Why the Variety of Tick Hosts in Your Back Yard Might Be a Good Thing
A simple reason why New England gets its reputation for beautiful fall foliage is because trees are everywhere: Woodland dominates the landscape, and most of the towns and cities spread throughout the region are nestled into the forests. That same dynamic is also at play, however, in the region’s abundance of ticks and tick-borne disease.
Researchers studying the spread of Lyme disease have closely investigated the dynamics involved among ticks, their animal hosts, humans, and the habitats in which they all live. One principle that has come to be understood is the dilution effect, which suggests that the abundance of the pathogen Borrelia burgdorferi (the bacteria that causes Lyme disease) falls as the diversity of host species for ticks increases. Looking at landscapes such as those in New England, it stands to reason that host species diversity would decrease in fragmented habitats, where residential areas encroach on forests, as compared to uninterrupted woodland. And that, in turn, would boost the presence of B. burgdorferi in those fragmented habitats—except that’s not how it actually plays out.
In a new study published Thursday in the Journal of Medical Entomology, scientists at the Connecticut Agricultural Experiment Station say they’ve found just the opposite, that ticks’ host-species diversity is higher in residential areas than in woodland areas in Connecticut, leading to lower levels of B. burgdorferi infection among mice in residential areas compared to those in the forests. Specifically, says lead author Megan Linske, Ph.D., residential areas favor an abundance of alternative hosts for blacklegged ticks (Ixodes scapularis), ones that aren’t as effective reservoirs for B. burgdorferi as white-footed mice.
“Abundance of alternative host species was the best predictor for pathogen presence. As alternative-host-encounter abundance increased, pathogen presence decreased,” Linske says. “We showed that this phenomenon was driven by abundance of reservoir-incompetent host species and not general diversity of hosts. Hosts need not be increasingly diverse to dilute tick-borne pathogens, but rather the dichotomy of reservoir-competent to -incompetent hosts needs to favor a greater proportion of the latter.”
While white-footed mice are primary reservoirs for B. burgdorferi, animals such as white-tailed deer, opossums, squirrels, chipmunks, birds, cats, and dogs can all carry B. burgdorferi and serve as hosts for ticks but are less likely to transmit the pathogen to feeding ticks. In simple terms, the more ticks can get their bloodmeals on animals that don’t readily pass B. burgdorferi to them, the less the ticks spread it around.
The research pokes some holes in the assumption that fragmentation equates to habitat loss. “While this may be the case in some circumstances, in others fragmentation can create a mosaic of open land, woodland, edge habitat, and landscaped islands that are capable of supporting much greater diversity than our overly mature woodlands. Furthermore, many wildlife species have adapted and accommodated to anthropogenic changes to the landscape,” Linske says.
The study was conducted across 18 woodland sites and 30 residential properties in Connecticut in 2015 and 2016. Researchers used motion-triggered cameras to capture images of host species at the locations and live trapping to capture small mammals such as white-footed mice. Captured mice were then sampled for antibodies to B. burgdorferi in their blood (a way to infer if they’d been exposed to the pathogen) and to count attached ticks.
The number of unique species measured was 50 percent higher in residential areas than in woodland areas, while the number of mice captured was roughly equal between the two. The number of feeding juvenile ticks on each mouse, however, averaged 2.4 in the woodland areas but just one per mouse in residential areas. And the infection rate of B. burgdorferi in mice averaged 80.9 percent at woodland test sites compared to 58.5 percent at residential ones.
For the average person hoping to avoid tick-borne disease, it’s a strong reminder to take adequate precautions when venturing into woodland terrain, but for ecologists, public-health officials, and other stakeholders, the research raises some questions about how our ecosystems are managed.
“Our study is not condoning the concept of further fragmenting the landscape but calls to recognize the need for managing our woodlands to restore some of that diversity to properly reflect what our ecosystem should look like as well as restore its proper functionality,” Linske says.
The research further illustrates the complex array of factors driving the spread of Lyme disease. “There is not going to be a silver-bullet solution that will work instantaneously,” says Linske. “Our ecosystem has been disrupted on several occasions over a long period of time and will require an equal or greater amount of time to be restored to a balanced state. Going forward, we will need to engage multiple disciplines to manage for Lyme disease on a broader scale. Wildlife biology, entomology, vector biology, forestry, and human dimensions, among other fields, are going to have to collaborate to bring about real change to our environment.”
“Ixodes scapularis (Acari: Ixodidae) Reservoir Host Diversity and Abundance Impacts on Dilution of Borrelia burgdorferi (Spirochaetales: Spirochaetaceae) in Residential and Woodland Habitats in Connecticut, United States”
Journal of Medical Entomology