Arboviruses Cause Serious Illness in Thousands of People in the U.S.

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. However, several other arboviruses also cause sporadic cases and seasonal outbreaks. A recent CDC report summarizes surveillance data reported to the CDC in 2014 for WNV and other nationally notifiable arboviruses, excluding dengue. In 2014, the CDC received reports of 2,327 cases of nationally notifiable arboviral disease, among which 1,453 (62%) were classified as neuroinvasive disease.


Forty-two states and the District of Columbia (DC) reported 2,205 cases of WNV disease. Of these, 1,347 (61%) were classified as WNV neuroinvasive disease (e.g., meningitis, encephalitis, or acute flaccid paralysis), for a national incidence of 0.42 cases per 100,000 population. After WNV, the next most commonly reported cause of arboviral disease was La Crosse virus (80 cases), followed by Jamestown Canyon virus (11), St. Louis encephalitis virus (10), Powassan virus (8), and Eastern equine encephalitis virus (8). WNV and other arboviruses cause serious illness in substantial numbers of persons each year. Maintaining surveillance programs is important to help direct prevention activities.


In the United States, most arboviruses are maintained in transmission cycles between arthropods and vertebrate hosts (typically birds or small mammals). Humans usually become infected when bitten by infected mosquitoes or ticks. Person-to-person transmission also occurs rarely through blood transfusion and organ transplantation.


The majority of human arboviral infections are asymptomatic. Symptomatic infections most often manifest as a systemic febrile illness and, less commonly, as neuroinvasive disease. Most endemic arboviral diseases are nationally notifiable and are reported to the CDC through ArboNET, a national arboviral surveillance system managed by the CDC and state health departments.


Using standard definitions, human cases with laboratory evidence of recent arboviral infection are classified as neuroinvasive disease or nonneuroinvasive disease. Cases reported as encephalitis, meningitis, or acute flaccid paralysis are collectively referred to as neuroinvasive disease; others are considered nonneuroinvasive disease. Acute flaccid paralysis can occur with or without encephalitis or meningitis. In this report, any case reported as acute flaccid paralysis (with or without another clinical syndrome) was classified as acute flaccid paralysis and not included in the other categories. Because of the substantial associated morbidity, detection and reporting of neuroinvasive disease cases is assumed to be more consistent and complete than that of nonneuroinvasive disease cases. Therefore, incidence rates were calculated for neuroinvasive disease cases using U.S. Census 2014 mid-year population estimates.


Overall, 1,589 patients were hospitalized, and 97 died. The median age of patients who died was 75 years. Three states reported two thirds (66%) of the neuroinvasive disease cases: California (561 cases), Texas (253), and Arizona (80). California reported a record number of disease cases, 83% more than the next highest year (2005). Within California, 70% of all neuroinvasive disease cases were reported from just two counties (Los Angeles and Orange). These findings highlight the focal nature of WNV outbreaks.


Over 90% of arboviral disease cases occurred during April–September, emphasizing the importance of focusing public health interventions during this period. Arboviruses continue to cause substantial morbidity in the United States, although reported numbers of cases vary annually. Cases occur sporadically, and the epidemiology varies by virus and geographic area. The weather, zoonotic host and vector abundance, and human behavior are all factors that can influence when and where outbreaks occur. Because of this complex ecology, 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.


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. Because human vaccines against domestic arboviruses are not available, prevention depends on community and household efforts to reduce vector populations (e.g., applying insecticides and reducing breeding sites), personal protective measures to decrease exposure to mosquitoes and ticks (e.g., use of repellents and wearing protective clothing), and screening of blood donors.


See the CDC Report


See also Medical Law Perspectives, April 2014 Report: Danger and Controversy: Lyme Disease Liability Risks


See also Medical Law Perspectives, March 2014 Report: Blood Draws, Testing, Transfusions: Venipuncture Injury, Inaccurate Results, Tainted Blood - The Liability Risks


See the Medical Law Perspectives July 27, 2015, Blog: Problems of Proving a Lyme Disease Diagnosis