In the United States, 2021 was the fourth hottest year on record. In 39 out of 49 states, excluding Hawaii, winter was recorded as the fastest-warming season, reports Aliya Uteuova for the Guardian. With warmer winter months in the U.S. becoming commonplace, ticks are expanding their reach, and with them, Borrelia burgdorferi, the microbe that causes Lyme disease.
“They’re emerging earlier in the spring, and they’re staying active later in the fall,” said Theresa Crimmins, the director of the USA National Phenology Network and University of Arizona biologist, to the Guardian. “That’s a longer period of time that they could potentially be interacting with humans and potentially biting and spreading diseases.”
In the last two decades, cases of Lyme disease in the U.S. have tripled, Science reports. In one year, 476,000 individuals come down with flu-like symptoms accompanied by a distinct bulls-eye rash associated with the infection. The disease-causing bacteria can also invade the brain, nerves, heart and joints, causing arthritis or permanent nerve damage if left untreated, according to the Centers for Disease Control and Prevention.
Of more than 600 wild black-legged ticks sampled over three winter seasons, 79 percent of infected ticks survived cold temperatures, Laura Ferguson, an ecoimmunologist at Dalhousie University in Nova Scotia, explained at the annual meeting. In contrast, only 50 percent of uninfected ticks survived, reports Lauren Barry for Audacy. Each tick was placed in a vial and left outside during the winter at temperatures ranging minus 0.4 degrees Fahrenheit to 68 degrees Fahrenheit and checked again four months later to see which ones survived.
The increase in survival for infected ticks during the winter means there could be higher disease rates come springtime, Science reports. In a second experiment, Ferguson also experimented to see how changing winters could affect the reach of the ticks. For this experiment, the team took both infected and uninfected ticks into the lab and subjected them to three temperature conditions: freezing temperatures, temperatures of 37.4 degrees Fahrenheit, and temperatures predicted to happen because of climate change, per Science. Under lab conditions, a laser beam was used to detect when the ticks awoke and became active in various temperatures.
Ticks carrying the Lyme disease–causing pathogen had the most activity at fluctuating temperatures and woke up about four days a week. Uninfected ticks and ticks kept at stable temperatures woke up one to two days a week, Science reports. Lyme-carrying ticks increased their activity after a cold spell, whereas uninfected ticks were less active.
“Winter conditions may favor the ability of infected ticks to find hosts and continue to spread disease,” Ferguson told Science.
Previous research has suggested that B. burgdorferi makes infected ticks more active and more likely to bite. An expansion of where ticks reside geographically makes encountering an infected tick more likely.
“With climate change, there are going to be real consequences, and we need to tease these apart to make [the best] public health decisions,” says Laura Zimmerman, an ecoimmunologist at Millikin University, not involved with the study, told Science. “We tend to think what when it’s cold, nothing happens … more work like this is needed to find out what it means for disease transmission.”