Prognosis

EEEV is the most deadly encephalitic arbovirus in North America. Case fatality is typically 30% to 50%.[2][11] Death mainly occurs 2 to 10 days after symptom onset, but can occur later.[15]

Although some patients with EEEV infection may be asymptomatic, it is common for patients to experience symptoms resembling influenza or dengue fever (i.e., a prodrome), which may have an abrupt onset. Neurological (encephalitic) symptoms may also develop in some patients. Neurological/central nervous system involvement is most commonly seen in those aged over 50 years or under 15 years. However, neurological involvement due to Madariaga virus (MADV) infection is more prevalent in children.[16] In the absence of neurological symptoms, febrile illness usually resolves in 1 to 2 weeks.[5] If neurological symptoms are present, they tend to appear several days into the course of illness, and include irritability, drowsiness, altered mental status, seizures, cranial nerve palsies, focal weakness, photophobia, and meningismus. Severe cases result in coma and death, which usually occurs 2 to 10 days after symptom onset. Of those who recover from EEEV infection, approximately 50% have neurological sequelae.[2] Prognosis may be better in patients over 40 years of age, and in patients who experience a longer prodrome (e.g., >3 days).[2][44] The size of radiographic lesions in the brain does not appear to be associated with outcome in patients with EEEV infection; however, pleocytosis in cerebrospinal fluid and severe hyponatraemia have been correlated with poor outcomes.[36]

Long-term sequelae

Neurological sequelae may be long-term, consisting of gross and fine motor impairment, memory impairment, behavioural changes, weakness, seizures, or tremors.[16][45]

EEEV infection appears to confer life-long immunity.[15]

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