Land development that breaks up forests appears to be a driving factor in Pennsylvania's rising number of Lyme disease cases. By creating new communities that displace wildlife in these areas, people become more likely targets for ticks than other potential hosts.
Lyme disease cases reported in the U.S. have doubled over the last two decades, with as many as 476,000 Americans now diagnosed each year, according to the U.S. Centers for Disease Control and Prevention. Pennsylvania's 8,413 confirmed cases last year led the nation for the 11th time in the last 12 years. Neighboring states like New Jersey and New York also have high rates of Lyme disease.
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Research has pointed to several likely causes for this increasing prevalence, including milder winters that have made the Northeast more hospitable to black-legged ticks year-round. These ticks, also known as deer ticks, pick up the bacteria that cause Lyme disease by feeding on other mammals, most commonly mice and deer.
A report from Spotlight PA, published Monday, suggests that land development in forested areas not only makes humans more prone to tick bites, but also increases the likelihood that ticks will pick up Lyme disease. That's because when people clear forests to build roads, shopping centers and housing developments, only some of the species that ticks normally feed on stick around. Those that do — especially mice — are more likely to harbor the bacteria that cause Lyme disease.
Pennsylvania lost an estimated 300,000 acres of forest land to commercial development between 2014 and 2019, according to Spotlight PA. Most of those losses occurred in the western and eastern parts of the state, which accounted for about half of Pennsylvania's Lyme disease cases last year.
Lyme disease can cause a fever, headaches, fatigue, joint pain and a round, expanding rash. It is treated with antibiotics. Untreated cases can lead to facial paralysis, nerve pain, arthritis and heart palpitations.
Scientists trace the boom in Lyme disease cases in the Northeast to shifting land uses since the colonial era, long before the disease was identified in Connecticut in the 1970s. Colonists felled large swaths of forest land for agriculture and also hunted greater numbers of deer. This depleted tick populations, since one of their main animal hosts was not as abundant.
When the nation's agricultural base moved to the Midwest in the 1800s, large parts of the Northeast gradually saw reforestation that enabled deer and tick populations to recover. But while more forest land helps ticks thrive on a variety of wood-dwelling species, ecologists believe Lyme disease is propelled by fragmenting these forests and reducing the options for ticks to find hosts. They're more likely to bite animals that harbor the disease and then bite humans who venture into their backyards.
"When you fragment that forest and place housing developments and strip malls and agriculture and stuff in the midst of that forested landscape, then you make matters worse," Richard Ostfeld, a disease ecologist at New York's Cary Institute of Ecosystem Studies, told PBS last year.
In southeastern New York, Ostfeld and his colleagues collected ticks from various forest patches to study whether the size of woodlands surrounding developments affects the presence of ticks carrying Lyme disease. His team found that in the smaller forests, the density of ticks in the nymph stage was about three times higher than in the larger forests. About 70% of the nymphs from the smaller forests carried Lyme disease bacteria, compared to 48% of those from the larger forests.
Researchers have found that ticks in the nymph stage are the primary risk factor for Lyme disease. People often don't realize they have been bitten by these tiny ticks.
Tick populations tend not to decline when forests are cleared because various disease-carrying species tend to remain in these areas. They feed on the hosts that haven't moved, or those that have moved in — like people.
Other factors also may help explain the growth in tick populations in the Northeast. Some research suggests declining red fox populations have reduced a common predator for mice, chipmunks and shrews — all of which carry Lyme disease bacteria. The dwindling number of red foxes has been linked to the emergence of coyotes in some parts of the Northeast. Coyotes not only hunt red foxes, but are less likely to eat rodents that contribute to the spread of Lyme disease.
Suzy Yetter, an ecologist with the environmental group ClearWater Conservancy, told Spotlight PA that municipalities and rural communities should think more strategically about how they divvy up forests.
"There's no easy solution," Yetter said. "Because if you do one thing, you might increase the chance of something else, but thinking in terms of forest blocks, if your municipality has a huge section of forest, don't develop that."
Black-legged ticks carry Lyme disease and five other pathogens, including Powassan virus, a rare neurological disease that has become more prevalent in recent years. The American dog tick and the lone star tick, also present in Pennsylvania, carry other pathogens that commonly cause diseases in people – but not Lyme disease.
The lone star tick has garnered growing attention because it has been linked to the spread of alpha-gal syndrome, which can cause a potentially life-threatening allergy to red meat and other animal products like cow's milk. During a recent five-year stretch, the number of people diagnosed with alpha-gal syndrome was more than twice as high as the nine years preceding it, according to CDC research.
There has been a renewed push to develop another Lyme disease vaccine now that reported cases are at an all-time high. Shots for Lyme disease have not been available since 2002, when the last vaccine, Lymerix, was pulled off the market.
Pennsylvania's high rates of Lyme disease are beginning to prompt more action in Harrisburg. State Sen. Michelle Brooks, a Republican from Mercer County, has called for better testing and tracking efforts to monitor the disease.
"Pennsylvania, unfortunately, ranks the highest in the country for the number of confirmed Lyme disease cases. It's a pervasive threat to public health, but hasn't gotten the attention or resources it deserves," Brooks said in May. "We must fight back against Lyme and other tick-borne diseases and ensure that patients have access to coverage for comprehensive testing, diagnosis and treatment."