The critters run through forest floors throughout the eastern United States, snatching up acorns and other tree seeds, berries and bugs.
White-footed mice — known for their wide eyes and ears, long tails and snow-white bellies and the feet from which they get their name — are often overlooked by humans, hiding out by the billions in U.S. forests, shrubby thickets and even wooded wetlands. But there's one creature that knows them well: the tick.
Scientists say white-footed mice, which are primary carriers of the Lyme bacterium Borrelia burgdorferi, are a highly popular host of black-legged ticks — which consequently makes them a key culprit in the spread of Lyme disease.
For Lyme disease transmission, "essentially, the only way people can get infected is through a tick bite," said Richard Ostfeld, senior scientist at Cary Institute of Ecosystem Studies.
Scientists say that white-footed mice are posing a particularly high risk to humans this year. A bountiful acorn harvest a couple of years ago gave them the sustenance needed to reproduce in greater numbers and climate change may be pushing them to expand their range toward the north.
"That's something of a worry because where the mice go, so too go the infected ticks," said Ostfeld, who is co-heading the Cary Institute's Tick Project, along with his wife, Felicia Keesing, a biology professor at Bard College in New York.
Ostfeld said there are areas in the United States where Lyme disease is rare and, in those places, few or none of the white-footed mice are infected. But in an endemic area such as one that extends from Virginia to Maine, at least half and sometimes up to 90 percent of the mice are infected with Lyme bacteria.
There are about 30,000 reported cases of Lyme disease in the United States each year, according to the Centers for Disease Control and Prevention. However, because many cases are never reported, studies suggest that the number may be closer to 300,000, according to the health protection agency.
The CDC reported that in 2015, 95 percent of Lyme disease cases that were reported came from 14 eastern states. The disease is typically spread when immature ticks, which are called nymphs, bite mice that are infected and then bite humans.
Ticks typically attach themselves to hidden areas on the human body such as the groin, armpits and scalp, and must maintain the bite for 36 to 48 hours before they can spread the infection, according to the CDC.
What are white-footed mice?
Unlike typical gray house mice, white-footed mice, known as Peromyscus leucopus, are tawny brown, aside from the white that covers their bellies and feet. They are fast. And they are small, with tails as long as their bodies.
Ostfeld said these rodents are "generalists" when it comes to what they choose to eat and where they live.
"There's this very interesting and rather complicated set of ecological connections where the mice and, of course, ultimately the oak trees, sit at the center of this important risk to human health," said Clive Jones, emeritus terrestrial ecologist at the Cary Institute of Ecosystem Studies.
Jones, who has studied the connection between acorn production and Lyme disease in oak forests in the eastern United States, said that the more acorns there are in the fall, the greater the risk of Lyme disease to humans two years later. Because 2015 was a good year for acorns, 2017 could be a bad year for humans.
Here's how it works: Adult ticks, which mostly feed upon white-tailed deer, drop off and lay their eggs on the forest floor, Jones said. The eggs hatch out the next year into larvae, which at that point are generally not infected by the bacteria that causes Lyme disease. The larvae get infected when they feed on an animal that carries the bacteria, which is most often white-footed mice.
Jones said the larval ticks then molt into infected nymphs that can infect humans the following year.
How many acorns there are in the fall determines how many mice there are the next year that can infect larval ticks — acorns are a major food source for mice, Jones said. He said heavy acorn production two years ago resulted in many mice a year ago that became hosts for infected many larval ticks. That resulted in many infected tick nymphs this year and a high risk of Lyme disease for humans, he said.
What makes the mice such excellent carriers?
Lax grooming habits. Passive immune systems. Endless offspring that can potentially carry on disease, scientists say.
"For reasons we don't understand, mice are what we call extremely tolerant of both tick infestations and Lyme bacteria in their bodies," said Ostfeld.
"The mice just aren't fastidious. They just don't really care about being bitten by lots of ticks," he added.
In addition, white-footed mice have bloodstreams that Ostfeld calls "a breeding ground for all kinds of infectious agents," including pathogens that cause babesiosis and anaplasmosis, other tick-borne diseases.
How? Because, he said, "mice simply don't mount a strong immune response once they get exposed. So the mice are not protecting themselves physiologically against either ticks on the outside of their bodies or against these microbial pathogens that get inside their bodies and swim around in their bloodstreams."
That said, predators of white-footed mice, such as owls, hawks, bobcats, foxes and weasels, won't get Lyme disease by eating an infected rodent because the bacteria cannot survive digestive tracts of birds or mammals.
It's not clear why his particular rodent is so tolerant of microbes, but it may be in part to conserve energy.
"Why spend your precious energy mounting an expensive immune response to avoid sickness if you could spend that same energy cranking out babies as fast as you can before you drop dead in the talons of an owl or the mouth of a fox?" Ostfeld said. "So we think it's an evolved strategy to allocate energy toward faster reproduction."
Indeed, they do.
Research shows that white-footed mice reach maturity at about one month and can then start to reproduce.
After fertilization and a gestation period of about 22 days, mother mice can have anywhere from three to six babies.
Ostfeld said that as soon as a day after birthing a litter, female mice can be ready to reproduce again, meaning they may be nursing one litter while carrying another. These mice can do this again and again throughout their lives, which typically last about six to 10 months, scientist say.
"They have a live fast, die young kind of lifestyle," Ostfeld said.
What does it all mean for humans?
"We know that this cryptic animal that most people don't even know is around plays a critical role in our health," Ostfeld said.
The scientist said a key to protecting human health is to take measures to try to control the white-footed mouse population, and one way to do that is by supporting natural predators by keeping their habitats intact.
"It's a tough nut to crack because these mice are everywhere and are hard to control, hard to manage," he said. "But I'm optimistic that we can reduce rates of Lyme."