By Hannah Foster
The onset of spring and summer means barbecues, camping, hiking, and a plethora of other outdoor activities. However, warmer weather also means ticks. Tick bites spread numerous pathogens that can cause serious diseases. Among these pathogens is the bacterial species Borrelia burgdorferi. The blacklegged tick (Ixodes scapularis) carries and transmits B. burgdorferi to humans, causing Lyme disease. With the rate of Lyme disease infections increasing over the past two decades, more and more people are concerned with how they can prevent tick bites and eliminate the pests and the risk of disease.
In order to help people make informed decisions about the best way to protect themselves, Dr. Lars Eisen and Marc Dolan of the Centers for Disease Control reviewed scientific literature reporting the effectiveness of various methods at controlling blacklegged ticks. Their findings are published in the Journal of Medical Entomology. Lyme disease is usually spread to humans by bites from nymphal ticks (as opposed to larval or adult ticks), so Eisen and Dolan focused on studies concerned with the control of host-seeking nymphs with B. burgdorferi. Methods of tick-bite prevention, black-legged tick control, and confining the spread of disease, fall into two main categories: 1) personal protection against tick bites, and 2) controlling ticks and the spread of B. burgdorferi in the environment.
1) Personal protection
The best line of defense against tick bites for individuals is to wear long-sleeves, socks, shoes, and some form of insect repellent. Eisen and Dolan report that typical synthetic insect repellents containing deet reduce ticks crawling on subjects by more than 80 percent. Not only are synthetic repellents effective, but natural products, such as nootkatone (citrus ingredient), carvacrol (essential oil of oregano), and essential oil combinations were equally, and in some cases, even more effective. Unfortunately, few researchers have studied the effectiveness of wearing insect repellent with typical summer attire, so we have little information regarding insect repellents applied directly to skin.
A similar method of personal protection involves the use of permethrin-treated clothing. Permethrin is a contact toxicant that cannot be applied directly to skin. The chemical does not repel ticks very well, but permethrin kills ticks that come into contact with treated clothing. Using treated clothing was similarly effective to other repellents at preventing tick bites, since most ticks died within a few hours of attaching to people wearing permethrin-treated clothes.
For more information on ways you can prevent tick bites, check out http://www.cdc.gov/lyme/prev/on_people.html.
2) Controlling ticks and the spread of B. burgdorferi in the environment
Personal protection is an important aspect of tick-bite prevention, but many homeowners want to reduce tick populations and B. burgdorferi infections on their property. This can be accomplished using landscape management, environmental treatments with synthetic or natural compounds to kill ticks, or with host-targeted treatments. Researchers are confronted with three main challenges when considering these types of treatments. One obstacle is that treatments must maintain their efficacy for the majority of the 2-3 months when nymphs are active, in spite of weather and outdoor conditions, in order to have a lasting impact. The second challenge involves the fact that most homeowners usually have access only to low-pressure application systems like handheld and backpack sprayers, which are less likely to reach ticks hidden in the undergrowth. Lastly, most synthetic chemicals cannot be applied near water, wellheads, wetlands, or plants that will be eaten. All three of these must be taken into consideration when a home-owner is choosing a treatment method, application system, and number and frequency of treatments.
Landscape management can help reduce the number of ticks in a person’s yard even without the use of chemical treatments. Removal of leaf litter from yards by raking or leaf blowing can reduce the abundance of host-seeking nymphs by nearly 80 percent. Burning vegetation may also reduce tick populations, but authors reported variable results for this method. Since ticks tend to be more abundant around the edges of yards or in wooded areas surrounding yards, researchers have tested the effectiveness of creating meter-wide barriers, made from substances like gravel or wood chips, between the forest and lawn edges. Of the substances tested, the use of Alaska yellow cedar sawdust may be the most promising so far, since its efficacy continues even after four weeks of outdoor exposure. However, researchers have not quantified the effectiveness of barriers, nor have they determined an appropriate barrier width.
Treating ground substrate and vegetation with either synthetic or natural acaricides (toxins used to kill ticks or mites) can also help to reduce tick populations. In terms of synthetic acaricides, Eisen and Dolan discuss the effectiveness of organophosphates, carbamates, and pyrethroids. All three types proved effective at reducing host-seeking nymph abundance up to eight weeks. However, most studies have only looked at high-pressure application of these chemicals. Of those that were studied with low-pressure application methods, pyrethyroids gave very promising results, showing a reduction by 85 percent of host-seeking nymphs for up to seven weeks.
Many homeowners are reluctant to apply synthetic chemicals to their property. Thus, studies are underway to determine the effectiveness of natural acaricides. By far, the most well-studied of these are pyrethrin and nootkatone, but results are quite mixed from one study to the next. The effectiveness of pyrethrin and nootkatone seems to be mainly limited by their loss of efficacy within one to four weeks after application. One study did find that high-pressure application of nootkatone may reduce host-seeking ticks for up to six weeks, but a similar study saw a reduction of two weeks tops. New research suggests that two well-timed applications of a natural acaricide, even with a low-pressure sprayer, might have a more substantial effect on tick populations.
A number of less conventional tick-control methods are currently being developed and tested. TickBot, a robotic device that carries a permethrin-treated cloth along a guide wire, has proven effective at suppressing another tick species for up to 24 hours. However, it has not been tested with the blacklegged tick. Biological agents, such as a fungi that infect blacklegged ticks or parasitoid wasps that lay their eggs in tick larvae or nymphs, may also be somewhat effective at reducing host-seeking nymph populations. However, the use of biological agents shows similar limitations to the application of natural acaricides, as they are greatly affected by weather and environmental conditions.
Host-targeted control methods
Targeting the hosts of the blacklegged tick is another environmentally-based method of controlling the spread of Lyme disease. Black-legged ticks have two main hosts: the white-footed mouse (Peromyscus leucopus) or other small mammals or birds during the larval and nymphal stages, and white-tailed deer (Odocoileus virginianus) in the adult stage. Deer are considered the main reproductive host for the blacklegged tick. Thus, control of deer populations or deer exclosures can significantly limit blacklegged tick numbers. Treating deer with acaricides also appears to be relatively effective at reducing host-seeking blacklegged tick nymphs.
Much of the research on host-targeted control methods has focused on the white-footed mouse, since these rodents serve as reservoirs for B. burgdorferi. The mice can be treated with with acaricides, antibiotics, or vaccines. Multiple methods of rodent-targeted acaricides are under investigation, but the widest-studied is the Damminix® Tick Tube. These tubes contain permethrin-treated cotton balls, which mice use as nesting material. In some cases, studies showed that the tubes nearly entirely eliminated larvae and nymphs on white-footed mice. However, in other studies, results were much less convincing. The mixed results may have to do with the quantity of available alternate hosts (other small mammals or birds) for the ticks.
Other possible treatments include rodent-targeted baits with antibiotics to treat the bacterial infections, or vaccines to prevent infection. However, few studies have been published on these methods. Some major obstacles include the possibility of the development of antibiotic-resistant bacteria if antibiotics are used to treat the mice. A preliminary study looking at an oral vaccine for mice against Lyme disease showed promising results, but currently, no such vaccine is commercially available.
For more information on ways to prevent ticks in your yard, check out this CDC web page.
Eisen and Dolan stress that research on Lyme disease prevention and blacklegged tick control is far from over.
“One of the most important outcomes from this review of the existing evidence base was to clarify the outstanding knowledge gaps, and they are substantial,” Eisen said.
It is unclear at this point what the tick-control method of the future will be, but Eisen suggests that, in the absence of a human vaccine for Lyme disease, “no single method will be sufficient to substantially reduce Lyme disease.” Therefore, a main goal of researchers is not only to develop new treatment methods, but to assess the effectiveness of combined treatments. As Eisen puts it, “The challenges are great, but we are working hard to define the best way forward to close knowledge gaps and generate the critical data needed to provide evidence that certain single or integrated methods do indeed reduce human bites by blacklegged ticks and Lyme disease.”
Read more at:
– 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
Hannah Foster is a freelance writer and a postdoctoral researcher in bacteriology at the University of Wisconsin-Madison. She studies protein biochemistry in microbes, and enjoys writing about science and non-science alike. You can follow her on Twitter at @Foster_HR and read her blog about boxing as it pertains to life at theblowbyblow.com. She is also a frequent contributor to Harvard Science in The News Flash and to The Bitter Empire.