Black-legged ticks in forests of the Northeast and Midwest have a variety of options for the three blood meals they consume in their lifetime: In their earlier stages, the ticks feed on at least 41 species of mammals, from chipmunks to black bears, plus 57 species of birds and 14 species of lizards. In adulthood, they will hop onto at least 27 species of mammals and one type of lizard.
These are the ticks that transmit pathogens that are harmful to humans, including the bacterium that causes Lyme disease. As tick-borne diseases become increasingly common in the U.S., scientists are scrutinizing these external parasites’ relationship with their hosts with the goal of figuring out how to fight the spread of Lyme. Ticks aren’t born with the pathogens that cause the main tick-borne diseases in humans; they get them from feeding on animals that act as reservoirs of the bacteria and parasites. Scientists are especially interested in the host that seems to be by far the best at harboring Lyme disease: the white-footed mouse. And some researchers have noted that an abundance of the mice one year is linked to more Lyme disease in humans the next — suggesting that this relationship could matter a lot when it comes to reducing Lyme disease.
Tick-borne Lyme disease is now the most common disease transmitted to humans via blood-sucking pests in the U.S., and the problem is expected to get worse. The Centers for Disease Control and Prevention estimates the number of cases at about 300,000 per year.1 Lyme disease, which causes fever, fatigue and headaches, can usually be successfully treated with antibiotics; when untreated, it can become a chronic, debilitating condition.2 Previously unknown tick-borne pathogens — at least a dozen since 1967 — are being discovered as well. And “there’s no reason to think we would have found everything,” said Ulrike Munderloh, a professor of entomology at the University of Minnesota.
The white-footed mouse is not only the most efficient known reservoir of Borrelia burgdorferi, the bacterium that causes Lyme disease, but the rodent is expanding its range, which currently covers 41 states. The narrowing footprint of forests, fragmented by roads and developments and sliced into backyards, often leaves the white-footed mouse as king of the forest, able to scamper about and reproduce without fear of predators, perhaps contributing to increased rates of tick-borne diseases in humans.
Scientists who study ticks and Lyme disease are keeping a close eye on how what happens in the forest affects the spread of the disease. A few years ago, scientist Rick Ostfeld of the Cary Institute of Ecosystem Studies in Dutchess County, New York, noticed a key pattern tied to a favorite mouse meal: lots of acorns = lots of mice = lots of Lyme disease in people. So when he and Bard College biology professor Felicia Keesing noted a bumper crop of that favorite mouse snack in the areas where they work in eastern New York in the summer of 2015, they weren’t surprised by an influx of mice in 2016. And now they’re bracing for more cases of Lyme and other tick-borne diseases there and possibly in nearby southern New England and the mid-Atlantic.
The possibility of breaking that chain of events led Keesing and Ostfeld to embark on a five-year experiment they call The Tick Project.3 Keesing and Ostfeld recruited almost 1,000 households in eastern New York that are at high risk for Lyme disease — they think of them as red zones, Keesing said, where people are reluctant to buy homes for fear of the disease — for the project and set up each yard with a bait box and fungal insecticide spray. The bait boxes, which attract mostly mice but also other small rodents such as chipmunks and shrews, deliver a dose of a pesticide that kills ticks that try to feed on the animals for the next couple of months. Vegetation in the yards is doused with a fungal spray that also kills ticks. While Keesing and Ostfeld are just beginning to collect data, they say that if people encounter fewer ticks or there are fewer cases of Lyme disease in the area they’re studying, the methods could be implemented elsewhere after the study concludes in 2020.
“We could spend our whole careers researching what not to do, but what do you tell your friends and neighbors in high-risk areas?” Keesing said. “They may have been interested [in our other research], but it didn’t really help them in any profound way.”
Kevin Esvelt, an evolutionary engineer at MIT, is also targeting the white-footed mouse, although his idea to tackle the tick-borne diseases is taking a more radical route: editing the mice’s DNA to make them unable to pass bacteria and viruses to ticks.
“When engineering a complex system, you should always make the smallest possible change that could solve the problem,” Esvelt said. “For tick-borne disease, that means preventing the ticks from being infected by white-footed mice. If we’re looking exclusively at Lyme disease, the contrast is clear: immunize every at-risk human indefinitely or heritably immunize the mouse population once? The latter is arguably more parsimonious, but reasonable people may prefer one or the other depending on their values.”
His early-stage efforts — his lab is still identifying the genes that would be edited — have been part of ongoing ethical debates about manipulating wild animals in communities where he would conduct the experiment. But Esvelt said he would proceed only with residents’ blessing. At first, that would be limited to islands, where the mice would be able to spread the new genetic code. That’s at least seven years out, and introducing the genetically engineered mice to the mainland would require a massive effort and tack on several years. But choosing the mouse was a no-brainer. “By immunizing the mice against tick saliva, we may be able to block babesiosis, anaplasmosis, ehrlichiosis and Powassan virus, as well as the Borrelia that cause Lyme disease,” he said.