A new study estimates almost 15% of people worldwide have had Lyme disease. Side effects can include life-threatening heart inflammation and arthritis.

Scientists in a new study estimate that almost 15% of the world’s population have had Lyme disease, a tick-borne condition that can cause debilitating complications if symptoms aren’t recognized and treatment is delayed.

Lyme disease can be misdiagnosed because initial symptoms are flu-like, and blood tests aren’t always reliable, particularly early in the illness.

A common misconception is that the disease can be distinguished by a bull’s-eye rash; about a quarter of people don’t get any skin lesions at all.

Writing in BMJ Global Health on Monday, researchers from China estimated 14.5% of the world’s population had caught the illness.

The team analyzed 89 studies on Lyme disease that included the blood tests of more than 158,000 people.

The condition seems to be growing in prevalence, the researchers said. In the US, confirmed Lyme-disease cases increased by 44% from 1999 to 2019, according to Centers for Disease Control and Prevention data.

The authors said the results were preliminary and warranted further studies.

Lyme disease starts with flu-like symptoms
Early symptoms of Lyme disease include fever, severe tiredness, headache, muscle and joint aches, and swollen lymph nodes.

Most people with a rash caused by Lyme disease will get a uniformly red or blue lesion that expands to more than 2 inches in size. About 20% of Lyme-disease lesions have a bull’s-eye appearance. Doctors can use a rash to diagnose the condition without a blood test.

Lyme disease is caused by bacteria spread by ticks
Lyme disease is spread by infected ticks. They can attach to any part of the body to transmit Lyme-disease-causing bacteria but are often found in the groin, armpits, and scalp.

In the Northeastern, mid-Atlantic, and north-central US, the blacklegged tick — also known as the deer tick — spreads the bacteria, while the western blacklegged tick spreads it on the West Coast.

Not all tick bites will transmit the Lyme-disease-causing bacteria, and uninfected tick bites and other insect or spider bites won’t expand.

Lyme disease can cause disabling complications
If the bite becomes infected and is left untreated, or if treatment is unsuccessful, the bacteria can travel through the bloodstream and cause a similar rash in other parts of the body. It can also trigger what is known as disseminated Lyme disease.

At this point disabling complications can include life-threatening heart inflammation called carditis, cognitive impairment, a facial droop, meningitis, swollen knees, and arthritis.

A two-part blood test that detects disease-fighting antibodies against the bacteria is used to diagnose the condition at this stage. A negative test doesn’t rule it out, though — in some Lyme disease cases it may never turn positive.

According to the CDC, early diagnosis and treatment with
antibiotics
can help prevent the late stages of Lyme disease.