April 14, 2022 – Two widely used and environmentally safe tick control measures may slash the number of bloodthirsty bugs in your neighborhood, but they won’t reduce your chances of contracting Lyme disease, researchers have found.
The results of a 4-year study, which appears in the May 2022 issue of Emerging Infectious Diseases, showed a silver lining: The measures, while not helpful for people, did lead to a sharp drop in tick-borne disease for their outdoor pets.
The failure of the measures to reduce Lyme disease for people is “an unwelcome answer,” says Richard Ostfeld, PhD, one of the study researchers and a disease ecologist at the Cary Institute of Ecosystem Studies in Millbrook, NY.
The study involved a lot of people, was randomized, placebo-controlled, and double-masked – “the gold standard of study designs,” Ostfeld says. “We have to take that result seriously.”
The results led the researchers to speculate that, contrary to popular belief, people are more likely to attract Lyme-transmitting ticks when they’re away from home, and that those ticks bide their time before biting.
The longstanding assumption has been that “people encounter the tick that makes them sick when they’re in their yards,” says another study researcher, Felicia Keesing, PhD, a disease ecologist at Bard College in Annandale-on-Hudson, NY. “The evidence is not that solid.”
Ticks at Home
Lyme disease is caused by bacteria that certain ticks can transmit from infected animals like rodents to humans. About 30,000 cases are reported to the CDC in the U.S. each year, but recent estimates find that Lyme may strike 476,000 Americans yearly.
Symptoms can be mild and over quickly if treated by antibiotics, or long-term and crippling if untreated.
Experts have long advised people to try several measures to avoid tick bites, and a common one has been to decrease the number of the critters crawling around.
“Much of the literature claims that it is para-domestic – it’s around people’s homes,” Ostfeld says. His team tested that idea between 2017 and 2020 in 24 residential neighborhoods in Dutchess County, NY, “smack dab in the epicenter of the Northeastern Lyme epidemic.”
The average neighborhood in the study covered 68 acres and 118 properties, about three-quarters of which were covered in trees, shrubs, and grass where ticks thrive. An average of 34% of households in each neighborhood took part.
The researchers chose two products to test. One was Met52, a tick-killing fungal spray that Keesing says doesn’t tend to waft; the other was the Tick Control System, a collection of baited boxes that attract small mammals, which serve as hosts for the ticks.
Once inside the box, the animals get a dab of the insecticide fipronil, the active ingredient in products like Frontline, a flea and tick treatment for pets. The researchers chose the products because they are relatively harmless and available.
“We didn’t want to do research to document the efficacy of something that [people] couldn’t use,” Ostfeld says.
The study was placebo-controlled and double-masked, meaning some homes got both treatments, some got one of each, and some got nothing, but neither homeowner nor researcher knew the assignment.
The study found that the fungal spray didn’t have any effect on the number of either attached ticks or ticks actively seeking a host. But the tick boxes resulted in roughly a 50% decrease in ticks found on rodents.
Concerningly, according to the researchers, the reduction in ticks didn’t lead to a drop in illness in people. Overall, people in the study reported 130 cases of tick-borne illness, and the researchers found no statistically significant difference for people whose properties got the fungal spray or tick boxes, compared to those who didn’t.
The researchers did find “a significant reduction” in the number of illnesses people reported for their outdoor pets. “They go into every nook and cranny of our yard,” Keesing says.
‘It’s Too Bad’
“The good news is that you can protect your pets from the ticks, and in doing so protect yourself,” says Holly Ahern, an associate professor of microbiology at SUNY Adirondack in Queensbury, NY.
Lyme can cause kidney and heart disease in pets, says Ahern, who’s also a Lyme disease activist whose daughter got the illness in 2002. “It’s too bad [the mitigation] doesn’t do more to prevent tick-borne Lyme disease in humans.”
After all these years, Ahern says she found comfort in understanding that mitigation measures may not be enough, and that there are other avenues for research to protect against the disease.
“It is frustrating that these broad-scale prevention studies have consistently been unsuccessful in preventing Lyme disease,” Wendy Adams, the research grant director for the Bay Area Lyme Foundation in Portola Valley, CA, said in a statement.
One topic for future research is learning where, if not at home, people encounter the tick that ultimately made them sick. “We have very, very little solid data,” Ostfeld says.
Keesing and Ostfeld, who are married, say people may have picked it up on a hike. “The tick will crawl around for hours, even a day, before they decide where they want to stick their mouthparts in your skin,” Ostfeld says.
“Most people don't spend all their time on their own property,” says Dorothy Leland, vice president of the patient advocacy group LymeDisease.org. “We walk our dogs around the neighborhood, our children play at public parks and athletic fields, many adults work in outdoor occupations.”
Corrections: An early version of this story mistakenly said Lyme disease can spread from deer to humans. Deer are reservoirs for Lyme disease, but not carriers. Also, Met52 is a fungal spray, not an antifungal.