West Nile Virus Neuroinvasive Disease On the Rise

Marcia Frellick

August 08, 2019

West Nile Virus (WNV) neuroinvasive disease incidence was up nearly 25% in 2018 from the average incidence from 2008 to 2017, according to the Centers for Disease Control and Prevention's latest Morbidity and Mortality Weekly Report.

Emily McDonald, MD, with the CDC's arboviral diseases branch of the National Center for Emerging and Zoonotic Infectious Diseases, was lead author on the report published online today.

WNV in 2018 caused 94% of domestically acquired arboviral disease (which are diseases spread by mosquitoes and ticks), the authors note. But other arboviruses — such as eastern equine, Jamestown Canyon, La Crosse, Powassan, and St. Louis encephalitis viruses — cause cases and outbreaks of neuroinvasive disease as well.

"Health care providers should consider arboviral infections in patients with aseptic meningitis or encephalitis, perform appropriate diagnostic testing, and report cases to public health authorities," the authors write.

The authors note that for the first time since 2013, WNV transmission via organ transplantation was reported. Also new is the first documented case of Powassan virus transmission by blood transfusion.

Of the WNV cases, 1658 (63%) were classified as neuroinvasive disease (such as meningitis, encephalitis, and acute flaccid paralysis), putting the national incidence at 0.51 cases per 100,000 population, 25% higher than the median incidence of 0.41 from 2008 to 2017 (range = 0.13 [2009] - 0.92 [2012]).

Surveillance is critical because arboviruses can cause serious illness and there are no vaccines or definitive treatment.

In 2018, 48 states and the District of Columbia reported 2813 cases of domestic arboviral disease, including 2647 WNV cases. Only Hawaii and New Hampshire had no reports of the viruses.

Some Western states, such as Arizona and California, which previously had large numbers of cases, saw a drop in incidence in 2018, whereas several Northeastern states, such as New Jersey, New York, and Pennsylvania, had increases.

The highest incidence rates were in North Dakota (7.89 per 100,000), Nebraska (6.43), South Dakota (5.33), Montana (2.35), and Iowa (1.87).

Understanding the epidemiology of the arboviruses and when and in what regions the cases are concentrated is important for differentiating them from other neurologic infections, the authors write. Most human infections are asymptomatic.

According to the report, the great majority (93%) of arboviral disease cases occurred during April to September of 2018, consistent with past years' peak seasons.

The report excluded dengue, chikungunya, and Zika viruses because they are mostly nondomestic viruses acquired through travel.

The authors have disclosed no relevant financial relationships.

MMWR. 68(31);673-678. Full text

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