https://www.cidrap.umn.edu/lyme-disease/want-predict-lyme-disease-risk-count-acorns-not-deer
Want to predict Lyme disease risk? Count the acorns, not the deer
Laine Bergeson
Today at 11:49 a.m.
Lyme Disease
Tick-borne Disease
That’s one of several surprising findings to emerge from more than 30 years of tracking ticks, mice, deer, and weather in a Lyme disease hot spot. The findings, published this week in PNAS, challenge long-held ideas about what drives Lyme disease risk.
For the study, researchers from the Cary Institute of Ecosystem Studies and Bard College analyzed decades of data from a forested area in New York’s Hudson Valley. The long span of the study enabled researchers to track trends over time.
Studies of this depth and length are incredibly rare.
“Studies of this depth and length are incredibly rare,” says study coauthor and co-director of the Cary Institute of Ecosystem Studies, Shannon LaDeau, PhD, in an institute news release. “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,” she added.
4 findings that upend risk assumptions
Lyme disease is the most frequently reported vector-borne disease in temperate zones across the globe, with more than 450,000 cases reported annually in the United States and more than 200,000 annually in Western Europe, the authors state. It affects millions of people across North America, Europe, and Asia, and, while rarely fatal, it can result in chronic, debilitating illness.
Lyme disease is transmitted to humans by black-legged ticks (Ixodes scapularis), also known as deer ticks. But while white-tailed deer often carry many adult ticks, more deer don’t mean more Lyme risk. That’s one of several surprises, listed below, that the researchers uncovered.
#1. More deer don’t mean more dangerous ticks
It’s long been assumed that more deer equal more ticks, because when deer are abundant, more female ticks are able to feed and lay eggs on them. That would lead to more larval ticks the following summer, the theory goes—and, in turn, more nymphal ticks two years later. Nymphal-stage ticks are responsible for most human infections in North America.
But after observing more than a sixfold variation in deer abundance over the course of the study, the researchers found no relationship between the number of deer and the density of nymphs.
“We had detected a weak positive relationship between deer and subsequent nymph abundance over short subsets of the data,” says lead author Richard Ostfeld, PhD, of the Cary Institute, in the news release, “but the whole data set in its entirety shows no such thing.”
Another surprise helped explain why: More ticks at one stage of the life cycle did not reliably translate into more ticks at the next. “Simple life history theory predicts that abundant populations of larvae should lead the next year to abundant nymphs, which in turn should lead to abundant adults, a phenomenon called demographic forcing,” the researchers write. But “significant demographic forcing was seen only for the transition from nymphal to adult stage.”
#2. Mice matter, but so do squirrels and skunks and opossums…
White-footed mice are very good at passing the bacterium that causes Lyme disease (Borrelia burgdorferi) to larval ticks, so researchers expected that mouse-abundant years would lead to more infected nymphal ticks.
But in the study, the number of mice didn’t predict the proportion of nymphal ticks infected with B burgdorferi. Instead, the rate of infection appeared to depend on how larval ticks were distributed across the entire community of small mammals who serve as hosts for the bacteria.
In years when mice are abundant, other hosts, like skunks and squirrels, might be more abundant, too. But those other hosts aren’t very good at spreading B burgdorferi to ticks. So the higher their numbers, the lower the rate of Lyme bacteria in nymphal ticks.
Mice did, however, strongly predict the number of nymphal ticks. A strong mouse year was associated with approximately 40% more nymphs the following year.
#3. Acorns play an outsized role
Mice feed on acorns that fall from large red oaks. When a large number of oak trees drop their acorns at the same time, it creates an endless buffet for mice. All those hearty meals help boost the mouse population the following year and, in turn, the number of nymphal ticks the year after that.
Acorns are therefore postulated to be both a driver and a leading indicator of Lyme disease risk in regions where oaks are abundant.
“Acorns are therefore postulated to be both a driver and a leading indicator of Lyme disease risk in regions where oaks are abundant,” write the researchers.
#4. Warmer weather doesn’t simply mean more ticks
Previous laboratory studies have shown that extreme heat and cold can kill black-legged ticks, which suggests that severe seasonal temperatures may help determine tick abundance. But ticks outsmart frigid winters and steamy spring days by burrowing into the soil, write the researchers. The current findings suggest that neither extremely cold winter days nor extremely warm spring days predicted nymphal tick density.
But in the warmest years, mice had roughly 45% fewer nymphal ticks than in colder years. “Cumulative degree-days above freezing, a measure of overall warmth, was positively associated with mouse abundance but negatively associated with nymphal tick abundance,” write the researchers.
These observations suggest that [density of nymphs] is more likely to be predictable from abundance of white-footed mice than from that of white-tailed deer.
This suggests that, all else being equal, tick populations might decline in the future with the warming climate. “These observations suggest that [density of nymphs] is more likely to be predictable from abundance of white-footed mice than from that of white-tailed deer, a result directly relevant to efforts to manage Lyme disease risk by managing host populations,” the researchers note.
The factors that contribute to Lyme disease risk are complex but predictable, conclude the authors, and knowing more about what contributes to increased risk can help public health officials anticipate seasons with elevated Lyme disease risk and guide prevention strategies.
Want to predict Lyme disease risk? Count the acorns, not the deer
Laine Bergeson
Today at 11:49 a.m.
Lyme Disease
Tick-borne Disease
If you want to know how bad a future tick season will be, it may be more useful to count the number of acorns on the ground than the number of deer in the forest.That’s one of several surprising findings to emerge from more than 30 years of tracking ticks, mice, deer, and weather in a Lyme disease hot spot. The findings, published this week in PNAS, challenge long-held ideas about what drives Lyme disease risk.
For the study, researchers from the Cary Institute of Ecosystem Studies and Bard College analyzed decades of data from a forested area in New York’s Hudson Valley. The long span of the study enabled researchers to track trends over time.
Studies of this depth and length are incredibly rare.
“Studies of this depth and length are incredibly rare,” says study coauthor and co-director of the Cary Institute of Ecosystem Studies, Shannon LaDeau, PhD, in an institute news release. “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,” she added.
4 findings that upend risk assumptions
Lyme disease is the most frequently reported vector-borne disease in temperate zones across the globe, with more than 450,000 cases reported annually in the United States and more than 200,000 annually in Western Europe, the authors state. It affects millions of people across North America, Europe, and Asia, and, while rarely fatal, it can result in chronic, debilitating illness.
Lyme disease is transmitted to humans by black-legged ticks (Ixodes scapularis), also known as deer ticks. But while white-tailed deer often carry many adult ticks, more deer don’t mean more Lyme risk. That’s one of several surprises, listed below, that the researchers uncovered.
#1. More deer don’t mean more dangerous ticks
It’s long been assumed that more deer equal more ticks, because when deer are abundant, more female ticks are able to feed and lay eggs on them. That would lead to more larval ticks the following summer, the theory goes—and, in turn, more nymphal ticks two years later. Nymphal-stage ticks are responsible for most human infections in North America.
But after observing more than a sixfold variation in deer abundance over the course of the study, the researchers found no relationship between the number of deer and the density of nymphs.
“We had detected a weak positive relationship between deer and subsequent nymph abundance over short subsets of the data,” says lead author Richard Ostfeld, PhD, of the Cary Institute, in the news release, “but the whole data set in its entirety shows no such thing.”
Another surprise helped explain why: More ticks at one stage of the life cycle did not reliably translate into more ticks at the next. “Simple life history theory predicts that abundant populations of larvae should lead the next year to abundant nymphs, which in turn should lead to abundant adults, a phenomenon called demographic forcing,” the researchers write. But “significant demographic forcing was seen only for the transition from nymphal to adult stage.”
#2. Mice matter, but so do squirrels and skunks and opossums…
White-footed mice are very good at passing the bacterium that causes Lyme disease (Borrelia burgdorferi) to larval ticks, so researchers expected that mouse-abundant years would lead to more infected nymphal ticks.
But in the study, the number of mice didn’t predict the proportion of nymphal ticks infected with B burgdorferi. Instead, the rate of infection appeared to depend on how larval ticks were distributed across the entire community of small mammals who serve as hosts for the bacteria.
In years when mice are abundant, other hosts, like skunks and squirrels, might be more abundant, too. But those other hosts aren’t very good at spreading B burgdorferi to ticks. So the higher their numbers, the lower the rate of Lyme bacteria in nymphal ticks.
Mice did, however, strongly predict the number of nymphal ticks. A strong mouse year was associated with approximately 40% more nymphs the following year.
#3. Acorns play an outsized role
Mice feed on acorns that fall from large red oaks. When a large number of oak trees drop their acorns at the same time, it creates an endless buffet for mice. All those hearty meals help boost the mouse population the following year and, in turn, the number of nymphal ticks the year after that.
Acorns are therefore postulated to be both a driver and a leading indicator of Lyme disease risk in regions where oaks are abundant.
“Acorns are therefore postulated to be both a driver and a leading indicator of Lyme disease risk in regions where oaks are abundant,” write the researchers.
#4. Warmer weather doesn’t simply mean more ticks
Previous laboratory studies have shown that extreme heat and cold can kill black-legged ticks, which suggests that severe seasonal temperatures may help determine tick abundance. But ticks outsmart frigid winters and steamy spring days by burrowing into the soil, write the researchers. The current findings suggest that neither extremely cold winter days nor extremely warm spring days predicted nymphal tick density.
But in the warmest years, mice had roughly 45% fewer nymphal ticks than in colder years. “Cumulative degree-days above freezing, a measure of overall warmth, was positively associated with mouse abundance but negatively associated with nymphal tick abundance,” write the researchers.
These observations suggest that [density of nymphs] is more likely to be predictable from abundance of white-footed mice than from that of white-tailed deer.
This suggests that, all else being equal, tick populations might decline in the future with the warming climate. “These observations suggest that [density of nymphs] is more likely to be predictable from abundance of white-footed mice than from that of white-tailed deer, a result directly relevant to efforts to manage Lyme disease risk by managing host populations,” the researchers note.
The factors that contribute to Lyme disease risk are complex but predictable, conclude the authors, and knowing more about what contributes to increased risk can help public health officials anticipate seasons with elevated Lyme disease risk and guide prevention strategies.