A cluster study of predictors of severe West Nile virus infection.

OBJECTIVE: To assess the value of multifocal chorioretinitis and of clinical manifestations and biologic parameters in the diagnosis of West Nile virus (WNV) infection.

PATIENTS AND METHODS: We conducted a prospective, controlled case series study during an outbreak of WNV infection between August 15 and October 24, 2003, of 64 consecutive patients who presented with clinical manifestations consistent with WNV disease. In each patient, standardized clinical and biologic data were collected. An ophthalmologic examination searching particularly for multifocal chorioretinitis was performed.

RESULTS: Of 64 patients who presented primarily with meningitis and/or encephalitis, 36 had IgM antibodies against WNV. The WNV-infected patients tended to be older (median age of 54 years vs 46 years in WNV infection and control groups, respectively) and more frequently had diabetes (30% vs 7% in WNV infection and control groups, respectively; P = .03). Multifocal chorioretinitis was found in 75% of WNV-infected patients but in no patient in the control group (P = .001). Blood glucose and amylase levels were higher in WNV-infected patients, whereas serum sodium levels were lower. The cerebrospinal fluid leukocyte count and protein levels were significantly higher in WNV meningitis or encephalitis. Overall, multifocal chorioretinitis had 100% specificity and 73% sensitivity (88% when only patients with meningitis or encephalitis were analyzed) for the diagnosis of WNV. Multivariate analysis disclosed multifocal chorioretinitis as the only predictor of WNV infection (odds ratio, 62; 95% confidence interval, 6-700; P = .001).

CONCLUSION: Multifocal chorioretinitis appears to be a specific marker of WNV infection, particularly in patients who present with meningoencephalitis. An ophthalmologic examination should be part of the routine evaluation of such patients.