Arboviral Infections in the United States — Not Just West Nile
October 1, 2015 4 minutes read
In 2014, 2327 cases of arbovirus infection were reported to the CDC, mostly via ArboNET, by the District of Columbia and all the states with the exception of Alaska, Delaware, Rhode Island, and Vermont. The vast majority — 2205 (95%) — were due to West Nile virus (WNV), and 1347 (61%) of these caused neuroinvasive infections (see Figure) such as meningitis (42% of those with neuroinvasive disease), encephalitis (46%), and acute flaccid paralysis (10%) — accounting for a national incidence of 0.42 per 100,000 population. The incidence in the six states with the highest rates (Nebraska, North Dakota, California, South Dakota, Louisiana, and Arizona) ranged from 2.2 to 1.2 per 100,000. The incidence in California was significantly increased from previous years, and two California counties, Los Angeles and Orange, accounted for 70%. Of the total cases, 90% had onset during July to September, with a peak in late August. Almost two-thirds occurred in males. Hospitalization was thought to be necessary for 72%. Overall, 4% of the patients died — the median age of those who died was 75 years, while it was only 57 years for the total cohort. The incidence of neuroinvasive disease increased with increasing age, and all but 4% of patients with neuroinvasive disease were hospitalized; 6% died. An important feature of WNV is its potential transmission by blood transfusion and organ transplantation from infected asymptomatic donors.
The other 5% of reported arboviral infections included La Crosse virus (80 cases), Jamestown Canyon virus (11), St. Louis encephalitis virus (10), Powassan virus (8), Eastern equine encephalitis virus (8), and unspecified California serogroup virus (5). The 80 cases of La Crosse virus disease, 95% of which were neuroinvasive, were reported from nine states. While 28 of the 90 were reported by South Atlantic states, Ohio had the highest incidence (0.26 per 100,000 population), just beating out North Carolina (0.23 per 100,000). The demographics were almost the opposite of those of WNV disease: The median age was only 8 years, and 53% were female. All but one were hospitalized, and three (4%) died.
Four states (Massachusetts, Minnesota, Tennessee, and Wisconsin) reported a total of 11 cases of Jamestown Canyon virus disease; none were fatal. Jamestown Canyon virus, like La Crosse virus, belongs to the California serogroup virus. There were an additional five cases of California serogroup disease for which the specific virus was undetermined. No fatalities were reported among the 10 cases of St. Louis encephalitis or the eight cases of Powassan virus disease, while three of the eight patients with Eastern equine encephalitis died.
COMMENTARY
As pointed out by CDC, this report is not the whole story, and the true number of cases is likely much greater. Since the majority of infections are asymptomatic and many of the rest are associated with only a nonspecific febrile illness, a diagnosis is only likely to be made with more severe disease, especially those causing neurological manifestations. They estimate that there are actually 40,000 to 94,000 infections annually, with only a few percent identified. In addition, other arboviral infections, including chikungunya, dengue, and Colorado tick fever, are not notifiable on a national level. Finally, novel viruses, such as Heartland1 and Bourbon2 viruses, continue to be discovered.
Heartland virus is a phlebovirus that has been identified in a small number of patients in Missouri and Tennesse and is believed to be transmitted by the bite of the Lone Star tick, and has been fatal in at least one instance.1 The Bourbon virus, a thogotovirus, was first recovered in a patient from Bourbon County, KS, with a history of tick bites.2 The patient presented with fever, thrombocytopenia, and leukopenia, went on to develop multi-organ system failure, and died.
In the United States, annual federal funding for arbovirus surveillance decreased by 39% between 2006 and 2012, when it reached a low of only $9.3 million.3 In that same year, the United States experienced the highest incidence of confirmed WNV neuroinvasive disease since 2003, as well as the highest number of confirmed deaths ever. With reports of arbovirus vectors appearing in new areas and the identification of novel arboviruses, we likely will have more such infections due to a variety of viruses in the future.
REFERENCES
- Kosoy OI, Lambert AJ, Hawkinson DJ, et al. Novel thogotovirus associated with febrile illness and death, United States, 2014. Emerg Infect Dis 2015;21:760-764.
- Muehlenbachs A, Fata CR, Lambert AJ, et al. Heartland virus-associated death in Tennessee. Clin Infect Dis 2014;59:845-850.
- Hadler JL, Patel D, Nasci RS, et al. Assessment of arbovirus surveillance 13 years after introduction of West Nile Virus, United States. Emerg Infect Dis 2015;21:1159-1166.
Source: Centers for Disease Control and Prevention.
West Nile virus accounted for 95% of arbovirus infections in 2013 reported to CDC, with the majority causing neuroinvasive disease.
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