Tick-borne illnesses are becoming more common in the United States, and Lyme disease remains the most commonly reported illness. Nearly 500,000 Americans are diagnosed each year. The infection is caused by bacteria Borrelia burgdorferiwhich usually affects humans through the bite of an infected blacklegged tick. Antibiotics can treat Lyme disease, but without treatment, the infection can affect the joints, heart, and nervous system. Some patients also have symptoms that persist over time.
Now, one of the longest-running studies on the ecology of Lyme disease challenges 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 complex relationships between ticks, mice, oaks, predators and weather. Their research revealed unexpected trends that could help predict when people will be more exposed 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 results show how decades of observations can reveal connections that shorter studies might miss.
“Overall, these surprises have enriched our knowledge of this system, helping 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 has been essential in separating meaningful ecological patterns from coincidences and statistical fluctuations.
“Studies of this depth and length are incredibly rare,” LaDeau said. “This research followed an ecological community – including oaks, mammals and microorganisms – examining how interactions such as predation, parasitism and competition shape the system over time. The evolving understanding summarized in this paper is something you simply cannot get without such a long-term, system-focused study.”
Deer may play less role than expected in Lyme disease risk
When Lyme disease was first recognized in the United States in the 1980s, white-tailed deer were considered a likely factor in increasing tick populations. Hunters frequently encountered deer carrying many adult ticks, suggesting that areas with more deer might also produce more ticks.
The reasoning seemed simple. Although deer cannot transmit Lyme disease bacteria to ticks, they provide blood meals to adult female ticks. Scientists suspected that larger deer populations would allow more female ticks to reproduce, increasing the number of young ticks capable of spreading infection.
But decades of monitoring have produced a different picture. On the Cary Institute’s 2,000-acre forestry research property in Dutchess County, New York, researchers found no statistically significant link between deer numbers and the abundance of nymphal ticks. Nymphs are immature ticks responsible for much of the transmission of Lyme disease to humans.
“We had detected a weak positive relationship between the abundance of deer and subsequent nymphs on short subsets of the data,” Ostfeld said, “but the entire data set shows nothing like that.”
Mice are a much stronger predictor of tick outbreaks
While deer populations failed to predict nymphal tick abundance, mouse populations showed a much clearer relationship.
Years with particularly large mouse populations were followed by about 40% more nymphal ticks the following year. White-footed mice are particularly important because they provide excellent blood meals for tick larvae and are very efficient at transmitting Borrelia burgdorferi to those who feed on it.
Once infected as larvae, ticks can retain the bacteria as it pupates. When these nymphs take another blood meal, they can transmit the infection to humans.
The results suggest that monitoring mouse populations could provide a valuable early warning of unusually high tick numbers.
How acorns can predict tick populations two years in advance
One of the most unexpected discoveries involves something that might seem completely unrelated to Lyme disease: acorns.
Oak trees periodically produce exceptionally large crops of acorns in synchronized events known as acorn masting. These exceptionally productive years provide an abundance of food for forest rodents, allowing mouse populations to grow over the following year.
This increase in the number of mice then supports a greater number of young ticks. As a result, researchers found that a particularly abundant acorn harvest can reliably predict an increase in nymphal ticks two years later.
This 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 increased exposure to Lyme disease.
Why more mice don’t always mean more infected ticks
Although mouse abundance helps predict how many nymphal ticks will appear, researchers have found that it does not consistently predict what percentage of those ticks will carry the Lyme disease bacteria.
Initially, scientists expected both measurements to increase simultaneously. Because mice are effective transmitters of B. burgdorferian increase in the number of mice seemed likely to produce not only more ticks, but also a greater proportion of infected ticks.
Initial observations seemed to support this idea. However, as the data accumulated, the apparent relationship gradually disappeared.
“It kept me up at night,” Ostfeld said, “because it meant my understanding of how the system worked was incomplete or wrong.”
The explanation seems to involve the other animals that share the forest with the mice. Skunks, squirrels, and opossums also serve as feeding hosts for ticks, but most are much less effective at transmitting Lyme disease bacteria to them.
When these animals are abundant, more tick larvae can feed on hosts unlikely to infect them. This can reduce the percentage of nymphal ticks carrying the bacteria, even when mouse populations are high.
“So the prevalence of infection in nymphs really depends on how all the meals of the tick larvae are distributed across all the hosts,” Ostfeld explained. “It would have been convenient if a host predicted this to us, but that wasn’t the case.”
This finding highlights an important distinction between the total number of ticks in an environment and the proportion carrying the pathogen. Predicting infection risk requires understanding the broader community of animals that ticks feed on, not just counting mice.
Extreme heat and cold may not kill ticks in nature
The weather also produced unexpected results.
Previous laboratory experiments suggested that exposure to intense heat or intense cold could kill blacklegged ticks. Yet observations in natural habitats have revealed that ticks are surprisingly capable of surviving these conditions.
Researchers believe ticks protect themselves from extreme temperatures by retreating to deeper layers of soil or hiding under fallen leaves, where conditions are less severe than on the surface.
However, the long-term data also revealed a more complicated relationship with temperature. Overall warmer years tend to be followed by fewer nymphal ticks than usual, despite each tick’s ability to withstand short periods of extreme heat.
At the same time, warmer conditions generally favor mouse populations, which can lead to increased tick numbers. These opposing effects make it difficult to predict how temperature changes will ultimately influence Lyme disease risk.
What 35 years of research means for Lyme disease prevention
Taken together, these findings show that Lyme disease risk depends on a complex but not entirely unpredictable network of ecological interactions.
Mice abundance, for example, provides a stronger indicator of future nymphal tick numbers than deer abundance at the study site. Likewise, particularly harsh winters may not reduce tick populations as dramatically as expected.
By understanding these trends, public health officials may be better equipped to anticipate periods of increased exposure to ticks and develop more effective prevention strategies.
Yet important questions remain, including how climate change will affect ticks, the animals they feed on and the oak trees that influence rodent populations.
“You would expect that global warming, which improves the lives of mice, would eventually improve the lives of ticks, but we haven’t seen that yet,” Ostfeld said. “We need to know what to expect from different manifestations of climate change, which are sometimes warmer and drier, and sometimes hotter and wetter. The time of year when it is very hot is also important for the production of ticks, oaks and acorns.”
The Dutchess County research site has undergone significant environmental changes since monitoring began in 1991. Climate change, changes to the surrounding landscape, and the arrival of invasive species have all altered the conditions in which forest plants and animals live.
The researchers plan to continue tracking 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 tick-borne infections.
Gn Health
