Vital Signs

Trends in Reported Vectorborne Disease Cases — United States and Territories, 2004–2016

Ronald Rosenberg, ScD; Nicole P. Lindsey, MS; Marc Fischer, MD; Christopher J. Gregory, MD; Alison F. Hinckley, PhD; Paul S. Mead, MD; Gabriela Paz-Bailey, MD; Stephen H. Waterman, MD; Naomi A. Drexler, MPH; Gilbert J. Kersh, PhD; Holley Hooks, MPH; Susanna K. Partridge, MPH; Susanna N. Visser, DrPH; Charles B. Beard, PhD; Lyle R. Petersen, MD

Disclosures

Morbidity and Mortality Weekly Report. 2018;67(17):496-501. 

In This Article

Conclusions and Comments

These data indicate persistent, locality-specific risks and a rising threat from emerging vectorborne diseases, which have increasingly encumbered local and state health departments tasked with preventing, detecting, reporting, and controlling them. The overall case number masks two distinct trends. Epidemics characterize the mosquitoborne viruses. WNV transmission is effectively limited to the continental United States, whereas most dengue, chikungunya, and Zika virus transmission occurred in the territories. By contrast, the increasing reports of tickborne disease, which occurs almost exclusively in the continental United States, has been gradual. The area at risk for Lyme disease has been expanding.[14] Although Lyme disease accounts for 82% of all reported tickborne diseases, spotted fevers, babesiosis, and anaplasmosis/ehrlichiosis have become increasingly prevalent. Diseases caused by pathogens that were relatively uncommon during the 13-year analysis period remain important because of their historical potential to cause epidemics (e.g., St. Louis encephalitis virus), their high case fatality rates (e.g., eastern equine encephalitis virus), or their potential as bioterror agents (e.g., plague and tularemia).

The reported data substantially underestimate disease occurrence. NNDSS relies on a person seeking care, a clinician requesting appropriate tests, and providers or laboratories reporting to public health authorities. Recent data from clinical and laboratory diagnoses estimate that Lyme disease infects approximately 300,000 Americans yearly, eight- to tenfold more than the number reported.[15,16] Many arbovirus infections result in minimal symptoms. It has been estimated that 30–70 nonneuroinvasive arboviral disease cases occur for every WNV neuroinvasive disease case reported.[17] Based on the number of neuroinvasive disease cases reported in 2016, between 39,300 and 91,700 nonneuroinvasive disease cases of WNV would have been expected to occur, but only 840 (1%–2%) were reported.[17]

The dynamics of vectorborne pathogen transmission are significantly influenced by the characteristics of vector, reservoir, and host. Tickborne pathogens rarely cause sudden epidemics because humans are typically incidental hosts who do not transmit further, and tick mobility is mostly limited to that of its animal hosts. For ticks, the prolonged life cycle and widely separated blood feeds limit opportunities for pathogen transmission. Ixodes scapularis, for example, an important vector of B. burgdorferi, might feed on blood once in a year, but this is compensated for by their broad host preferences and the ability of single ticks to transmit multiple pathogen species. In contrast, the more mobile female mosquitoes feed on blood every 48–72 hours. Dengue, Zika, and chikungunya viruses are typically transmitted directly between humans by the mosquito, Aedes aegypti, after about a week's extrinsic incubation period, resulting in explosive epidemics. WNV is one of the few purely zoonotic vectorborne pathogens with epidemic potential; humans are only at risk from mosquitoes that have fed on viremic birds. There must be a coincidence of flocks with a high prevalence of infection near humans when vector mosquito species are abundant. Bird movement was responsible for WNV's rapid spread across the United States after its introduction to New York City in 1999.

The presence of competent vector species does not alone assure transmission. Ae. aegypti, whose range has been expanding, might now be present in up to 38 states,[18] but despite the frequent arrival of travelers infected with dengue, chikungunya, or Zika viruses, autochthonous transmission has been rare. No local transmission of malaria resulted from the importation of about 1,500 cases annually, even though Anopheles mosquitoes are present in much of the United States. Although the range of Ixodes scapularis extends over much of the eastern United States, transmission of Lyme disease, B. microti babesiosis, and Powassan virus are rare outside of the Northeast and upper Midwest regions. Whatever the biologic, economic, behavioral, or land use reasons for these differences, the presence of vectors with proven or possible capacity to transmit a wide range of pathogens leaves the United States susceptible to outbreaks of exotic vectorborne diseases, as demonstrated by the limited local transmission of dengue and Zika viruses in Florida and Texas.

The findings in this report are subject to at least three limitations. First, underreporting might have substantially limited the number of cases analyzed. As noted, the number of Lyme disease cases reported to NNDSS is estimated to represent a fraction of incident cases. In addition, because many patients with dengue, nonneuroinvasive West Nile, and Zika virus infections experience mild symptoms, they might not seek medical attention. Second, not all the diseases described in this report were reportable for the full 13-year analysis period or from all states and territories; babesiosis data are only available from 2011 from some states. Finally, although CDC collected national dengue data before 2011, the first year it was officially designated as notifiable, it is possible a higher proportion of cases were reported after reporting became mandatory. Overall, it is likely the actual number of vectorborne disease cases substantially exceeds those described in this report.

In the face of increasing incidence and threat from novel pathogens, the burden on local and state public health departments has increased. Critical to effectively preventing or responding to disease outbreaks is sensitive disease and vector surveillance, backed by well-organized, well-prepared, and sustained vector control operations. Good surveillance and reporting depend on rapid, accurate diagnostic confirmation; more sensitive and specific tests that can be used locally are needed. Vaccines against Lyme disease, dengue, chikungunya, and Zika, goals of intense research and development, could reduce risk from those major threats. The tools for vector control are limited but can be effective when implemented rapidly. Ticks have been especially difficult to control,[19] increasing the responsibility for personal protective measures. Nearly all public vector control operations in the United States are locally funded and operated. Networks of vector control operatives are essential to support threat reduction and counter outbreaks, yet in a recent national survey 84% of 1,083 local mosquito control organizations reported lacking one or more of five core vector control competencies.[20] Resources available to assist state and local health departments could be used to develop vector control program competencies.

Reducing vectorborne disease incidence and responding to outbreaks is a large and complex challenge. CDC is using two strategies to mitigate vectorborne threats: advancing innovation and discovery and rebuilding comprehensive vector control programs that have eroded over time.[20] CDC works with states, territories, and tribal councils to compile surveillance data, develop strategies and guidance, and educate the public about specific threats and prevention measures for populations at risk. Expanding sustainable vectorborne disease prevention programs is needed to respond to the ongoing and increasing threat of vectorborne disease.

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