Dr. John Aucott loves to let his dog go off-trail when he hikes. But as the director of the Johns Hopkins Rheumatology Lyme Disease Research Center in Baltimore, he knows better than to do it in June and July — the height of Lyme Disease season, when tiny nymph-stage ticks can move, undetected, from wild host (a mouse or deer) to a dog or human. While dogs can’t directly transmit Lyme disease to their owners, they can harbor ticks capable of doing the job.

People who get Lyme disease suffer from unpleasant symptoms like a rash, facial paralysis and swollen knees. But it isn’t always easy to detect and if left untreated, can progress to complications like memory problems, heart rhythm irregularities and chronic arthritis. A small minority of people with Lyme disease may even suffer symptoms like fatigue and joint pain for months after treatment.

This year, because of the East Coast’s unusually warm winter, ticks seem to be making an earlier appearance, which could make people unknowingly vulnerable to getting Lyme disease. Aucott says he is already finding ticks on his dog.

“I just pulled an engorged tick off [the dog] in February, which would be very unusual if the ground was snow-covered and it was 30 degrees,” he said. “But there’s no snow, and it’s been 60 and 70 degrees for some reason this winter.”

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One implication of the warm weather is that it attracts mice, which also harbor the ticks and bacteria that cause Lyme disease: 2017 is expected to be a very risky Lyme disease season, based on the surge of mice in New York measured in 2016, experts Felicia Keesing of Bard College and Rick Ostfeld of the Cary Institute of Ecosystems Studies told NPR this week. Aucott wasn’t surprised to hear this.

Local health departments, state university researchers and local doctors in other high-risk areas are also sounding the alarm in their respective communities about the rise of Lyme disease and tick sightings in their area this year.

“The mice of the previous year are important because they’re the ones infecting the larvae, and [they turn into] the nymphs that are feeding the following spring,” Aucott explained. “So it make intuitive sense — more mice, more infected larvae, more Lyme disease.”

However, just because there are a lot of mice in New York, doesn’t mean there are a lot of mice in other areas where Lyme disease is present.

“It’s really highly unlikely that the same variables in play in New York are in play in Virginia, Nova Scotia or Maryland,” Aucott said. “In other words, predicting one area doesn’t do a good job of predicting what’s going on in an adjacent region.”

Lyme disease is the most commonly reported vector-borne infection in the U.S., and is concentrated mostly in the northeast and upper midwest regions of the country. There were about 28,000 confirmed cases of Lyme disease in 2015, but the actual number could be as much as ten times higher, as not all cases are reported to the Centers for Disease Control and Prevention. In fact, CDC researchers estimate that the true number of Lyme disease infections is at around 300,000 cases every year, and occur mostly in the 14 states that make up over 96 percent of all reported cases: Connecticut, Delaware, Maine, Maryland, Massachusetts, Minnesota, New Hampshire, New Jersey, New York, Pennsylvania, Rhode Island, Vermont, Virginia and Wisconsin.

Mice aren’t the problem ― climate change is

Despite regional variation, it is generally true that climbing temperatures encourage the reproduction of mice, which are both natural reservoirs for the Borrelia burgdorferi bacteria that cause Lyme disease and carriers of the ticks that spread the infection to humans. Keesing and Ostfeld also point out that when people cut down trees and set their homes up in increasingly fragmented forests, that creates even better conditions for mice to multiply.

What’s more, warm weather generally means more time spent outdoors.

“If the weather’s nice, then people are out working in yard, or hiking,” Aucott said. “But if the weather’s terrible, people don’t go out and hike on the weekends.”

This isn’t the first attempt to study how Lyme disease season might change as a result of warming weather, associated with climate change. A 2015 analysis by researchers at Johns Hopkins and the National Center for Atmospheric Research predicts that the start of Lyme disease season, which ranges from about mid-April to the end of June (week 16 to week 26 of the calendar year) depending on the region, could shift an average of half a week earlier as soon as 2025, and two weeks earlier by 2065.

High-risk Lyme disease regions are growing

It’s also clear that Lyme disease risk is expanding regionally. From 1993 to 1997, there were 43 counties in the northeastern U.S. with a high incidence of Lyme disease. The number of high-incidence counties increased more than three-fold to 182 counties by 2008 to 2012. And in north-central states during the same time period, the number of high-incidence counties increased 2.5-fold, from 22 to 78, according to a 2015 analysis by the CDC.

CDC A map from the 2015 CDC analysis reveals the spread of high-incidence Lyme disease regions over time.

“The disease is clearly expanding north, south and west off the Eastern seaboard,” Aucott said. But whether that’s significantly influenced by climate change isn’t settled, says Dr. Eugene Shapiro, a professor of epidemiology at the Yale School of Medicine. Shapiro does think it’s clear that warmer temperatures play some kind role in incrementally increasing Lyme disease in areas where it was once rare.

“None of this is hard science yet,” Shapiro said. “What we do know is that the incidence is increasing in areas where there didn’t used to be Lyme disease. I’m not sure it’s attributable only to climate change, but it seems likely that it has at least a partial role.”

While Lyme disease risk is on the rise, the average person’s response to it shouldn’t change

While trying to predict the spread and severity of Lyme disease by region is an interesting question for researchers, Aucott believes that it doesn’t make much of a difference for people who are trying to avoid an infection. In the 14 states where Lyme disease is endemic, risk is always higher than the national average, and people should act accordingly no matter what regional predictions are made for any given year.

“There’s always a huge risk,” Aucott said. “It’s not like a risk of a tornado, where the tornado is either there or not.”

Here are some pointers on how to avoid tick bites and spot the symptoms of Lyme disease:

How To Cut Your Risk Of Lyme Disease See Gallery