Wednesday, 27 July 2016

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

DOI: tjw103 First published online: 20 July 2016


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.
  • Borrelia burgdorferi
  • Ixodes scapularis
  • blacklegged tick
  • Lyme disease
  • risk management
In the early 1980s, the blacklegged tick, Ixodes scapularis Say (including the junior synonym Ixodes dammini Spielman, Clifford, Piesman & Corwin), was implicated as a vector to humans in the eastern United States of the Lyme disease spirochete Borrelia burgdorferi (Burgdorfer et al. 1982; Spielman et al. 1985; Piesman et al. 1987a,b). Rodents, particularly the white-footed mouse, Peromyscus leucopus (Rafinesque), were recognized as primary enzootic spirochete reservoirs (Levine et al. 1985, Donahue et al. 1987, Mather et al. 1989) and the white-tailed deer, Odocoileus virginianus (Zimmerman), was shown to be the principal host for the adult stage of I. scapularis (Piesman et al. 1979, Main et al. 1981). These findings led to pioneering field studies aiming to reduce the risk for transmission of B. burgdorferi to humans by directly attacking host-seeking ticks with acaricide applied to the ground substrate and vegetation (Schulze et al. 1987), interrupting the enzootic transmission cycle by killing immatures infesting rodent reservoirs by means of acaricide-treated nesting material (Mather et al. 1987a), or reducing the abundance of white-tailed deer to suppress tick numbers (Wilson et al. 1988). Nearly three decades later, 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 through environmentally based control methods have emerged.
We review the evidence for personal protective measures to reduce human contact with I. scapularis and for environmentally based control methods to suppress host-seeking nymphs and B. burgdorferi infection in nymphs and rodent reservoirs. Published literature was queried by searching the Scopus database, last done in December 2015. The search spanned the years 1960 to present and used the following key words: 1) “Ixodes scapularis” and 2) “Ixodes dammini”. Additional searches using the same key words were conducted in PubMed and the Armed Forces Pest Management Board’s Literature Retrieval System. The snowball technique, which identifies additional publications based on referenced materials, was then employed to identify additional publications of interest. Because most human infections with B. burgdorferi in the eastern United States are considered to result from bites by infected I. scapularis nymphs (Spielman et al. 1985; Piesman 1987a; Falco et al. 1996, 1999; Mead 2015), we focus primarily on the impact of personal protective measures against nymphal tick bites, and the impact of environmentally based interventions on the abundance of host-seeking nymphs, infection rates of host-seeking nymphs with B. burgdorferi, and the abundance of infected nymphs. As used in this paper, data for abundance or density of host-seeking ticks (e.g., <0.1 nymphs/100 m2) generated by drag or flag sampling should be interpreted as relative abundance and relative density rather than as absolute estimates of the nymphal population present. Prospects for current personal protective measures and environmentally based tick and pathogen suppression methods to reduce Lyme disease will be discussed in a separate forthcoming paper.