As rodent populations grow, ticks — and Lyme disease — are coming indoors

Posted June 19, 2017, at 2:58 p.m.
Last modified June 30, 2017, at 5:04 p.m.

Twice in the same week, Lois Wood woke to find ticks crawling over her bare leg in her New Hampshire home. A few nights later, she spotted a mouse running across her bed.

A mother of seven, Wood tries to shrug off her tiny bedfellows. “It’s a common rural problem,” she says, although she admits that she has “never experienced anything like this in my own bed.”

The recent appearance of vermin and pests in Wood’s bedroom coincides with the warming temperatures related to climate change. The past three years have been the planet’s hottest on record, and it is in this changing climate that many pests thrive, negatively affecting human health.

Forty to 90 percent of white-footed mice carry Borrelia burgdorferi, the spirochete bacterium that causes Lyme disease, and they provide the first blood meals for blacklegged ticks, also known as deer ticks, which can transmit the disease to humans.

White-footed mice are typically considered outdoor animals. But in suburban and rural areas near forested land, they easily squeeze through cracks and holes the size of a dime and often nest inside walls and in garages.

Wood’s garage, which is built into a hillside amid old cow pastures that are being reclaimed by young forests, rests directly beneath her bedroom, and it’s not uncommon for her to hear the scratching of mice within the walls. Dozens of poppy-seed-size blacklegged tick nymphs could hitch a ride indoors on a single white-footed mouse, then find a blood meal on house mice, other rodents or even members of Wood’s family.

“We’re diligent about doing tick checks when we come in from outside,” Wood says, “but my son and one of our family dogs have had Lyme. It’s scary to think that mice might be bringing ticks inside my house.”

One hundred fifty miles south of Wood’s home, Peggy Siligato, co-owner of Narragansett Pest Control in Rhode Island, has seen more mice over the past five years than during the rest of her 40-year career. “Seventy-five percent of our business is mice,” she said, and it had increased about 25 percent “from what it used to be.”

Science is offering a possible explanation for what Siligato is witnessing: Warming temperatures and milder winters have increased not only the population of the white-footed mouse but also its range.Sheila Haddad, vice president of sales for Bell Laboratories, which manufactures rodent control technology, agrees: “Rodent pressure is increasing. Mice used to seasonally enter homes primarily in the fall and winter months in New England, but now it’s a year-round problem. Warmer winters mean that more mice survive; it never gets cold enough to kill them.”

Haddad said she has noticed the same kind of increase in mouse populations as Siligato. In Atlanta, for example, “there’s been an increase in reported venomous snakebites, which means that there are more snakes feeding on rodents.”

“Everything is changing year after year,” Haddad says. “Our rodenticide sales to distributors have increased about 15 percent over the past two years.”

Siligato adds: “West Nile virus has killed many birds of prey along the East Coast, meaning there are more mice. To say it’s just climate change isn’t exactly accurate, but it’s probably a part of the story.”

The Cary Institute of Ecosystem Studies, a research and education organization in New York’s Dutchess County, has predicted that there will be a rise in reported Lyme disease cases in 2017 along the Eastern Seaboard because there was a bumper crop of acorns in 2015. Acorns are a favorite food of the white-footed mouse, and the population of the species has been shown to increase two years after a surge of the nuts. More mice means more opportunities for tick nymphs to have their first blood meals.

Families such as the Woods, who have a large oak tree that may have dropped up to 10,000 acorns just 100 feet from their home last fall, might notice even more mice around or even inside their homes.

The dangerous trifecta of mice, ticks and Lyme disease might seem like a problem for only rural and suburban areas, but climate change is increasingly making it an issue in cities as well. “We know we’re at risk in the country for getting Lyme,” Wood says, “although people living in concrete jungles should know that they can get it there, too.”

Large city parks provide preferred habitat for white-footed mice: small parcels of land, grassy knolls, shady oak trees, brush for safely making nests, and few natural predators.

Leo Galland, an internist practicing in New York City, says he treated a woman for acute Lyme after she found a blacklegged tick in her Park Avenue apartment. She had not left the city for months, but she often spent hours in nearby Central Park with her young child, and Galland says he believes a tick may have come home with her. “I also treated two other patients with acute Lyme who hadn’t left New York City, but they had each spent time in Riverside Park,” he says.

The website of the city health department warns that blacklegged ticks have been collected in four of the city’s five boroughs, with some of them testing positive for B. burgdorfergi. But how many New Yorkers would think to look at the website to learn about ticks? And even if they did, how many would also consider the mice that transmit Lyme?

In addition to surging populations of white-footed mice and the expected increased prevalence of blacklegged ticks in 2017, rising temperatures may be introducing invasive species that might also transmit disease.

“We’re seeing a new species of rodent” in Rhode Island, Siligato says. “Invasive species – perhaps harvester mice, but we’re not sure – can now survive here because the year-round temperatures aren’t all that different from what they’re used to.”

Haddad said a few nonnative mice have been found near Cape Cod. “They might have been brought on boats from the tropics, although I haven’t heard that tests have conclusively identified the species,” she says. “When we’re talking about mice, we have to remember that there are hundreds of species.”

Humans could suffer grave consequences because of the recent population explosion and spread of the white-footed mouse. And older methods of mouse eradication might not be enough to decrease the growing prevalence of Lyme disease. The Tick Project, a five-year study by the Cary Institute, is examining whether focusing on the elimination of ticks rather than mice might reduce cases of Lyme.

In 2016, the institute identified the neighborhoods in Dutchess County that were hot spots for Lyme disease. This April, two tick-killing methods began to be tested in these areas, and treatments will continue each spring and summer through 2020.

One method uses a small box that attracts mammals such as mice and chipmunks with bait. A wick coated with a chemical called fipronil rubs over the animals’ backs as they enter the box, killing any ticks attached to the animal. The second method, a fungus that is sprayed over grass and other vegetation, has been shown to kill ticks waiting to feed. Each method has been shown to be safe to pets, people and the environment.

Time will tell if either method will decrease reported Lyme disease cases, but it stands to reason that if ticks can be killed outside homes, then mice and any other species carrying the Lyme bacteria might be less of a health risk to humans, whether indoors or outdoors.

Although Wood’s family isn’t a part of the Tick Project, learning that new methods are being developed for controlling tick populations is reassuring to her.

“Climate change means that people aren’t safe from Lyme whether they live in the city or the country,” she says, “but knowing we can fight the disease means we can live in awareness, not fear.”

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