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Just Curious

Why Has Lyme Disease Become so Prevalent?

By Andrea Appleton
John Aucott is an assistant professor of medicine at Johns Hopkins who has spent more than a decade studying Lyme disease. He is the founder and director of the new Lyme Disease Clinical Research Center in the Division of Rheumatology at Johns Hopkins.
Lyme disease is ancient. Scientists recently discovered that Ötzi, a 5,300-year-old mummy found in the Italian Alps, probably had it. Yet the tickborne illness wasn’t named until the 1970s, after a number of children in Lyme, Connecticut, mysteriously developed what was initially thought to be juvenile rheumatoid arthritis. Since then, it has spread, reaching epidemic levels in temperate regions across the Northern Hemisphere. Lyme is now one of the most commonly reported infectious diseases in the United States, particularly on the East Coast. Why has Lyme disease spread so much in recent decades?
 
Changes in our natural surroundings are to blame, says John Aucott, founder and director of Johns Hopkins’ new Lyme Disease Clinical Research Center. Lyme is a zoonosis, a disease normally found in animals that can also afflict humans. Borrelia burgdorferi, the bacterium responsible for it, depends on three principal animals: deer ticks, mice, and deer. Tick larvae feed on mice, ingesting the bacteria and infecting other mice with it. As adults, the ticks need abundant blood to reproduce. Though deer do not carry Lyme disease themselves, they serve as an ample food source for the ticks that do. The ticks multiply in response to this easily accessible source of energy. 
 
At the turn of the 20th century, deer were nearly extinct. Hunters had deci­mated their populations and Lyme disease was likely rare as a result. Over the last century, deer populations have exploded and our land use has changed. “People moved from the city to the suburbs because they like to live on one-acre lots that allow the forest to intermingle, and deer are happy to cohabitate with people,” Aucott says. (Anyone who has had a garden destroyed by hungry deer knows how brazen they can be.) Meanwhile, warmer winters may be expanding the geographic range of animals associated with Lyme, helping to explain the spread of the disease in places like Canada. 
 
“You have to have the mice harboring the bacteria, the deer feeding the ticks, [and] the people living in beautiful areas that are inhabited by ticks, deer, and mice—which is, of course, the entire Eastern seaboard of the United States now, from Maine to Virginia,” Aucott says. Increasingly, infected ticks chance upon a meal in the form of a human. 
 
The only sure way to avoid Lyme is to stick to the pavement, Aucott says. But precautions help. He suggests staying on trails when hiking in nature and treating clothing with the tick-killing chemical Permethrin. It’s often thought that a welt in the shape of a bull’s-eye accompanies Lyme infections, but Aucott says that’s usually not the case. Tick bites can be misdiagnosed as a spider bite as a result. “People often miss the chance to make an early diagnosis because they’re looking for the target lesion,” he says, “when it’s often really just round and red.”  
Illustration of a deer, rat and crab with a silhouette of a human head behind them
Barry Falls

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