Rationale:

• Increased age and immunocompromise may predispose to more severe disease.

Evidence:

• Advanced age is the most important risk factor for death according to a case series of 393 patients with WNV meningoencephalitis from Romania and 326 such patients from Israel (23; 24). Surveillance data indicate that the case-fatality rate of persons with WNV neuroinvasive disease increases from 1% among persons under age 40 to 36% among persons aged 90 years and older (25).
• A 1999 case series of 59 patients in New York showed that older age is also a risk factor for encephalitis with muscle weakness, and history of diabetes is a risk factor for death (26).
• Encephalitis and history of diabetes and immunosuppression are possible risk factors for death and poor neurologic outcome according to two case series, one of 59 patients in New York in 1999 and one of 326 patients in Israel in 2000 (24; 26).
• The incidence of neuroinvasive disease and the probability of death after acquiring neuroinvasive disease is higher in men than in women (25).
• Organ transplant recipients are at extreme risk of developing neuroinvasive disease after WNV infection. A seroprevalence study carried out in patients of Canadian outpatient transplant clinics following a WNV epidemic in 2002 indicated that approximately 40% developed neuroinvasive disease following infection, a rate approximately 40 times that of the population at large (27; 28).

Comments:

• Patient age has been shown to be the most important overall risk factor for developing neuroinvasive disease and for mortality due to WNV infection.
• Organ transplant patients are at extreme risk for developing neuroinvasive disease. Case reports suggest that patients with advanced malignancies, particularly hematologic malignancies, may also be at increased risk.