Credit CDC
# 8765
As the chart above indicates, even before West Nile arrived in 1999, North America already had a relatively long list of major mosquito-borne encephalopathies to deal with; SLE (St. Louis Encephalitis), EEE (Eastern Equine Encephalitis), WEE (Western Equine Encephalitis), LAC (LaCrosse Virus), and POW (Powassan Encephalitis).
While the number of human infections each year was relatively low, what we lacked for in patient counts we made up for in variety.
To these neuroinvasive (i.e., meningitis, encephalitis, or acute flaccid paralysis producing) arboviruses we can add additional mosquito-borne diseases such as dengue, malaria, and the new threat on the block; Chikungunya. While North America is hardly a hotbed of mosquito borne diseases, they are decidedly less rare today than they were two or three decades ago.
And given the regular importation of new vector-borne diseases via international travelers, this seems to be a trend likely to continue (see The Global Reach Of Infectious Disease).
.
Yesterday the CDC’s MMWR published an update on the 2013 West Nile/Arbovirus season (excluding dengue). While far from being the worst year we’ve seen (WNV was down by about 50% over 2012), 2013 provided its fair share of arboviral misery.
- 2,469 cases of WNV disease
- 85 cases of La Crosse virus (LACV)
- 22 cases of Jamestown Canyon virus (JCV
- 15 cases of Powassan virus (POWV)
- 8 cases of eastern equine encephalitis virus (EEEV)
Despite the reputation for Florida and the Southeastern states as being a mosquito haven, those states ranked among the lowest in reports of neuroinvasive WN disease.
Incidence* of reported cases of West Nile virus neuroinvasive disease, by state — United States, 2013
It is too soon to know what kind of year 2014 will be for mosquito-borne diseases in North America. With both dengue and chikungunya threatening to make inroads, this year’s mosquito season will be watched closely across the country. In the meantime, some excerpts from last year’s surveillance report below, but follow the link to read it in its entirety.
West Nile Virus and Other Arboviral Diseases — United States, 2013
Weekly
June 20, 2014 / 63(24);521-526Nicole P. Lindsey, MS1, Jennifer A. Lehman1, J. Erin Staples, MD1, Marc Fischer, MD1 (Author affiliations at end of text)
Arthropod-borne viruses (arboviruses) are transmitted to humans primarily through the bites of infected mosquitoes and ticks. West Nile virus (WNV) is the leading cause of domestically acquired arboviral disease in the United States (1). However, several other arboviruses also cause sporadic cases and seasonal outbreaks of neuroinvasive disease (i.e., meningitis, encephalitis, and acute flaccid paralysis) (1). This report summarizes surveillance data reported to CDC in 2013 for WNV and other nationally notifiable arboviruses, excluding dengue. Forty-seven states and the District of Columbia reported 2,469 cases of WNV disease. Of these, 1,267 (51%) were classified as WNV neuroinvasive disease, for a national incidence of 0.40 per 100,000 population. After WNV, the next most commonly reported cause of arboviral disease was La Crosse virus (LACV) (85 cases), followed by Jamestown Canyon virus (JCV) (22), Powassan virus (POWV) (15), and eastern equine encephalitis virus (EEEV) (eight). WNV and other arboviruses continue to cause serious illness in substantial numbers of persons annually. Maintaining surveillance remains important to help direct and promote prevention activities.
<SNIP>
Discussion
In 2013, WNV was the most common cause of neuroinvasive arboviral disease in the United States. However, LACV was the most common cause of neuroinvasive arboviral disease among children. More JCV cases were reported in 2013 than in any previous year and included the first cases reported from eight states. This increase is likely related to the initiation of routine immunoglobulin M testing for JCV at CDC in 2013 and suggests that the incidence of JCV infection in prior years might have been underestimated. EEEV disease, although rare, remained the most severe arboviral disease, with four deaths among eight patients. More than 90% of arboviral disease cases occurred during April–September, emphasizing the importance of focusing public health interventions during this period.
Reported numbers of arboviral disease cases vary from year to year. Weather (e.g., temperature and precipitation), zoonotic host and vector abundance, and human behavior (e.g., repellent use, outdoor activities, and use of air conditioning or screens in the home) are all factors that can influence when and where outbreaks occur. This complex ecology makes it difficult to predict how many cases of disease might occur in the future and where they will occur. Increased numbers of reported cases and the identification of cases in new locations might reflect actual changes in incidence and epidemiology or increased disease awareness.
The incidence of WNV neuroinvasive disease declined substantially in 2013 (incidence of 0.40 per 100,000 population) compared with 2012 (0.92 per 100,000 population), when a large multistate outbreak occurred, with incidence nearing the levels observed in 2002 and 2003 (4). However, the incidence in 2013 was similar to that during 2004–2007 (median = 0.43; range = 0.39–0.50) and was higher than that during 2008–2011 (median = 0.18; range: 0.13–0.23) (3–5). WNV activity remained focalized in 2013, with more than half of the neuroinvasive disease cases being reported from just six states.
The findings in this report are subject to at least two limitations. First, ArboNET is a passive surveillance system that relies on clinicians to consider the diagnosis of an arboviral disease and obtain appropriate diagnostic tests, and on health-care providers and laboratories to report laboratory-confirmed cases to public health authorities. Second, testing and reporting are incomplete, leading to a substantial underestimate of the actual number of cases (6). For example, data from previous studies suggest there are 30–70 nonneuroinvasive disease cases for every reported case of WNV neuroinvasive disease (7–9). Extrapolating from the 1,267 WNV neuroinvasive disease cases reported, an estimated 38,000–88,500 nonneuroinvasive disease cases might have occurred in 2013. However, only 1,202 (1%–3%) were diagnosed and reported.
Arboviruses continue to cause substantial morbidity in the United States. However, cases occur sporadically, and the epidemiology varies by virus and geographic area. Surveillance is essential to identify outbreaks and guide prevention efforts aimed at reducing the incidence of these diseases. Health-care providers should consider arboviral infections in the differential diagnosis of cases of aseptic meningitis and encephalitis, obtain appropriate specimens for laboratory testing, and promptly report cases to public health authorities (2). Because human vaccines against domestic arboviruses are not available, prevention of arboviral disease depends on community and household efforts to reduce vector populations (e.g., applying insecticides and reducing mosquito breeding sites), personal protective measures to decrease exposure to mosquitoes and ticks (e.g., use of repellents and wearing protective clothing), and screening blood donors.
Acknowledgment
ArboNET surveillance coordinators in state and local health departments.
