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Nearly 15% of People Worldwide Have Had Lyme Disease, Study Says

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Lyme disease was first named nearly 50 years ago in Lyme, Connecticut, but the tick-borne disease is now found around the world. A new study published in BMJ Global Health estimates that 14.5% of the world’s population has, at some point, been infected with Lyme disease, which can cause short-term symptoms including a skin rash, fever, headache, and fatigue—as well as long-term ones, including damage to the joints, heart, and nervous system.

The scourge also appears to be getting worse. Lyme prevalence has doubled from 2010 to 2021, compared to 2001 to 2010.

The study, led by a team of researchers from the Institute for Tropical Medicine at Kunming Medical University in China, was a meta-analysis of 89 studies dating as far back as 1984 until 2021. That cache of papers included blood samples from nearly 160,000 people who were tested for Borrelia burgdorferi, the spiral-shaped bacterium that causes Lyme disease. Blood samples were examined by any one of a range of methods, including the ELISA test, which uses enzymes to detect the presence of antibodies in the blood; and the IFA test, which uses a fluorescence method to do the same job. Considered more reliable than both techniques is the Western blotting method, which looks for proteins in the blood as markers of infection with a target bacterium or virus. The Western blotting method is thought to reduce the incidence of false positive results, but it has a major drawback: it’s less sensitive in the early stages of Lyme infection than ELISA or IFA. Of the 89 studies selected, 58 used Western blotting.

That nearly 15% of people worldwide have had Lyme is striking. “[Lyme disease] has become the most common tick-borne zoonotic disease worldwide,” the researchers wrote. “There is a need for preventive measures, which necessitates understanding the dynamics of tick-borne disease transmission and the lack of effective disease-prevention strategies.”

As for why the rate of Lyme has doubled in the last decade, climate change is thought to play the leading role. Higher temperatures and longer springs and summers increase ticks’ range and the amount of time people spend outside. “Tick populations,” the researchers write, “have expanded globally and geographically in recent years, thereby greatly increasing the risk of human exposure.”

Read More: We Used to Have a Lyme Disease Vaccine. Are We Ready to Bring One Back?

Geography was key to determining who was at greatest risk of contracting Lyme disease. The highest incidence of infection was found in central Europe, with 21% of the population affected; eastern Asia came in second at 16%, followed by western Europe at 13.5%. Lower risk sites were Oceania at 5.5%, southern Asia at 3%, and the Caribbean at 2%. The Americas finished somewhere in the middle at 9.4%. Part of the reason for the variance is the differing presence of Lyme-bearing ticks in various parts of the world, but other factors played a role in who is likely to contract the disease and who isn’t.

People 50 and older were at higher risk than younger groups. Of the people who tested positive in the study, 18.1% were in this age group, compared to 17.6% in the 40 to 49 group and 9.5% in the 39 and younger group. The researchers did not speculate about the reason for the finding, but weaker immune systems could play a role, or merely the fact that older people have had more years to be exposed to Lyme than younger people.

Not surprisingly, people who lived in rural areas were at higher risk than those who lived in urban ones, with 12.6% of positive tests coming from rural populations compared to 8.1% from urbanites. The researchers did not break their data out by occupation, but they speculated that high-risk jobs include farmers, soldiers, and homemakers, all of whom might have greater exposure to the outdoors and to tick-bearing animals like dogs and sheep.

The varying methods used to test for Lyme infection muddy the results somewhat, with Western blotting tests generally producing lower overall figures. The 14.5% topline number, for example, is an average of a 9.8% rate confirmed with Western blot and 17.5% using ELISA and IFA. There is a similarly wide range when it comes to age, with ELISA and IFA showing an 18% rate in the 50 and over group compared to just 8.8% with Western blot. The researchers point to newer, still-in-development molecular diagnostic techniques as potential ways to test more reliably for Lyme and reduce such statistical ambiguity in the future.

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Write to Jeffrey Kluger at jeffrey.kluger@time.com