In 2021, the Philadelphia five-county area reported 1,246 cases of Lyme disease, which can affect the nervous system if left untreated, according to data from the Pennsylvania Department of Health. That's 500 cases more than it had 30 years prior, in 1992.
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Bucks County and Chester County had the highest Lyme disease incidence in the area, with about 62 and 63 cases per 100k residents, respectively. Ticks are most commonly found in rural areas, particularly in places with tall grass or lots of trees. But they also show up in cities.
Last year, Philadelphia had blacklegged ticks carrying at least five different pathogens, including the ones that cause Lyme disease, Anaplasmosis and Babesioisis. These ticks can crawl onto a human directly or hitch a ride on a household pet before landing on its owner.
Katie Krebs, a veterinarian at University of Pennsylvania's Ryan Veterinary Hospital, noted an increase in animals with tick bites this year due to the warm winter: "We didn't get cold enough for there to be a die-off of ticks," she said.
Sustained cold weather is an effective defense against ticks and the illnesses they carry: Blacklegged ticks are less active at temperatures below 45 degrees Fahrenheit, according to the Environmental Protection Agency. But the parasites recover quickly when the weather starts warming up. To kill ticks, temperatures must remain below 10 degrees for several days. That didn't happen in Philadelphia this past winter.
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Tickborne illnesses aren't just a local issue, and their increased prevalence isn't a one-off event. According to Climate Central, winter cold snaps have gotten shorter and less frequent in cities across the country over the last 50 years. Throughout the United States -- and especially in the Northeast -- warmer winters have brought more ticks to areas where they already lived, spread ticks to new places where they previously couldn't survive and allowed ticks to thrive during times of year that used to be too cold for them. As ticks have prospered, so have the diseases they spread to their hosts.
Click the play button below to see Lyme disease's geographic spread in recent decades, and hover over states for their yearly case rates.
Nationwide, Lyme disease case rates more than tripled in the 25 years from 1993 to 2018, according to data from the Centers for Disease Control and Prevention.
Emergency room visits for tick bites typically peak in May, CDC data shows. This month, emergency rooms across the U.S. are seeing about 98 visits for tick bites out of every 100,000 visits. The Northeast sees nearly double that rate.
Dr. Anjuli Gans, an attending physician at the Children's Hospital of Philadelphia, said it's important to remove a tick with tweezers as soon as possible to avoid disease transmission.
"Then look for a rash or fever or irritation at the site over the next week or two and check in with your care team if you have any questions," Gans advised.
She added that people spending time outdoors in areas with ticks should cover up as much skin as possible with lightweight clothing and then apply insect repellent. She also advised parents to check their children for ticks regularly.
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Krebs cautioned pet owners to put dogs and outdoor cats on a year-round tick preventative and search them for ticks after coming in from a walk.
Sarah Larson of Fitler Square said she protects herself from ticks by wearing long pants and showering after walks with her dog. She also inspects her dog's body thoroughly.
"Just yesterday she was running through some crazy weeds and stuff, tall grass, and afterwards you've got to check every inch," Larson said.
Kaitlyn Nyhuis of University City said she checks her dogs for ticks as she pets him, and when she finds them, they're usually around his face.
"I was actually surprised that there were so many ticks in the city, 'cause I actually grew up in the woods where it was pretty bad," she said.
For people who are not used to ticks, it's important to be aware of where and when they are likely to live. But as the world warms, those places and times of year are expanding and extending.