Evidence for Personal Protective Measures to Reduce Human Contact With Blacklegged Ticks and for Environmentally Based Control Methods to Suppress Host-Seeking Blacklegged Ticks and Reduce Infection with Lyme Disease Spirochetes in Tick Vectors and Rodent Reservoirs

Abstract

In the 1980s, the blacklegged tick, Ixodes scapularis Say, and rodents were recognized as the principal vector and reservoir hosts of the Lyme disease spirochete Borrelia burgdorferi in the eastern United States, and deer were incriminated as principal hosts for I. scapularis adults. These realizations led to pioneering studies aiming to reduce the risk for transmission of B. burgdorferi to humans by attacking host-seeking ticks with acaricides, interrupting the enzootic transmission cycle by killing immatures infesting rodent reservoirs by means of acaricide-treated nesting material, or reducing deer abundance to suppress tick numbers. We review the progress over the past three decades in the fields of: 1) prevention of human–tick contact with repellents and permethrin-treated clothing, and 2) suppression of I. scapularis and disruption of enzootic B. burgdorferi transmission with environmentally based control methods. Personal protective measures include synthetic and natural product-based repellents that can be applied to skin and clothing, permethrin sprays for clothing and gear, and permethrin-treated clothing. A wide variety of approaches and products to suppress I. scapularis or disrupt enzootic B. burgdorferi transmission have emerged and been evaluated in field trials. Application of synthetic chemical acaricides is a robust method to suppress host-seeking I. scapularis ticks within a treated area for at least 6–8 wk. Natural product-based acaricides or entomopathogenic fungi have emerged as alternatives to kill host-seeking ticks for homeowners who are unwilling to use synthetic chemical acaricides. However, as compared with synthetic chemical acaricides, these approaches appear less robust in terms of both their killing efficacy and persistence. Use of rodent-targeted topical acaricides represents an alternative for homeowners opposed to open distribution of acaricides to the ground and vegetation on their properties. This host-targeted approach also provides the benefit of the intervention impacting the entire rodent home range. Rodent-targeted oral vaccines against B. burgdorferi and a rodent-targeted antibiotic bait have been evaluated in laboratory and field trials but are not yet commercially available. Targeting of deer—via deer reduction or treatment of deer with topical acaricides—can provide area-wide suppression of host-seeking I. scapularis. These two deer-targeted approaches combine great potential for protection that impacts the entire landscape with severe problems relating to public acceptance or implementation logistics. Integrated use of two or more methods has unfortunately been evaluated in very few published studies, but additional field evaluations of integrated tick and pathogen strategies are underway.

The Blacklegged Tick, Ixodes scapularis : An Increasing Public Health Concern

“…These approaches include: tick repellents and permethrin-treated clothing to prevent human–tick contact; synthetic chemicals, natural products, and biological agents to suppress host-seeking ticks; deer reduction to suppress tick populations; topical application of pesticides to reduce tick burdens on rodents and deer; and antibiotic treatment or vaccination of rodent reservoirs against Lyme borreliosis spirochetes [27]. However, very few approaches have been evaluated with tick-borne diseases as an outcome measure, and we lack evidence for any currently available personal protective measure or environmentally-based tick/pathogen-control method to consistently reduce I. scapularis -borne infections [28,29].…”

“…Previous reviews have addressed (i) personal protective measures to reduce human contact with I. scapularis ticks and environmentally based control methods to suppress host-seeking ticks and reduce infection with Lyme disease spirochetes in tick vectors and rodent reservoirs [25–27,101]; (ii) the evidence base for such measures, and methods to reduce Lyme disease [28,29,102,103]; and (iii) the prospect for a human Lyme disease vaccine to re-emerge in the wake of the rise and fall of Lymerix, an effective licensed vaccine that was removed from the US market in 2003 [104,105]. Despite the emergence of a wide array of approaches to avoid contact with ticks through personal protective measures, suppress host-seeking I. scapularis, or disrupt enzootic B. burgdorferi transmission, we unfortunately still lack robust evidence for any method other than a human Lyme disease vaccine to reduce disease cases.…”

“…This can be achieved by the use of tick repellents on skin and clothing, or the use of permethrin-treated clothing [27,116], combined with regular checks for crawling ticks, changing clothes when coming inside (and drying removed clothing articles at high heat), and taking a shower (ensuring removal of clothes worn outside, increasing the likelihood of detecting crawling ticks, and perhaps also dislodging crawling ticks while showering) [25,29,95]. The main problem is that all of these actions require a level of daily vigilance and effort that is hard to keep up for the period of 2–3 months during which I. scapularis nymphs are most active.…”

“…Even further distant from the human infection, we can intervene by minimizing contact with host-seeking ticks (Figure 5) through (i) avoidance of habitats with a high risk for tick contact (easier said than done if it includes your backyard), (ii) reduction in longevity or survival of desiccation-sensitive host-seeking I. scapularis ticks in the peridomestic environment (e.g., xeriscaping, hardscaping, and vegetation management, including keeping grass short, clearing brush and removing leaf litter), or (iii) direct killing of host-seeking ticks with acaricides or biological control agents [24,25,27,29,117]. Here we introduce increased complexity by relying on solutions that are highly sensitive to human movement patterns.…”

“…Here we introduce increased complexity by relying on solutions that are highly sensitive to human movement patterns. For example, controlled experimental spring applications of pyrethroids typically reduce host-seeking I. scapularis nymphs by >85% [27]. Nevertheless, a recent effort to reduce tick bites and human I. scapularis -borne infections by spraying pyrethroids along the lawn–wood interface on residential properties, rather than treating all the wooded and brushy high-risk habitat on the properties, achieved 50–70% suppression of host-seeking I. scapularis nymphs within the areas sprayed but failed to reduce either tick bites or human infection [118].…”