][5]
• La Crosse encephalitis — primarily affects school-aged children; low mortality (~1%) [1][5]
• St. Louis encephalitis — flavivirus; similar geographic overlap [5]
• Bacterial meningitis — CSF with neutrophilic predominance early in EEE can mimic bacterial infection; low glucose favors bacterial etiology [1]
• Acute ischemic stroke — focal deficits (hemiparesis, dysarthria) may initially mimic stroke, as occurred in reported cases [4]
• Autoimmune encephalitis (anti-NMDA receptor, etc.) — subacute onset, psychiatric features
• Acute disseminated encephalomyelitis (ADEM) — post-infectious demyelination; may overlap clinically [4]
• Tuberculous meningitis — subacute, basilar meningeal enhancement, low glucose

Key distinguishing feature of EEE: Early, prominent basal ganglia and thalamic involvement on MRI differentiates EEE from HSV encephalitis, which typically spares these structures early. [6]

• Prior arboviral infections or vaccinations (may cause cross-reactive serologies) [1]
• Immunosuppressive conditions or medications (transplant, rituximab, chemotherapy) [1]
• Seizure history (baseline vs. new-onset)
• Chronic neurologic conditions affecting baseline function
• Prior residence in or travel to endemic areas

• Vitals: Fever (100% in one series), tachycardia; monitor for Cushing response (hypertension + bradycardia suggesting elevated ICP) [7]
• Neurologic:
  • Mental status: GCS assessment; confusion → obtundation → coma
  • Meningismus: Nuchal rigidity, Kernig/Brudzinski signs
  • Focal deficits: Hemiparesis, facial droop, dysarthria, cranial nerve palsies [3-4]
  • Seizure activity: Observe for subtle/nonconvulsive seizures
• Fundoscopic: Papilledema (elevated ICP)
• Skin: No characteristic rash (unlike some other arboviruses); look for mosquito bites

• CSF analysis (critical):
  • Pleocytosis — initially neutrophilic (mean ~60% neutrophils), later lymphocytic [1][7]
  • Elevated protein (median ~100 mg/dL) [7]
  • Normal glucose (distinguishes from bacterial meningitis) [1][3]
  • May have elevated RBCs [3]
• Serum: CBC, CMP (watch for hyponatremia ≤130 mmol/L as poor prognostic marker), blood cultures, coagulation studies [5-6]
• EEEV-specific IgM (serum and CSF) by ELISA or microsphere immunoassay — appears within first week of illness [1]
  • Caution: Commercially available immunofluorescence assay has reduced sensitivity early in infection; false negatives reported [1]
  • Repeat IgM testing ≥8 days after symptom onset if initial test negative and suspicion remains high [1]
• Confirmatory: Plaque reduction neutralization test (PRNT) through public health laboratory [1]
• RT-PCR: Limited utility (viremia typically resolves by onset of neurologic symptoms); consider in immunocompromised patients who may not mount antibody response [1]
• Diagnosis is often delayed — median 24.5 days in one outbreak series due to initially negative serologies [9]

• MRI brain with contrast (imaging modality of choice):
  • T2/FLAIR hyperintensities in basal ganglia, thalami, and mesial temporal lobes — the hallmark pattern [1][6]
  • May also involve brainstem (43% on MRI), cortical regions [6]
  • Some cases show hyperintense ring encapsulation around basal ganglia/thalamus [3]
  • Meningeal enhancement and periventricular white matter changes less common [6]
• CT head: Less sensitive; abnormal in ~66% of patients (vs. 100% on MRI in comatose patients) [6]
  • May show focal basal ganglia lesions (56%) and diffuse cerebral swelling [6]
  • Useful for rapid initial assessment to rule out hemorrhage or mass lesion
• Prognostic imaging features: Numerous and symmetrical abnormalities predict worse neurologic outcome [1]

The following figure from a landmark NEJM study demonstrates the characteristic basal ganglia and thalamic lesions on MRI and CT in a fatal case of EEE:

• EEG: δ and θ frequency slowing consistent with encephalopathy; epileptiform discharges; may reveal nonconvulsive status epilepticus (seen in all 4 patients in one series) [1][3]
  • [1]
• Continuous EEG monitoring recommended for encephalitic patients given high seizure burden
• Modified Rankin Scale (mRS): Used to track functional disability over time [7]
• Public health notification: Report suspected cases to state/local health department immediately to trigger mosquito surveillance and community prevention measures [1][8]

• No specific ECG findings associated with EEE
• Obtain baseline ECG to assess for arrhythmias related to electrolyte abnormalities (hyponatremia) or medication effects (QTc prolongation from antiemetics)
• Monitor telemetry in ICU setting given critical illness

EEE is the most lethal arboviral encephalitis in the US. Among 193 cases reported to the CDC from 2005–2024, 99% were neuroinvasive and 41% died. [1] Death typically occurs within the first 3 weeks but can occur months later. [1] Among survivors, >50% develop long-term sequelae including cognitive impairment, aphasia, personality disorders, seizures, paralysis, and cranial nerve dysfunction. [1-2]

Severity stratification

• Mild/systemic illness only (fever, myalgias without CNS involvement) — generally self-limited, resolves in 1–2 weeks [1]
• Neuroinvasive disease (meningitis/encephalitis) — high mortality and morbidity; requires ICU-level care

Poor prognostic indicators: Hyponatremia (Na ≤130), high CSF WBC count (>500/mm³), symmetric/extensive MRI abnormalities, severely depressed EEG, immunosuppression, age extremes [1][5-6]

Initial stabilization

• ABCs — early intubation if GCS declining or inability to protect airway (50% required mechanical ventilation in one series) [3]
• IV access, continuous monitoring, ICU admission

Empiric therapy (until EEE confirmed)

• IV acyclovir 10 mg/kg q8h (cover HSV encephalitis)
• Broad-spectrum antibiotics (cover bacterial meningitis — e.g., vancomycin + ceftriaxone ± ampicillin if age >50 or immunocompromised)
• Dexamethasone if bacterial meningitis suspected

EEE-specific supportive care

• Seizure management: Levetiracetam first-line; continuous EEG monitoring; treat nonconvulsive status epilepticus aggressively [3]
• ICP management: Head of bed elevation, osmotic therapy if signs of elevated ICP; neurosurgical consultation for refractory cases [1]
• Fluid/electrolyte management: Avoid hypotonic fluids; correct and monitor hyponatremia closely [5]
• Antiemetics and analgesics for meningeal symptoms [1]

Immunomodulatory therapy (limited/inconclusive evidence)

• IVIG: May be considered early in hospitalization; shorter time to administration correlated with decreased long-term disability in one retrospective series (N=17) [1][7]
• High-dose IV methylprednisolone: Used in some cases; no proven survival benefit for viral encephalitides [1][7]

Admission criteria (essentially all neuroinvasive cases)

• Any altered mental status, seizures, focal neurologic deficits, or meningismus → ICU admission [2]
• 98% of reported cases required hospitalization; average stay ~10 days among survivors [3][8]

Observation/floor admission

Discharge criteria

• Neurologically stable, seizure-free, able to protect airway, adequate oral intake
• Many survivors require discharge to rehabilitation facilities for long-term neurologic recovery [1][3]

Specialist consultation triggers

• Neurology (seizure management, EEG interpretation)
• Infectious disease (diagnostic workup, IVIG/steroid consideration)
• Neurosurgery (refractory elevated ICP)
• Critical care (ventilator management)
• Public health department notification (mandatory) [1][8]

• Survivors require long-term neurologic follow-up: Cognitive rehabilitation, seizure monitoring, physical/occupational/speech therapy [1-2]
• Neurologic sequelae may include: cognitive/memory deficits, global aphasia, personality changes, seizure disorder, paralysis, cranial nerve dysfunction [1]
• One case demonstrated complete resolution of MRI abnormalities at follow-up, indicating some recovery potential [3]
• Return precautions: New or worsening seizures, declining mental status, new focal deficits, fever recurrence
• Prevention counseling: EPA-registered insect repellents (DEET, picaridin, IR3535), long-sleeved clothing, avoiding outdoor activity at dawn/dusk in endemic areas, eliminating standing water [1-2]
• No licensed human vaccine currently available; veterinary vaccines exist for horses [1][11]
• Counsel that humans are dead-end hosts — no person-to-person transmission risk [1]

1. Eastern Equine Encephalitis in the US. — Staples JE, Gould CV. The Journal of the American Medical Association. 2025.

2. What Is Eastern Equine Encephalitis?. — Roberts K. The Journal of the American Medical Association. 2025.

3. The Clinical and Radiographic Features of Eastern Equine Encephalitis: A Single-Center Retrospective Case Series. — Garcia-Dominguez MA, Moonis M, Kipkorir V, Srichawla BS. Medicine. 2024.

4. Eastern Equine Encephalitis Virus: A Case Report and Brief Literature Review of Current Therapeutic and Preventative Strategies. — Ruiz C, Gibson G, Rojas S, Friend K. Vector Borne and Zoonotic Diseases. 2024.

5. Viral Encephalitis: Familiar Infections and Emerging Pathogens. — Whitley RJ, Gnann JW. Lancet. 2002.

6. Clinical and Neuroradiographic Manifestations of Eastern Equine Encephalitis. — Deresiewicz RL, Thaler SJ, Hsu L, Zamani AA. The New England Journal of Medicine. 1997.

7. Eastern Equine Encephalitis and Use of IV Immunoglobulin Therapy and High-Dose Steroids. — Wilcox DR, Collens SI, Solomon IH, Mateen FJ, Mukerji SS. Neurology Neuroimmunology & Neuroinflammation. 2021.

8. Eastern Equine Encephalitis Virus in the United States, 2003-2016. — Lindsey NP, Staples JE, Fischer M. The American Journal of Tropical Medicine and Hygiene. 2018.

9. Clinical Characteristics of the 2019 Eastern Equine Encephalitis Outbreak in Michigan. — Ladzinski AT, Tai A, Rumschlag MT, et al. Open Forum Infectious Diseases. 2023.

10. Association of Human Eastern Equine Encephalitis With Precipitation Levels in Massachusetts. — Mermel LA. JAMA Network Open. 2020.

11. Notes From the Field: Increase in Eastern Equine Encephalitis Virus Activity - Vermont, 2023-2024. — Strelau KM, Pareles E, Kelso P, et al. MMWR. Morbidity and Mortality Weekly Report. 2026.