Stemming the Rising Tide of Human-Biting Ticks and Tickborne Diseases, United States

Lars Eisen

Disclosures

Emerging Infectious Diseases. 2020;26(4):641-647. 

In This Article

Shifting Sands of Ticks and Tickborne Diseases

The concept of tick control as an individual homeowner responsibility emerged, in part, from the knowledge gained about I. scapularis ticks, the Lyme disease spirochete (Borrelia burgdorferi sensu stricto), and tick encounter locations in the late 1980s and the first half of the 1990s, which made perfect sense at that time. Lyme disease was the near absolute focus among tickborne diseases, most of human infections occurred in the northeastern United States, and residential properties were pinpointed as the most common location for encounters with I. scapularis ticks in Lyme disease–endemic areas.[19,21,39,40] Moreover, as is still the case, both broadcast application of residual acaricides to the vegetation and placement of rodent-targeted tick control devices require physical access for control to be implemented on private properties. The difficulty in accessing these residential high-risk environments presented (and still presents) a major impediment for development of community-driven tick control, and the main focus was therefore on devising tick suppression approaches intended for use in backyards and tick-bite prevention measures for personal protection.[19] The notable exception was approaches targeting white-tailed deer, which were recognized as dominant hosts for the adult life stage of I. scapularis ticks and potentially represent a weak link in the life cycle of the tick.[41] With the exception of deer fencing, which can be used for single residential properties, deer-targeted tick control approaches (i.e., deer reduction or treatment of deer with topical acaricide) require area-wide implementation to be successful. There is broad consensus that the white-tailed deer is a main driver for the remarkable increase in I. scapularis ticks in the northern parts of the eastern United States over the past 40 years.[17,19,42,43] However, fierce debate continues about the specific thresholds required to be reached for either deer reduction (achieving a sufficiently low deer density) or topical treatment of deer with acaricides (achieving a sufficiently high level of treatment coverage in the deer population) to suppress I. scapularis tick populations to the point where we also see an effect on human tick bites and tickborne diseases.[17,19,43–45] Despite promising results in some studies,[43,45] neither deer reduction nor topical treatment of deer with acaricides has, to date, been widely used operationally to control I. scapularis ticks.

In the 25 years since control of human-biting ticks in the United States evolved into an individual homeowner responsibility, the sands of ticks and tickborne diseases have shifted dramatically, and we are no longer facing the same problems as in the 1990s. Although there is empirical evidence that I. scapularis tick bites still result most commonly from tick encounters on residential properties in suburban/exurban settings of the northeastern United States,[46] ongoing spread and population increase of this tick across the northern part of the eastern United States might have resulted in a more spatially diffuse risk for tick encounters as the density of host-seeking I. scapularis ticks reached levels across the landscape where even activities of limited duration (compared with the time spent in your own backyard) increasingly results in tick encounters. A recent systematic review and meta-analysis on spatial risk factors for I. scapularis tick bites and I. scapularis tick–associated diseases in eastern North America concluded that risk occurs in backyards, as well as in neighborhood green spaces and public lands used for recreation.[47]

Expanding ranges of other human-biting vector ticks contribute to a changing risk scenario for tick bites. Jordan and Egizi[48] reported that during 2006–2016, the vector tick species most commonly collected from humans and submitted to a passive tick surveillance system in New Jersey shifted from I. scapularis to A. americanum. Both A. americanum ticks and the Gulf Coast tick (A. maculatum) are spreading northward from their previous core ranges in the southeastern United States,[27–29] and we now also have the invasive H. longicornis tick to contend with along the Eastern Seaboard, as far north as New York state.[31]

Lyme disease is still by far the most commonly reported tickborne disease in the eastern United States, where 2 primary causative agents (B. burgdorferi sensu stricto across the eastern half and B. mayonii in the upper Midwest) are transmitted by I. scapularis ticks.[2] However, several other tickborne illnesses, as well as co-infections with Lyme disease, are on the rise and increasingly recognized as serious health threats. These illnesses include conditions caused by viral, bacterial, and parasitic pathogens transmitted by I. scapularis ticks (Anaplasma phagocytophilum, Babesia microti, B. miyamotoi, Ehrlichia muris eauclairensis, and Powassan virus), A. americanum ticks (E. chaffeensis, E. ewingii, Bourbon virus, and Heartland virus), and A. maculatum ticks (Rickettsia parkeri).[1,16,27,28]

In contrast to the situation in the Northeast and upper Midwest, I. scapularis ticks are only a minor public health threat compared with Amblyomma ticks in the Southeast. Moreover, the potential involvement of A. americanum ticks in red meat allergy is concerning because this notorious human-biter is not only abundant in the Southeast but also expanding its range north and thus affecting new human populations.[28,29] Finally, the American dog tick (Dermacentor variabilis) remains a threat across its wide geographic range as a vector of the agents causing Rocky Mountain spotted fever (R. rickettsii) and tularemia (Francisella tularensis).[1] Other vector tick species similarly are public health concerns in the Rocky Mountain region and the far western United States, including the western blacklegged tick (I. pacificus), the Rocky Mountain wood tick (D. andersoni), the Pacific Coast tick (D. occidentalis), the brown dog tick (Rhipicephalus sanguineus), and Ornithodoros spp. soft ticks.[1]

The strategies devised 2 decades ago to address I. scapularis ticks and Lyme disease spirochetes on residential properties in the Northeast are not necessarily well suited to address the current broader, more complex, and spatially diffuse threat of ticks and tickborne diseases in the United States. There is hope that a badly needed human Lyme disease vaccine will be found, but this will only solve 1 part of the overall problem with tickborne pathogens and it will not have any effect on tick populations. Because no silver bullets are on the near horizon to broadly address the increasing threat of ticks and tickborne diseases in the United States, we must reassess the problem and consider new shorter-term solutions.

One reasonable assessment, based on the experience over the past 25 years and the steadily worsening problem, is that the responsibility for tick and pathogen control must be shifted to include both individual persons (responsible for their own properties and use of personal protection measures) and local public health programs with professional staff (responsible for public outreach, assistance to homeowners with selection of appropriate tick control options, and control of ticks and tickborne pathogens in high-use risk areas, such as neighborhood green spaces and picnic areas and hiking trails on public lands). This 2-pronged concept for responsibility should be accompanied by a 2-pronged spatial concept: first, making the backyard a safe, tick-free zone; and second, achieving area-wide suppression of ticks and tickborne pathogens to reduce the risk for tick encounters in other high-use environments.

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