The incidence of neuroinvasive West Nile virus dropped sharply in 2013, but it and other arboviruses continue to cause serious disease, the CDC reported.
All told, the agency received reports of 2,469 cases of West Nile virus disease in 47 states and the District of Columbia, CDC investigators wrote in the June 20 issue of Morbidity and Mortality Weekly Report.
Of those, 1,267, or 51%, were classified as neuroinvasive, for a national incidence of 0.40 per 100,000 people, compared with 0.92 per 100,000 in 2012.
But the investigators cautioned that West Nile is probably under-reported.
The incidence of neuroinvasive cases, which are associated with substantial morbidity, is thought to be the most accurate indicator of disease activity. But previous research has suggested that for every neuroinvasive case, there are between 30 and 70 non-neuroinvasive cases.
Based on the 2013 numbers, that would mean as many as 88,500 non-neuroinvasive disease cases might have occurred — but only 1,202 were reported.
The figures come from the CDC’s ArboNET surveillance system, which gets reports on endemic arboviral diseases — those transmitted by mosquitoes and ticks — most of which are nationally notifiable.
In 2013, the agency was notified of 2,605 cases of arboviral diseases. In addition to the West Nile cases, the count included 85 caused by La Crosse virus, 22 by Jamestown Canyon virus, 15 by Powassan virus, eight by eastern equine encephalitis virus, five by unspecified California serogroup virus, and one caused by St. Louis encephalitis virus.
Of the total, 1,383 (or 53%) were reported as neuroinvasive disease, for a national incidence of 0.44 cases per 100,000 people.
The agency cautioned that the surveillance is passive, and relies on clinicians to make and confirm the diagnosis and on healthcare providers and labs to report confirmed cases to public health officials.
The incidence of West Nile in 2012 was elevated because of large outbreaks over several states.
The 2013 rate was more in line with levels seen in 2004 through 2007, where the median incidence was 0.43 per 100,000 people, but was higher than the median rate — 0.18 per 100,000 — observed in 2008 through 2011.
Viral activity remained “focalized” in 2013, the MMWR report said, with 51% of the neuroinvasive disease cases reported from six states — California with 237 cases, Texas with 113, Colorado with 90, Illinois with 86, North Dakota with 64, and Oklahoma with 60.
North and South Dakota had the highest incidence rates, at 8.9 and 6.8 per 100,000 people respectively.
West Nile was the most common cause of neuroinvasive arbovirus disease over all, but La Crosse virus was the most common among children, with a median patient age of 7 and 89% of patients under 18.
More Jamestown Canyon virus was reported in 2013 than in any previous year, the agency noted. Eight states — Georgia, Idaho, Massachusetts, Minnesota, New Hampshire, Oregon, Pennsylvania, and Rhode Island — reported their first cases last year.
Eastern equine encephalitis virus disease was seen in six states, two of which — Arkansas and Connecticut — had their first-ever cases. All eight cases had neuroinvasive disease and four died.
North American Correspondent for MedPage Today, is a three-time winner of the Science and Society Journalism Award of the Canadian Science Writers’ Association. After working for newspapers in several parts of Canada, he was the science writer for the Toronto Star before becoming a freelancer in 1994. His byline has appeared in New Scientist, Science, the Globe and Mail, United Press International, Toronto Life, Canadian Business, the Toronto Star, Marketing Computers, and many others. He is based in Toronto, and when not transforming dense science into compelling prose he can usually be found sailing.
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