The Need for a National Strategy to Address Vector-Borne Disease Threats in the United States

Vector-borne diseases (VBDs) are of great importance in the United States, resulting in significant morbidity and mortality each year. From 2004 to 2017, over 700,000 cases of 16 diseases caused by viral, bacterial, or parasitic agents were reported to the Centers for Disease Control and Prevention (CDC) (Centers for Disease Control and Prevention 2018, Curren et al. 2018, Rosenberg et al. 2018). In 2017, the top five reported VBDs were Lyme disease (42,743 cases), anaplasmosis/ehrlichiosis (7,718 cases), spotted fever rickettsiosis (7,718 cases), babesiosis (2,368 cases), and West Nile virus (WNV) disease (2,097 cases). Among the reported WNV disease cases, there were 146 deaths.

In terms of the total burden of illness associated with VBDs each year, it is important to note that reported cases represent only a small fraction of those sickened by these infections. The reasons for under-reporting vary by disease. For WNV infection, most reported cases have neuroinvasive disease (meningitis, encephalitis, and acute flaccid paralysis) as these patients are often hospitalized and tested. However, for every one case of WNV neuroinvasive disease, 30–70 WNV non-neuroinvasive cases occur (Burakoff et al. 2018). Yet only a tiny fraction of those are reported, in large part due to under-diagnosis. Many patients do not seek medical care, physicians may not consider WNV as a possible diagnosis, and diagnostic tests may not be ordered. For Lyme disease, under-reporting is not influenced so much by under-diagnosis as by the burden associated with the reporting process itself, which can require both laboratory and supportive clinical data for case classification. Recent studies indicate that the total number of annual cases likely exceeds 300,000 (Hinckley et al. 2014, Nelson et al. 2015), suggesting that national under-reporting for Lyme disease occurs by a factor of around 8- and 12-fold.

VBD Trends

VBD cases in the United States have steadily increased over the last two decades. From 2004 to 2016, the number of reported VBD cases tripled (Rosenberg et al. 2018). Two primary trends characterize the observed increase in reported cases. The first is a steady increase in tick-borne disease incidence over a broadening geographic distribution (Figs. 1 and 2). For Lyme disease, from 1993 to 2012, the number of high-incidence counties more than tripled in the northeastern United States and more than doubled in the north-central United States (Kugeler et al. 2015). Nearly all other tick-borne disease cases have also increased as well, as evidenced by record numbers of cases for all reportable tick-borne diseases in 2017 (Centers for Disease Control and Prevention 2018). Ixodes scapularis Say (Acari: Ixodidae), the most important tick vector in the United States, can transmit the pathogens that cause borreliosis, anaplasmosis, babesiosis, and Powassan virus disease. The number of counties where I. scapularis is established has more than doubled in the last 20 yr (Eisen et al. 2016).

A second observed trend accounting for the increase in VBDs in the United States over the last two decades is increasing intermittent outbreaks of mosquito-borne arboviruses. These include WNV, which has become endemic throughout the contiguous United States during this time, as well as exotic diseases like chikungunya and Zika, which have been more recently introduced to the western hemisphere. In the U.S. territories, dengue is a constant threat, resulting in widespread periodic outbreaks. Large outbreaks of dengue, chikungunya, and Zika in the Caribbean region have led to introductions into the continental United States with subsequent local mosquito-borne transmission (Brathwaite Dick et al. 2012, Kendrick et al. 2014, Likos et al. 2016). Most recently, during 2016 and 2017, there were 42,798 symptomatic cases of Zika reported in U.S. states and territories, including 37,032 locally acquired cases in Puerto Rico and 5,334 travel-related and 231 locally acquired cases, presumably mosquito-transmitted cases, in the continental United States (Centers for Disease Control and Prevention 2019a).

Drivers for VBD Emergence

The factors contributing to VBD introduction and emergence in the United States are complex and vary among diseases. Global travel and trade have been responsible for introduction of exotic arboviruses going all the way back to the slave trade in the 1600s when both yellow fever virus and its mosquito vector Aedes aegypti (L.) (Diptera: Culicidae) were presumably introduced from Africa (Barrett and Monath 2003). Shipments of used tires and lucky bamboo have transported Aedes albopictus (Skuse) (Diptera: Culicidae) throughout the Americas (Madon et al. 2002). Both Zika and chikungunya were introduced into the United States via infected travelers (Kendrick et al. 2014, Likos et al. 2016). Most recently, the Asian Longhorned tick Haemaphysalis longicornis Neumann (Acari: Ixodidae) has been documented as an invasive species now reported from nine states and detected biting a range of animals, including livestock, wildlife, pets, and humans (Beard et al. 2018). This tick is an important disease vector in other parts of the world and was presumably introduced into the United States through movement of pets or livestock.

Quite different factors have likely contributed to the emergence of arboviral and ‘arbobacterial’ diseases associated with I. scapularis and some of the other native tick species in the United States. The geographic expansion of Lyme disease and accompanying increase in human infection has been associated with changing land use patterns, chiefly with reforestation, expanding deer populations, changes in biodiversity, and growth of suburbia (Wood and Lafferty 2013). This combination of factors has placed an ever-increasing human population at greater risk for exposure to the bites of infected ticks. Climate change has no doubt influenced both the geographic distribution and seasonality of tick-borne diseases (Beard 2016). Temperature minimums define the northern distribution of arthropod vectors and influence their basic reproductive rate and subsequent population size (Ogden et al. 2006, Ogden et al. 2014). Milder winters, earlier springs, warmer summers, and longer growing seasons have been linked to changes in disease seasonality with Lyme disease (Moore et al. 2014, Monaghan et al. 2015, Beard et al. 2016).

Travel, trade, changing land-use patterns, growth of suburbia, and climate change are factors that are increasing at accelerating rates. Consequently, the effect of these factors as drivers for VBD emergence are likely to increase, strongly suggesting that in the absence of effective prevention and control tools and the capacity to implement them in a timely manner, the trends and patterns observed over the last two decades will continue and likely worsen.

Challenges to Preventing and Controlling VBDs

Several factors hinder prevention and control of VBDs. Currently, no licensed vaccines exist for any endemic human VBD in the United States. As a result, VBD prevention relies on vector control efforts and personal protection. Yet there is a dearth of high-quality data demonstrating the efficacy of vector control on reducing disease occurrence during seasonal exposure or outbreaks. For example, a randomized control trial involving over 2,700 homes, looking at a single springtime barrier acaricide application achieved 63% reduction in ticks between treated and untreated properties. There was no reduction, however, in the number of human tick encounters or physician-diagnosed Lyme disease cases (Hinckley et al. 2016). There is a similar lack of robust data from randomized control trials that demonstrate the efficacy of available interventions for control of mosquito-borne diseases (Wilson et al. 2015). High-quality data from rigorously designed studies that capture both entomologic and epidemiologic outcomes are badly needed for making sound, evidence-based public health recommendations. Large prospective community trials that are adequately powered to measure a reduction in disease incidence are very expensive, which is the reason why most data on the efficacy of vector control for prevention of VBDs come from observational studies.

Another hindrance to effective vector control is insecticide resistance. This was particularly important in Puerto Rico during the Zika outbreak where populations of Ae. aegypti from San Juan were resistant to multiple insecticides, including both Type I and Type II pyrethroids (Estep et al. 2017). Insecticide resistance varies by target species and can be highly localized. However, relatively little systematically collected information is available to guide effective vector control in the United States. A recent survey that evaluated 1,083 vector control organizations found that 84% failed to maintain at least one of the five core competencies for effective vector control (National Association of County and City Health Officials 2017). These competencies included such things as conducting routine mosquito surveillance and pesticide resistance monitoring and then using the resulting data to guide operational decisions. Maintaining high-quality information on vector population density and susceptibility to available pesticides is critical for effective vector control.

Still other challenges exist to VBD prevention and control activities such as public opposition to the use of synthetic pesticides. In Miami, Florida, there was significant public protest to aerial spraying of the insecticide Naled during the Zika virus outbreak in 2016, despite the occurrence of active local virus transmission (Karimi and Visser 2016). In Puerto Rico, during the same outbreak, public outcry resulted in a decision not to allow aerial spraying at all despite over 35,000 locally acquired Zika cases. The lack of a timely response can also compromise the effectiveness of vector control interventions. During the 2012 WNV outbreak in Texas, there were 1,868 reported cases including 844 cases of WNV neuroinvasive disease and 89 deaths (Centers for Disease Control and Prevention 2019b). Following a mild winter and early spring, the first positive mosquito pool was detected in the Dallas-Fort Worth area in late May close to when the first human case was reported (Chung et al. 2013). The number of cases rose precipitously from mid-June through early August. Aerial spraying was not implemented, however, until mid-August, after human case numbers were already declining. These data suggest that early vector-control interventions based on vector surveillance data are not always implemented effectively.

For tick control, the challenges are quite different. Interventions are seldom carried out by municipal governments or vector control districts but more frequently implemented by private property owners, chiefly homeowners. The deer that host adult I. scapularis and the rodents that harbor the various pathogens transmitted by this tick range freely between homeowner properties and adjacent green spaces, depositing ticks that can infect residents. Identifying interventions that are effective and affordable for homeowners has been a significant challenge for tick-borne disease prevention and control (Eisen and Eisen 2018). In the absence of a vaccine for any of these pathogens, safe and effective tick control methods must be developed and validated for use in local communities in order to reverse the upward trends in tick-borne disease incidence.

The Need for a National Strategy

The increasing disease incidence and other challenges noted above, together with eroding technical capacities in public health entomology at federal, state, and local levels (Hadler et al. 2014), leave the nation largely unprepared to respond to VBD threats. A recent call to action was issued to create a national strategy for addressing this need (Petersen et al. 2019). Both development and implementation of such a strategy will require collaboration between stakeholders at multiple governmental levels, together with partners from academics, industry, patient organizations, and communities. An effective national strategy should 1) develop a vision, mission, goals, and substrategies within the goals, and 2) describe accountability among the federal agencies and institutes. Once a national strategy has been created, detailed action plans can be developed and prioritized, which can be used to define key activities and specify success measures and timelines. Some of the specific needs that should be addressed and sustained in a national strategy to respond to VBD emerging threats include 1) basic understanding of VBD biology, ecology, and exposure risk, 2) vector and pathogen detection and diagnosis, 3) disease prevention and control tools and methods, 4) facilitated process to bring new vector control products to market and deploy them when needed, and 5) adequate workforce and resource capacity.

The complex nature of VBD transmission cycles, typically involving zoonotic reservoirs and vector populations, both of which are broadly dispersed in the environment, pose unique technological challenges for effective control. Consequently, successful efforts to establish and maintain an effective national system to detect and respond to emerging VBD threats will require a high level of coordination, cooperation, and perseverance involving all of the key stakeholders.

The findings and conclusions in this report are those of the authors and do not necessarily represent the official position of CDC.

References Cited