Climate and habitat changes affect tick population distribution

As ecosystems begin to change due to climate and human interference, tick species within them tend to experience a shift in distribution. The significance of this finding could change how ecosystem functions are interpreted.

As ecosystems begin to change, the tick species within them tend to experience a shift in distribution. As colder climates warm, warm climate species can expand their territory. The addition of human-caused habitat destruction has also caused a change in predator-prey relationships that often lead to the displacement of beneficial species. A real-time example of such changes can be seen through the observation of tick (Ixodes scapularis) population distribution changes over the last 10 to 20 years.

What’s mice got to do with it?

With a recent focus on tick research, scientists have confirmed that the white-footed mouse is the number one carrier of Lyme Disease in the world. Knowing this, researchers sought out a way to treat white-footed mice without eradicating the species from any given area. To do this, they set up three trial locations: a placebo location, a bait box location, and a bait box medicated with fipronil. Following laboratory trials, a single topical treatment of 0.75% fipronil was shown to effectively protect mice from being bitten by tick nymphs for 4 to 6 weeks6. The bait boxes were designed to be placed near foraging and nesting sites in order to passively treat white-footed mice while they foraged on the provided bait6.

Following the first week of bait box deployment in May 2002, 16.7% of boxes showed a decline in weight, indicating foraging activity6. During the following eight weeks, use increased steadily with approximately 91% of boxes being used at the end of the nine-week study period6. The findings of the study indicated a significant decline in tick nymph and larval infestations in the treated locations when compared to the placebo and non-treated locations.

Attempted Community Engagement of Mice Control

Another hypothetical solution to reducing the prevalence of Lyme Disease is the release of white-footed mice that do not have the capability to become infected through the process of selective breeding. The implementation of this solution is dependent upon community involvement in the decision-making process due to the resident’s inability to opt-out of participation in white-footed mice release activities1. Some of the highest rates of confirmed and probable cases of Lyme Disease in the United States occurs on the islands of Nantucket and Martha’s Vineyard, making the release of selectively bred white-footed mice an optimal tactic against tick-spread Lyme Disease. Although residents were unable to come to an agreement, the community engagement process was effective on the islands.

Lyme Disease Overview

Lyme disease is the most frequently reported vector-borne illness in the United States, with over 300,000 Americans becoming infected each year1. The black-legged (deer) tick is responsible for the transmission of Lyme Disease as well as babesiosis, anaplasmosis, ehrlichiosis and Powassan encephalitis1. While most infections are treatable, contracting the Powassan virus is lethal with infections becoming more widespread.

Modern Medicine

A vaccine against Lyme disease was approved in 1998 for high-risk individuals aged 15–70 with a 76% protection rate but was voluntarily withdrawn by its manufacturer due to a decline in sales1.

Calculating tick density versus predator density

Blacklegged ticks are born free of illness-causing pathogens and become carriers of disease upon taking a blood meal from infected hosts when in each stage of life: larva, nymph, and adult2. Ticks in the nymph stage are responsible for the majority of Lyme Disease infection and feed mostly on small mammals such as white-footed mice and eastern chipmunks2. To understand the relationship between small mammals and infection rates, the Cary Institute began monitoring six field plots on the institute’s grounds. Since the early 1990s, researchers have trapped small mammals every 3-4 weeks between May and November2. Approximately 90% of all captures are made up of white-footed mice and chipmunks2. Statistical analyses of mammal population densities over time has revealed that an abundance of acorns within the study plots resulted in population increases of small mammals the following year and was a reliable indicator of increased infection of nymphal ticks2.

To further assess the relationship between habitat and tick abundance, researchers turned their attention to predator-prey interactions. To accomplish this, camera traps were set up throughout Dutchess County in New York. Sites with high predator diversity were found to have lower infection prevalence in nymphal ticks2. Bobcat, foxes, and opossums were all associated with a reduction in tick infection2. The research team also found that climate could influence the ability of ticks to infiltrate non-native habitats where a warm, dry spring or winter has been associated with a reduction in the density of infected nymphs2.

Are guinea hens the answer to tick population control?

Guineas hens have become a popular alternative to tick population control. Since guineas prefer to function in tight-knit groups, their hunting abilities can result in quick eradication of disease and tick carrying species such as mice and small rats. Guinea hens have also been known to eat slugs and occasionally snakes3.

Alternative management methods

Habitat distribution for tick species can be correlated with changes in land use, environmental conditions, and human proximity to habitats5. The challenge of reducing the number of tick-human interactions could be solved in the residential backyard landscape. Since grasses dominate the vegetative components of most residential yards, its maintenance has long been thought to affect contact rates. A study conducted in Massachusetts found that lawn length affected pollinator populations but did have an impact on tick population numbers4. Further investigation found that less than 2% of ticks were collected from lawns with the majority collected from wood lots and other ecosystem boundaries4.

When exploring the prevalence of illnesses transmitted by ticks, the health of habitat and ecosystems weighs heavily. Changes in climate and human encroachment on habitat appears to be an indicator of tick population distributions and in turn, can indicate the prevalence of human-tick interaction. Proper habitat management practices have been found to be the most effective in controlling illness distribution and should be investigated further in order to guide proper management and regulatory activities.

References
1.            Buchthal, J., Evans, S. W., Lunshof, J., Telford, S. R., & Esvelt, K. M. (2019, March 25). Mice Against Ticks: an experimental community-guided effort to prevent tick-borne disease by altering the shared environment. Retrieved from https://royalsocietypublishing.org/doi/10.1098/rstb.2018.0105
2.            Cary Institute of Ecosystem Studies. (2018, July 9). Forest ecology shapes Lyme disease risk in the eastern US. Retrieved from https://www.caryinstitute.org/newsroom/forest-ecology-shapes-lyme-disease-risk-eastern-us
3.            Jacob, J. (2015, May 5). Raising Guinea Fowl. Retrieved from https://articles.extension.org/pages/67816/raising-guinea-fowl
4.            Lerman, S. B., & D’Amico, V. (2019, April 3). Lawn mowing frequency in suburban areas has no detectable effect on Borrelia spp. vector Ixodes scapularis (Acari: Ixodidae). Retrieved from https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0214615
5.            Pak, D., Jacobs, S. B., & Sakamoto, J. M. (2019, April 29). A 117-year retrospective analysis of Pennsylvania tick community dynamics. Retrieved from https://parasitesandvectors.biomedcentral.com/articles/10.1186/s13071-019-3451-6
6.            Schulze, T. L., Jordan, R. A., Williams, M., & Dolan, M. C. (2017, March 15). Evaluation of the SELECT Tick Control System (TCS), a Host-Targeted Bait Box, to Reduce Exposure to Ixodes scapularis (Acari: Ixodidae) in a Lyme Disease Endemic Area of New Jersey. Retrieved from https://academic.oup.com/jme/article/54/4/1019/3070958
As ecosystems begin to change due to climate and human interference, tick species within them tend to experience a shift in distribution. The significance of this finding could change how ecosystem functions are interpreted.

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