Top-down control: Fox and opossums were associated with a reduction in tick infection. Fox prey on the rodents know to transmit tick-borne disease to feeding ticks; opossums are efficient at killing ticks when they groom. Photo: Sandy Brown Jensen, Dennisicecap1

Stepping into virtually any vegetation in parts of North America can be an invitation to bloodsucking ticks jumping onto exposed flesh. But other times, a walk in the woods can yield none of the parasites.

Ecological conditions can predict tick populations, and also the pathogens they may harbor such as Lyme Disease, anaplasmosis and babesiosis, according to a study recently published in the journal Ecology.

The model of prediction is based off detailed observations over 19 years on parcels of land at the Cary Institute of Ecosystem Studies in Upstate New York.

“This is the first time that we’ve synthesized the effects of bottom-up and top-down control, together with climate, on infected tick abundance,” Richard Ostfeld, lead author, of the Cary Institute, said. “The length of this dataset, as well as the sample size of both collected ticks and captured small mammals, are unprecedented in this context.”

The observations began in the 1990s, and continued until recently. Six large wooded plots of land in southeastern New York were studied, with trapping of small mammals occurring every few weeks from May to November.

The take was massive: more than 78,000 captures over the period of the study, with 90 percent of the capture being mice and chipmunks. The number of these critters was cross-reference with relative counts of acorns and food sources.

Also studied: ticks, and lots of them. Drag cloths were used to scoop off the bloodsuckers in 450-meter transects. The total take: 11,115 nymphs and 147,238 larvae. Some rodents can host dozens of ticks, and be perfect reservoirs of the disease pathogens, according to the study.

Predators were also counted, using black-LED camera traps at dozens of sites across Dutchess County during two of the study period (2012 and 2013).

The dynamics they found established that there is a feedback loop in the ecosystem that determine tick numbers, and the frequency of pathogens like Borrelia burgdorferi.

  • Acorn abundance increases rodent populations the next year, and then a marked increase in infected nymph tick the year after that.
  • Diversity of predators is key to keep tick infection rates low. For instance, the locations that were dominated by coyotes alone had high prevalence of infected ticks. But those with a breadth of foxes, bobcats, fishers, raccoons and opossum had lower observed pathogens (some 7,000 nymphs overall were tested for germs during the course of the study). Opossum are especially efficient consumers of ticks, according to the paper.
  • Warm and dry climate–especially in the spring and winter–reduces tick populations and tick disease. Since ticks spend the vast majority of their time on the ground, they can easily dry out with the right combination of weather patterns.
  • A worst-case scenario would be a bumper crop of acorns, followed by a relative dearth of the food source. The rodent population would thus spike and then crash. The spring following that crash, there would be lots of infected ticks, and fewer rodents to feast on. The ticks would thus seek out other hosts such as people.

Overall, Ostfeld and the team felt they could devise a way of having a warning system so that people near areas at crucial times will know to be on the lookout for the bloodsuckers, and the germs they carry.

“Understanding disease systems can help us refine predictions of when and where risk will be especially high,” Ostfeld said. “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.”