Now, one of the longest-running studies of Lyme disease ecology is challenging several assumptions about what makes the disease more likely to spread. For more than 35 years, scientists at the Cary Institute of Ecosystem Studies in Millbrook, New York, have tracked the complicated relationships among ticks, mice, oak trees, predators, and weather. Their research has uncovered unexpected patterns that could help predict when people face greater exposure to infected ticks.

A new study published in Proceedings of the National Academy of Sciences brings together nine of the project's most surprising discoveries. The findings show how decades of observations can reveal connections that shorter studies might miss.

"On balance, these surprises have enriched our knowledge of this system, helping us to inform management of tick-borne disease risk," said lead author Richard Ostfeld, a disease ecologist at the Cary Institute who leads the research program alongside Shannon LaDeau.

According to Ostfeld, the extensive data collected over the years have been essential for separating meaningful ecological patterns from coincidences and statistical fluctuations.

"Studies of this depth and length are incredibly rare," said LaDeau. "This research has followed an ecological community -- including oak trees, mammals, and microorganisms -- examining how interactions like predation, parasitism, and competition shape the system over time. The evolution of understanding summarized in this paper is something you simply cannot get without such a long-term and system-focused study."

Deer May Play a Smaller Role in Lyme Disease Risk Than Expected

When Lyme disease first gained recognition in the United States during the 1980s, white-tailed deer were considered a likely driver of rising tick populations. Hunters frequently encountered deer carrying numerous adult ticks, suggesting that areas with more deer might also produce more ticks.

The reasoning seemed straightforward. Although deer cannot pass the Lyme disease bacterium to ticks, they provide blood meals for adult female ticks. Scientists suspected that larger deer populations would allow more female ticks to reproduce, eventually increasing the number of young ticks capable of spreading the infection.

But decades of monitoring have produced a different picture. On the Cary Institute's 2,000-acre forested research property in Dutchess County, New York, researchers found no statistically significant connection between deer numbers and the abundance of nymphal ticks. Nymphs are immature ticks responsible for much of the Lyme disease transmission to humans.

"We had detected a weak positive relationship between deer and subsequent nymph abundance over short subsets of the data," said Ostfeld, "but the whole data set in its entirety shows no such thing."

Mice Are a Much Stronger Predictor of Tick Surges

While deer populations failed to predict the abundance of nymphal ticks, mouse populations showed a much clearer relationship.

Years with especially large mouse populations were followed by approximately 40% more nymphal ticks the next year. White-footed mice are particularly important because they provide excellent blood meals for tick larvae and are highly effective at transmitting Borrelia burgdorferi to those feeding on them.

Once infected as larvae, ticks can retain the bacteria as they develop into nymphs. When these nymphs take another blood meal, they can pass the infection to humans.

The findings suggest that tracking mouse populations could provide a valuable early warning of unusually high tick numbers.

How Acorns Can Predict Tick Populations Two Years Ahead

One of the most unexpected discoveries involves something that might seem entirely unrelated to Lyme disease: acorns.

Oak trees periodically produce exceptionally large crops of acorns in synchronized events known as acorn masting. These unusually productive years provide an abundance of food for forest rodents, allowing mouse populations to expand during the following year.

That increase in mice then supports larger numbers of young ticks. As a result, the researchers found that a particularly heavy acorn crop can reliably predict an increase in nymphal ticks two years later.

The relationship reveals a remarkable sequence of events in which the reproductive cycle of forest trees helps determine future tick abundance. It also offers researchers another potential tool for anticipating periods of greater Lyme disease exposure.

Why More Mice Don't Always Mean More Infected Ticks

Although mouse abundance helps predict how many nymphal ticks will appear, the researchers discovered that it does not consistently predict what percentage of those ticks will carry Lyme disease bacteria.

Initially, scientists expected both measures to rise together. Because mice are efficient transmitters of B. burgdorferi, an increase in mice seemed likely to produce not only more ticks but also a greater proportion of infected ticks.

Early observations appeared to support that idea. However, as additional years of data accumulated, the apparent relationship gradually disappeared.

"It kept me up at night," said Ostfeld, "because it meant that my understanding of how the system works was incomplete or wrong."

The explanation appears to involve the other animals that share the forest with mice. Skunks, squirrels, and opossums also serve as hosts for feeding ticks, but most are much less effective at passing the Lyme disease bacterium to them.

When these animals are abundant, more larval ticks may feed on hosts that are unlikely to infect them. This can reduce the percentage of nymphal ticks carrying the bacteria, even when mouse populations are high.

"So the infection prevalence in the nymphs really depends on how all the larval tick meals are distributed across all the hosts," explained Ostfeld. "It would have been convenient if one host predicted that for us, but that has not been the case."

This finding highlights an important distinction between the total number of ticks in an environment and the proportion carrying the pathogen. Predicting the risk of infection requires understanding the broader community of animals that ticks feed on, not simply counting mice.

Extreme Heat and Cold May Not Kill Ticks in the Wild

Weather has also produced some unexpected results.

Earlier laboratory experiments suggested that exposure to intense heat or severe cold could kill blacklegged ticks. Yet observations in natural habitats revealed that ticks are surprisingly capable of surviving these conditions.

Researchers believe ticks protect themselves from temperature extremes by retreating into deeper soil layers or hiding beneath fallen leaves, where conditions are less severe than at the surface.

However, the long-term data also revealed a more complicated relationship with temperature. Years that are warmer overall tend to be followed by fewer nymphal ticks than usual, despite the ability of individual ticks to withstand short periods of extreme heat.

At the same time, warmer conditions generally favor mouse populations, which can lead to more ticks. These opposing effects make it difficult to predict how changing temperatures will ultimately influence Lyme disease risk.

What 35 Years of Research Mean for Lyme Disease Prevention

Taken together, the discoveries show that Lyme disease risk depends on a network of ecological interactions that is complicated but not entirely unpredictable.

Mouse abundance, for example, provides a stronger indicator of future nymphal tick numbers than deer abundance at the study site. Similarly, unusually harsh winters may not reduce tick populations as dramatically as previously expected.

By understanding these patterns, public health officials may be better equipped to anticipate periods of increased tick exposure and develop more effective prevention strategies.

Still, important questions remain, particularly about how climate change will affect ticks, the animals they feed on, and the oak trees that influence rodent populations.

"You would expect that a warming climate, which makes life better for the mice, would eventually make life better for the ticks, but we have not seen that so far," said Ostfeld. "We need to know what to expect from the different manifestations of climate change, which are sometimes warmer and drier, and sometimes warmer and wetter. The time of year when it's very warm also matters to ticks, oaks, and acorn production."

The research site in Dutchess County has undergone significant environmental changes since monitoring began in 1991. Climate shifts, changes to the surrounding landscape, and the arrival of invasive species have all altered the conditions in which the forest's plants and animals live.

Researchers intend to continue following these changes to determine how ticks and their hosts respond over time. Their work could provide increasingly valuable insights into how future environmental changes will influence human exposure to Lyme disease and other infections carried by ticks.