Rationale:

• Muscle weakness, including acute flaccid paralysis, and changes in mental status in patients with encephalitis are predictors of death and poor neurologic outcome.

Evidence:

• Only 28% of patients with WNV encephalitis and/or neuromuscular weakness in a Canadian series were discharged home without additional support, and most had persistent neurologic deficits 30 days after discharge (29).
• Three out of 16 hospitalized, WNV antibody-positive patients had poliomyelitis-like acute flaccid paralysis in a 2002 Louisiana series (30).

Comments:

• Case-fatality rates among patients hospitalized with neuroinvasive WNV illness during recent outbreaks of the currently circulating strain of WNV in the U.S., Canada, and Israel have ranged from 12% to 18%. These case-fatality rates are greater than those previously seen in the Middle East and Africa. This may be due to differences in the age structure or frequency of medical risk factors in the nonimmune population or to the underlying virulence of the WNV strain. Although the specific reason is not known, there has been a much higher incidence of encephalitis and a higher case-fatality rate than seen in previous outbreaks (6; 31).
• Case-fatality rates have been shown to be higher among patients under age 50 and in those with more severe disease (2; 24; 26).