Why Are Tick-Borne Illnesses Increasing in the U.S.?

As more people head outdoors this summer to go camping, hiking, or even gardening in their own backyards, they may face higher chances of being joined by some unwelcome visitors — ticks.

Reports have shown that the tick population has expanded in recent years, and the diseases they carry have become more prevalent. Tick-borne illnesses in the U.S. have more than doubled in the past two decades, due to factors such as increased awareness, a growing population, and environmental change.

As there are still many unknowns about tick-borne diseases, clinicians and researchers are concerned about the increasing public health threat.

Tick Distribution Expands

Each year, 30,000 to 40,000 Lyme Disease cases are reported to the CDC by state health departments. But that figure is much lower than what is actually diagnosed and treated. Approximately 476,000 cases each year were diagnosed and treated for the illness between 2010 and 2018, according to a recent CDC estimate.

The prevalence of all tick-borne illnesses has also jumped significantly in the past few years, with anaplasmosis, spotted fever rickettsiosis, babesiosis, tularemia, and Powassan virus disease all rising in recent years — although, these diseases are much rarer.

Experts say that increased awareness of tick-borne illnesses, as well as better public health surveillance, has contributed to higher numbers. However, environmental factors may also play a role.

One of these environmental factors is an expansion of tick populations to different geographic regions, said Alvaro Toledo, PhD, assistant professor of entomology at Rutgers University.

The blacklegged tick (Ixodes scapularis in the eastern U.S.; Ixodes pacificus in the west), which transmits the bacteria that causes Lyme Disease (mainly Borrelia burgdorferi), has crept into areas it historically hasn’t been seen. “We know that this tick has moved north and west quite significantly in the past 20 years,” Toledo said in an interview. Deer and mice populations — primary hosts for this tick — also play a role in expansion.

A wider distribution of ticks means that more people are encountering them. But while ticks have migrated, people have too. Previous wooded areas have been deforested and developed into suburban environments, pushing more people into tick habitats and increasing their chances of getting bitten, Toledo said.

Some experts have attributed the expansion of tick populations — and the subsequent increase in illness — partly to climate change. As temperatures get warmer and winters get shorter, ticks have extended time in the spring, summer, and fall to find hosts, said Jean Tsao, PhD, associate professor in the department of fisheries and wildlife at Michigan State University, who studies ticks and tick-borne diseases.

“Once you get enough ticks surviving, then you can get the pathogen cycle to be turning over as well,” Tsao told MedPage Today.

For example, the lone star tick (Amblyomma americanum) — a historically southern species that has been associated with a rare yet potentially life-threatening red meat allergy, among other illnesses — has also been moving steadily north over the last half century, most recently creeping up into places like Connecticut, Rhode Island, and southwestern Michigan, Tsao said.

“Wildlife hosts on which lone star and blacklegged ticks feed are common and abundant, and are not a barrier to the spread of these species. Along with a previously shorter ‘growing season’ to find hosts, the probability of surviving through the winter may have been lower for the lone star tick compared to the blacklegged tick,” she added. “Thus, a climate barrier that had been present is now being gradually reduced.”

A “Massive” Public Health Issue

As more people encounter ticks and develop tick-borne disease, clinicians and researchers have expressed concerns about the potential chronic health issues.

Among those diagnosed and treated for Lyme Disease annually, for example, clinicians see persistent symptoms in about 10% to 20% of patients who were treated with the standard course of antibiotics, said Shannon Delaney, MD, director of child and adolescent evaluation at Columbia University’s Lyme & Tick-borne Disease Research Center.

“That’s a massive public health problem,” Delaney said. “And, that’s just Lyme Disease. That doesn’t include other diseases you can get with a tick.”

Delaney told MedPage Today that one of the most concerning populations she sees in her clinical practice are those who had experienced delayed diagnosis and treatment. These patients may have never seen the tick attached, or never developed the tell-tale Lyme Disease symptoms, such as an erythema migrans rash or flu-like illness.

Experts still do not understand why some patients don’t develop Lyme Disease symptoms right after the initial exposure. “That does not mean that they will not develop symptoms, weeks, months to years down the road,” Delaney said. “That’s when it’s most problematic.”

But the most important research priority for Delaney is the pathophysiology of illness in these patients — specifically, what causes post-treatment Lyme Disease syndrome?

“Really, that’s the $64 million question,” Delaney said. “Why do people continue to have these persistent symptoms?”

Much like the COVID-19 long haulers, there is still much unknown about chronic Lyme, Delaney said. There’s a bit of a debate in the field about whether it’s a persistent infection that leads to lingering symptoms, or a post-infectious autoimmune phenomenon.

It’s Not Just Lyme

Lyme Disease is the most common tick-borne illness, but experts say clinicians should look out for other diseases that ticks can carry.

“The most common tick-borne agent is the bacteria that causes Lyme Disease,” said Rafal Tokarz, PhD, assistant professor of epidemiology at Columbia University’s Mailman School of Public Health. “But along with that, there are other pathogens that are present in the same ticks that transmit that bacteria.”

Blacklegged ticks, also known as deer ticks in the eastern U.S., can carry bacteria that cause diseases including anaplasmosis and babesiosis, as well as the Powassan virus, which is rare but carries a high fatality rate.

This tick species can also carry a new bacteria, Borrelia miyamotoi, which is similar to the bacteria implicated in Lyme Disease. Delaney, of Columbia’s Lyme Disease center, said that because this disease is so new (it was discovered in the U.S. in 2013), we do not know much about it, and may not test for it.

In a study that Delaney and colleagues conducted in 82 patients who sought a consult for long-term sequelae after suspected tick-borne illness, 26% tested positive for Borrelia miyamotoi antibodies. Yet, almost none of the patients (98%) were previously tested for this bacteria, indicating that physicians may not be testing for this emerging illness.

Co-infections, which occur when a tick transmits multiple pathogens to a host, are an emerging area of interest. These infections are rare — but experts say it is critical for physicians to know which illnesses to test for, and different courses of treatment. Anaplasmosis and Lyme Disease, for example, may receive the same course of treatment.

But for babesiosis, which is typically treated with anti-malarial drugs, the course of treatment is totally different. Experts say clinicians need to know when to test and treat for combined tick-borne illnesses.

“I think in general, doctors need to be aware of the existence of these coinfections,” Delaney said.

Prevention — The Key Public Health Intervention

There have not been many effective ways to control the tick population, so teaching people how to prevent tick bites in the first place is the optimal public health intervention, experts said.

“Right now, we are still stuck with personal prevention and awareness,” said Sam Telford, PhD, an infectious disease professor at the Tufts School of Veterinary Medicine in Grafton, Massachusetts.

Telford said that until a vaccine for Lyme Disease is available, it is critical to use repellents and wear long-sleeve, light-colored clothing before going outdoors for any extended period of time.

In addition to tick repellents (preferably those that contain DEET or IR3535, a milder alternative for pregnant women or kids), experts said that permethrin is an effective way to prevent tick bites, and is typically used on clothing.

And of course, there are tick checks. Experts recommend that people check themselves every 12 hours after outdoor activities, to prevent missing a potential bite.

Telford said that proper prevention and awareness should make people feel more secure about spending time outside.

“People should not be afraid of going outdoors, and enjoying their yards or enjoying hiking in the woods,” he said in an interview, noting that applying tick repellent should be like wearing a helmet while biking.

“Let’s promote that message, and that it’s healthy to be outdoors,” he said. “It’s not Armageddon.”

  • Amanda D’Ambrosio is a reporter on MedPage Today’s enterprise & investigative team. She covers obstetrics-gynecology and other clinical news, and writes features about the U.S. healthcare system. Follow

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