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WNV most common cause of reported neuroinvasive arboviral diseases in 2014
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Cases of West Nile virus were responsible for 95% of all U.S. arboviral disease reports during 2014, with neuroinvasive cases of the infection comprising more than half of these reports, according to recent data published in MMWR.
“Arboviruses continue to cause substantial morbidity in the United States, although reported numbers of cases vary annually,” the researchers wrote. “It is difficult to predict how many cases of disease might occur in the future and in what areas; therefore, surveillance is essential to identify outbreaks and guide prevention efforts.”
In 2014, there were 2,327 cases of nationally notifiable arboviral disease reported to the CDC through its ArboNET national surveillance system. Of these, 2,205 were West Nile virus (WNV), 80 were La Crosse virus, 11 were Jamestown Canyon virus, 10 were St. Louis encephalitis virus, eight were Powassan virus, eight were Eastern equine encephalitis virus, and five were unspecified California serogroup virus.
WNV cases were reported from 42 states and the District of Columbia, and peaked in late August. Seventy-two percent of patients with the infection were hospitalized, and 4% died.
Sixty-one percent of WNV cases were classified as neuroinvasive (n = 1,347), with 96% of neuroinvasive WNV patients hospitalized and 6% deceased. Neurologic presentations among these cases included encephalitis (46%), meningitis (42%), acute flaccid paralysis (10%) or other presentations (2%). National incidence of WNV neuroinvasive disease, as calculated using U.S. Census 2014 mid-year population estimates, was 0.42 per 100,000 population. States with the greatest incidence rates were Nebraska (2.2 per 100,000), North Dakota (1.6 per 100,000) and California (1.4 per 100,000), with California reporting 83% more cases of the infection than its previous highest year.
According to the researchers, these reported cases may not represent the total burden of WNV disease due to underdiagnoses and frequently unreported cases of non-neuroinvasive infection.
“Data from previous studies suggest there are an estimated 30 to 70 non-neuroinvasive disease cases for every reported case of WNV neuroinvasive disease,” they wrote. “Extrapolating from the 1,347 WNV neuroinvasive disease cases reported, an estimated 40,000 to 94,000 non-neuroinvasive disease cases might have occurred in 2014. However, only 858 (1%-2%) were diagnosed and reported.” – by Dave Muoio
Disclosure: The researchers report no relevant financial disclosures.
Perspective
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Lyle R. Petersen, MD, MPH
Fifteen years since its introduction to the United States, West Nile virus incidence shows no signs of abating, unfortunately, and now accounts for the vast majority of arboviral disease cases occurring in the United States. The 2014 total of 1,347 neuroinvasive disease cases was about the annual average since the first big outbreak in 2002. Nevertheless, the public health burden from these cases, many of whom will be left with long-standing neurological deficits, as well as that from the 40,000 to 96,000 estimated West Nile virus fever cases in 2014, was likely considerable.
There is some hope for reversing this trend. Good surveillance and prompt entomologic intervention are within our current reach, and several studies have demonstrated that this approach abates human outbreaks. Further down the road is the possibility of a human vaccine. Early phase human trials demonstrated safety and likely efficacy of at least three candidate human vaccines. The near-elimination of equine West Nile virus disease by vaccination suggests the possible benefit of this approach in humans.
The challenge remains to make these interventions cost-effective for this highly unpredictable disease. The alternative is to accept the 2014 statistics as the new norm.
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