Abstract
In 2013, the national surveillance case definition for West Nile virus (WNV) disease was revised to remove fever as a criterion for neuroinvasive disease and require at most subjective fever for non-neuroinvasive disease. The aims of this project were to determine how often afebrile WNV disease occurs and assess differences among patients with and without fever. We included cases with laboratory evidence of WNV disease reported from four states in 2014. We compared demographics, clinical symptoms and laboratory evidence for patients with and without fever and stratified the analysis by neuroinvasive and non-neuroinvasive presentations. Among 956 included patients, 39 (4%) had no fever; this proportion was similar among patients with and without neuroinvasive disease symptoms. For neuroinvasive and non-neuroinvasive patients, there were no differences in age, sex, or laboratory evidence between febrile and afebrile patients, but hospitalisations were more common among patients with fever (P < 0.01). The only significant difference in symptoms was for ataxia, which was more common in neuroinvasive patients without fever (P = 0.04). Only 5% of non-neuroinvasive patients did not meet the WNV case definition due to lack of fever. The evidence presented here supports the changes made to the national case definition in 2013.
Highlights
West Nile virus (WNV), a mosquito-borne flavivirus, is a leading cause of arboviral disease in the USA
A total of 977 patients who met the confirmed or probable laboratory criteria for WNV disease were reported from the four states during the project period
159 (50%) of the 320 patients with febrile non-neuroinvasive disease were hospitalised compared to two (13%) of the 16 patients without reported fever (P < 0.01). These data indicate that WNV disease without fever is uncommonly reported but does occur
Summary
West Nile virus (WNV), a mosquito-borne flavivirus, is a leading cause of arboviral disease in the USA. The Council of State and Territorial Epidemiologists (CSTE) collaborates with the Centers for Disease Control and Prevention (CDC) to define nationally notifiable diseases, including arboviruses like WNV. Starting in 2004, the CSTE case definition for arboviral diseases included documented fever as a required clinical criterion for both neuroinvasive and non-neuroinvasive arboviral disease [8]. After this case definition was implemented, some state and local epidemiologists expressed concerns about patients exhibiting clinical symptoms with convincing laboratory evidence for a diagnosis of WNV infection but without fever and not meeting the CSTE case definition
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