In the eastern US, risk of contracting Lyme disease is higher in fragmented forests with high rodent densities and low numbers of resident fox, opossum, and raccoons. These are among the findings from an analysis of 19 years of data on the ecology of tick-borne disease in a forested landscape, recently published in the journal Ecology.
Lyme disease is the most frequently reported vector-borne disease in the US. "Using nearly two decades of data on the forest food web, we were interested in untangling the ecological conditions that regulate the number of infected ticks in the landscape," explains Richard S. Ostfeld.
Long-term data were collected in Dutchess County, NY, an epicenter for Lyme disease. Variables monitored at six forested field plots (2.2 hectares each) on the grounds of the Cary Institute included: small mammals, blacklegged ticks, tick-borne pathogens, deer, acorns, and climate. Predator communities and tick infection rates were also recorded at 126 sites throughout Dutchess County over two years.
Taal Levi of Oregon State University, also a lead author, notes, "Our goal was to identify ecological indicators that could be used to protect public health. By analyzing these long-term data holistically, we can tease out how changes in things like predator populations and food resources shift the community structure of the forest ecosystem, and ultimately the abundance of infected blacklegged ticks searching for a meal."
The outsized role of rodents
Blacklegged ticks take a single blood meal at each of their three life stages: larva, nymph, and adult. They are born free of the pathogens that cause Lyme disease, anaplasmosis, and babesiosis. These are acquired when they take bloodmeals from infected mammals, with white-footed mice and eastern chipmunks being the most competent reservoirs of these tick-borne diseases.
Bottom-up control: Acorns
Seed baskets were used to monitor acorn production at the field sites. Oaks produce acorns in boom-and-bust cycles. Statistical analyses revealed that high acorn abundance on the plots boosted rodent populations the next year and were a reliable indicator of an increase in infected nymphal ticks two years after acorn boom years.
Ostfeld explains, "When acorn production is high, rodents feed on the seeds and are more successful at overwintering and reproducing. The spring following large acorn crops, rodent numbers increase. Questing larval ticks are more likely to feed on a white-footed mouse or chipmunk – animals that are very efficient at transmitting the bacterium that causes Lyme disease."
Top-down control: Predators
Coyote populations are expanding throughout the eastern US. Levi notes. "Coyotes can exclude foxes and other smaller carnivores, which should reduce predation rates on key small mammal hosts for pathogens. This can result in larger small mammal populations, reduced turnover rates that allow infected individuals to live longer and infect more ticks, and changes to rodent behavior that make questing ticks more likely to feed on rodents, amplifying the infection rates of ticks."
Putting it all together
The authors conclude that in oak-dominated forests in the eastern US, tick-borne disease risk is strongly tied to the fluctuating acorn supply and the structure of the predator community.
Ostfeld concludes, "Understanding disease systems can help us refine predictions of when and where risk will be especially high. Issuing warnings based on these specific predictions, rather than broad-stroke PSAs, will hopefully counter 'warning fatigue' and encourage people to become more proactive in taking self-protection measures."