Seasonal Variation of West Nile Virus, Associated Neuroinvasive Presentations in the US

Encephalitis caused by West Nile virus, computer illustration. West Nile virus (WNV) is known to cause encephalitis (inflammation of the brain) in humans. The WNV belongs to the flavivirus group, which are RNA (ribonucleic acid) viruses that are surrounded by an outer protein envelope. WNV is transmitted by mosquitoes and infects both humans and animals. Symptoms can range from a mild fever to spontaneous bleeding of the skin and circulatory failure, which are often fatal.
Study authors assessed location of WNV outbreaks and associated neuroinvasive presentations for patterns.

Although there has been stability in the incidence of West Nile virus (WNV) in the United States, sporadic peaks in activity across years in various regions makes outbreaks difficult to predict, according to a surveillance summary of WNV published by the Centers for Disease Control and Prevention in Morbidity and Mortality Weekly Reports.

Through a national arboviral diseases surveillance system, state and local health departments reported cases of WNV to the Centers for Disease Control and Prevention between 2009 and 2018. Study authors assessed location of outbreaks and associated neuroinvasive presentations for patterns.

During this surveillance period, 21,869 cases of WNV were reported, comprising 33% confirmed, and 67% probable cases. The WNV cases were classified as neuroinvasive (59%) or non-neuroinvasive (41%) diseases, and 5% were fatal. Cases did not associate with demographic characteristics.

Among the neuroinvasive cases, 53% were encephalitis, 37% were meningitis, 7% were acute flaccid paralysis, and 3% were unspecified.

The majority of cases (89%) were reported between July and September. WNV was identified in 60% of counties in all 50 states, Puerto Rico, and the District of Columbia. The cases reported in Alaska, Hawaii, and Puerto Rico were likely instances of travel-related exposures from the contiguous US.

Over time, the total number of cases was relatively stable (median, 1386; range, 1267-1658). However, regional spikes were common. Examples included a peak in 2012 in southcentral and upper Midwest, peaks in 2014 and 2015 in California, a peak in 2017 in Arizona, and a peak in 2018 in the northeastern US.

The 4 regions with the highest number of reported cases (California: n=2819, Texas: n=2043, Illinois: n=728, Arizona: n=632) accounted for nearly half (48%) of all neuroinvasive disease cases. Average yearly infections ranged from 3.16 per 100,000 in North Dakota to 0.01 per 100,000 in Alaska. States located in the West South Central, Mountain, and Pacific divisions tended to report high incidences during outbreak years, but had more year-to-year variation. New England, Middle Atlantic, and South Atlantic regions had consistently low incidence and Arizona, California, and Texas had consistently high incidence.

Among the 3142 counties in the US, 50% reported at least 1 neuroinvasive disease. Reporting was not widespread, however, because at least 50% of cases were reported by less than 10% of counties in the 10 highest-burden states. These high-infection counties were Los Angeles County (n=1092), Maricopa County (n=468), Cook County (n=432), Orange County (n=375), Harris County (n=304), and Dallas County (n=281), which together accounted for nearly a quarter of all cases (23%).

This summary was dependent on passive surveillance, only including data on patients who sought care from a clinician who suspected an arboviral disease.

The study authors concluded the WNV has become endemic in the contiguous US. The seasonality variation of outbreaks makes it difficult to predict future patterns of incidence, necessitating ongoing mosquito surveillance to curb infection across the country.


McDonald E, Mathis S, Martin S W, et al. Surveillance for west nile virus disease — united states, 2009–2018. MMWR Surveill Summ. 2021;70(1):1-15. doi:10.15585/mmwr.ss7001a1