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Daily News Blog

22
Jul

Deer Ticks Developing Resistance to Popular Tick Control Chemical: Implications of Lyme Disease

(Beyond Pesticides, July 22, 2021) A new study published in the Journal of Medical Entomology finds black-legged ticks (Ixodes scapulari) in New York are developing potential resistance to widely used tick-control pyrethroid insecticide, permethrin. The study suggests continuous use of area-wide, 4-poster devices (devices that attract deer and then apply pesticide to their head, ears, and neck) to apply insecticide treatments on deer to control tick populations promotes resistance.

Resistance is an ever-present issue among chemical compounds (i.e., antibiotics, antimicrobials, pesticides) used in medicine and agriculture, and threatening the ability to prevent disease outbreaks, such as Lyme disease. Furthermore, increasing populations of rodent and mammalian hosts, in addition to warmer temperatures prompted by the climate crisis, allows for disease-carrying ticks to flourish. Lyme disease is the most common vector disease and a primary concern for the general population. Therefore, studies like this highlight the need to assess resistance among disease-vector pest populations regardless of pesticide application methods. The researchers note, “Permethrin susceptibility of tick populations should be monitored from other 4-poster control areas so that guidelines for managing pesticide resistance in the field can be developed.”

Four-poster devices impart selective pressure on tick populations influencing reproduction and natural extinction of species. However, like mosquitoes, a subpopulation of ticks encountering chemical exposure naturally  alter gene function, which results in resistance to the chemical rather than death. To assess resistance among tick populations, researchers evaluated the susceptibility of deer ticks to permethrin exposure. Deer ticks used in this study came from Shelter Island, NY, and the Cary Institute of Ecosystem Studies (CIES) in Millbrook, NY. Researchers collected Shelter Island ticks from deer at sites where 4-poster devices are in operation. CIES tick collection, also from deer, took place in areas void of 4-poster devices. Researchers used deer tick larvae colonies reared from the Center for Disease Control and Prevention (CDC) laboratory as the control.

The study results find ticks at the Shelter Island site are less susceptible to permethrin treatments than ticks from CDC colonies. Shelter Island ticks are less sensitive to permethrin treatments than CIES ticks, but resistance ratios are small. Although CIES sites did not contain 4-poster devices, these areas still harbor ticks that are less susceptible to permethrin. From these findings, the researchers conclude that field populations of deer tick may be more resistant to permethrin insecticides than lab colonies.

Insecticide resistance has been an issue since the introduction of DDT (dichlorodiphenyltrichloroethane) in the 1940s. Although most countries currently ban DDT use, the compound is not the only chemical pesticide promoting pest resistance. Several current-use insecticides pose the same threat. Area-wide, indiscriminate spraying of insecticides is causing resistance to develop among many pests. Mosquitoes have become increasingly resistant to synthetic pyrethroids, in addition to other classes of insecticides, such as carbamates and organophosphates. In certain tropical regions, pesticide run-off from agricultural fields increases chemically resistant freshwater snails populations that are disease vectors for snail fever. However—unlike mosquito resistance—the occurrence of tick resistance lacks research regarding vector-disease implications and primarily focuses on domestic animal use. Tick resistance develops much slower than mosquito resistance due to a longer lifespan/life cycle and time between generations. However, studies show that cattle ticks are resistant to multiple pesticide chemical classes from direct applications to cattle. Furthermore, three-host species ticks in Florida and Texas demonstrate similar permethrin resistance. Therefore, existing disease control programs overlook possible disease outbreaks from inadequate tick resistance management and a heavy focus on domestic animal uses. 

Development of resistance is an entirely normal, adaptive phenomenon: organisms evolve, “exploiting” beneficial genetic mutations that give them a survival advantage. However, resistance is growing in all sectors of pest control, including critically needed agriculture and medicine. Whether it is antibiotics for bacterial infections, herbicides for weeds/pests, or insecticides to mitigate vector-borne diseases, organisms are becoming resistant to usually toxic compounds. Resistance developing in one of the “sectors” mentioned above can “crossover” to become problematic in another. Agricultural and veterinary uses of antibiotics significantly contribute to the resistance of certain bacteria or fungi to antibiotics that have historically knocked down such infections in humans. Pesticide Action Network North America (PANNA) notes, “The World Health Organization underscored the problem in their 2012 guidance on policymaking for Integrated Vector Management (IVM): ‘Resistance to insecticides is an increasing problem in vector control because of the reliance on chemical control and expanding operations…Furthermore, the chemical insecticides used can have adverse effects on health and the environment.’”

This study is the first to establish baseline susceptibility for deer ticks to permethrin and offer information on resistance emergence in tick populations under selective pressure from 4-poster devices. Unlike in mosquito control, area-wide pesticide applications for tick control are relatively unconventional. However, 4-poster devices act as an area-wide control method allowing deer to self-apply permethrin treatment via contact with devices while feeding. Although the study finds Shelter Island ticks are less susceptible to permethrin than CIES ticks, the difference in susceptibility is small enough to assert resistance can develop regardless of selective pressure. This assertion indicates permethrin resistance may occur among tick populations in real-world settings already seen among mosquito populations. Growing pesticide resistance often leads to an increase in chemical inputs to control pests.

Tick resistance can augment the use of chemical control methods, including the addition of toxic synergists like piperonyl butoxide (PBO), known to cause and exacerbate adverse health effects from exposure. Exposure to permethrin already has implications for human health, including cancer, endocrine (hormone) disruption, reproductive dysfunction, neurotoxicity, and kidney/liver damage. Furthermore, pets such as cats are extremely sensitive to synthetic pyrethroid insecticides, triggering seizures, tremors, muscle spasms that can lead to death. 

The study results demonstrate a need to address resistance among tick populations, regardless of selective pressure from 4-poster devices. The CDC previously reported that pesticides are ineffective at stopping the spread of Lyme disease, a principal health concern of increasing tick resistance. Therefore, public health advocates say that government and health officials must understand the mechanisms prompting pesticide resistance among tick populations to safeguard human health from widespread diseases lacking effective vaccines.

Jay Feldman, executive director of Beyond Pesticides, has noted, “We should . . . join together to address the root causes of insect-borne disease because the chemical-dependent alternatives are ultimately deadly for everyone.”

Beyond Pesticides maintains that management strategies to combat insect-borne diseases cannot be successful if they are chemical-intensive. More recent studies focusing on highly toxic permethrin-treated clothing, as a means of tick management, have not been accurately reported in the media. . These strategies ignore the underlying conditions that exacerbate the spread of the disease. Advocates urge that consumers avoid chemicals like DEET and permethrin. Instead, simple repellents like the oil of lemon eucalyptus, picaridin, and insect repellent IR 3535 can effectively deter ticks from finding and attaching to humans. 

The best method to prevent tick bites and the diseases they carry is to wear appropriate clothing (light-colored that covers one’s whole body), a hat, and consider tucking one’s pants into socks. Most important is to conduct regular tick checks as it is critically important to detach a tick from one’s skin as soon as possible after the bite to reduce the chance of disease transfer. If you have an outdoor pet, do not forget to check them as well. Safely kill tick larvae with non-toxic solutions: vacuum daily during flea season (changing bag often); groom pet daily with a flea comb (cleaning comb with soap-water between brushes); frequently bathe pets with soap and water; and frequently wash pet bedding, restricting pet to only one bed. For more information on how to manage ticks safely, see Beyond Pesticides ManageSafe webpage. Additionally, learn more about how to protect your pet from pesticides and the least-toxic controls for tick infestations. 

All unattributed positions and opinions in this piece are those of Beyond Pesticides.

Source: Lyme Disease Association, Journal of Medical Entomology

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