Clinical Characteristics of the West Nile Fever Outbreak, Israel, 2000

Abstract

West Nile VirusWest Nile (WN) virus was first isolated and identified from the blood of a febrile woman in Uganda in 1937 (1); fever was her only known symptom.In the early 1950s, several reports from outbreaks in Israel were the first to detail the clinical characteristic of this illness; the clinical picture that emerged was that of a benign febrile disease in young adults (2,3).In a later outbreak from Israel in 1957, a linkage between WN virus infection and severe central nervous system (CNS) disease was first noted, and a correlation between the age of patients and severity of disease was established (4).In the last decade, two outbreaks gained attention: the first from Romania in 1996, where a high percentage of CNS involvement was noted, and the second from New York in 1999, in which several cases of flaccid paralysis were described (5,6).The outbreak in New York was the first time WN fever was reported in the Western Hemisphere (7).From August to October 2000, 417 laboratory-confirmed WN fever cases occurred in Israel; 326 were hospitalized cases.We collected clinical data on 233 of the hospitalized population from 12 different hospitals throughout the country.We report the clinical characteristics of these patients. Methods Study PopulationA case of WN fever was defined as illness in a patient with a clinical picture consistent with WN fever and with anti-WN virus immunoglobulin (Ig) M antibodies detected in either a serum or cerebrospinal fluid (CSF) specimen.The study populat:ion was persons who were hospitalized and diagnosed from August 1, 2000, to October 31, 2000.Patients with headache and abnormal CSF findings, with no confusion or change in level of consciousness (i.e., drowsiness to coma), were diagnosed as having meningitis.Patients with an altered level of consciousness, confusion, or focal neurologic signs were diagnosed as having encephalitis, regardless of CSF findings.Patients with severe weakness of limbs or flaccid paralysis were diagnosed as having myelitis.Patients with fever and no CNS symptoms were diagnosed as having febrile disease.A total of 325 WN fever patients were hospitalized in 20 hospitals.Data were obtained from 12 (60%) of the hospitals and 233 (71%) of the 326 patients.The geographic location of hospitals that participated in the study was similar to that of nonparticipating hospitals.No exclusion was made based on patient characteristics.Demographic, epidemiologic, and clinical data were collected by infectious disease specialists in each hospital, using a structured questionnaire.Information was obtained by interviewing the patients and reviewing medical records or by reviewing the charts in cases in which the diagnosis was confirmed after the discharge of the patients. Serologic TestingSerologic diagnosis of WN virus infection was based on an IgM-capture enzyme-linked immunosorbent assay (ELISA), performed in serum or CSF samples.The assay, which was Clinical Characteristics of the West Nile FeverOutbreak, Israel, 2000