An especially bad tick season in the United States is probably hitting its peak, and experts are stressing the importance of taking personal precautions to protect against rising cases of tick-borne disease.
Diagnoses of Lyme disease - a bacterial infection spread by bites from blacklegged ticks, or deer ticks - were 17% higher in the first week of June than they were a year earlier, according to data from athenahealth, a health care technology company. There has been a sharp rise over the past three weeks, and trends from the past three years suggest that this year's seasonal peak is falling just before the Fourth of July holiday.
The findings are based on millions of patient records - about a fifth of all office visits in the US - and represent the share of office visits that result in a Lyme disease diagnosis.
"The blacklegged tick is the one that's public health enemy number one in terms of ticks because it's competent to vector many pathogens," said Emily Mader, program manager for the Northeast Regional Center for Excellence in Vector-Borne Diseases.
"They can pass a lot of different illnesses to people. They're very abundant in the Northeast, upper Midwest and Pacific Coast. And they really love to live in habitats where people recreate or live."
Lyme disease isn't the only pathogen linked to ticks. The US Centers for Disease Control and Prevention tracks at least four species of ticks and at least seven pathogens that they can spread.
A CDC report from March warned that cases of babesiosis are on the rise. Cases of the tick-borne disease - which can cause fever, muscle and joint pain and headache, and which can be fatal - grew 25% from 2011 to 2019.
Other surveillance data from the CDC shows that tick bites caused more than 100 out of every 100,000 emergency department visits in May and June, a higher rate than any other month since 2019.
A variety of factors are raising the risk for tick-borne disease, experts say. Among them are an expanding tick habitat and a changing human habitat.
The deer tick is spreading to the north and west, farther into the Northeast and Midwest. The Lone Star tick, which is typically found in the Southeast, is also moving north into middle Atlantic states, said Dr. Robert Smith, an infectious disease physician at MaineHealth and co-director of the Lyme Disease Initiative at Tufts University.
"So you have expanding range of two different species of ticks that are very effective at transmitting diseases to people into new areas," he said.
"And then you have human residential landscape change, with people moving into areas where there's been reforestation and fragmentation. You're creating environments which are suitable for the different hosts of a tick: rodent hosts, deer hosts and, of course, more people coming into contact, too."
Warmer weather also gives ticks a greater opportunity to extend their lifecycle, which gives them more chances to find hosts and spread disease.
Over a full lifecycle of two or three years, a tick has four stages: egg, larva, nymph and adult.
"That presents three opportunities in a tick's life where they can feed on an animal and catch a pathogen themselves. So each time they feed, the likelihood that they might have an infection that they could pass to a human increases," Mader said.
This part of the year, from late May until early July, is when the deer tick is typically in its nymphal life stage, she said - and a time when they can pose particular risk to humans.
In the nymph stage, ticks may already be carrying a disease from their first host, and they're so much tinier than adults - about the same as a poppy seed - that they could be missed.
"It's often the tick that you don't find that makes you sick because it's been on long enough to transmit illness," Smith said.
Increased awareness of tick-borne disease could also account for some of the rise in Lyme diagnoses, but experts encourage both doctors and patients to pay close attention to their symptoms.
"For several of these diseases, the typical presentation is what we would have thought of as 'summer flu' years ago. But patients come in with a fever and a headache and achiness in their joints, nothing that's very specific," Smith said.
"So it's important, first of all, that it be recognized that a number of these diseases can look the same way when somebody comes into the office. And then, for people to realize if they're in areas where they could be exposed to take note of symptoms like fever, headache, joint pains coming on acutely."
Controlling the tick population isn't very straightforward, experts say, making personal vigilance critical, especially within the first 24 hours of possible exposure.
"Ticks aren't like mosquitoes, for example, where you can see that there's a problem, apply pesticides or dump out water and control the populations. They're kind of ubiquitous across the landscape," Mader said. "Because of the widespread nature of the pest itself - it's jumping jurisdictional boundaries, private property boundaries, animals that transport ticks across wide geographies - it's just a really hard thing to contain."
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