A recent study in monkeys found that four weeks of antibiotic treatment didn’t eliminate all the Lyme disease-causing bacteria. What does this mean for people?

For most people with Lyme disease, a short course of antibiotics is enough to eliminate this common tick-borne infection. But for others, debilitating symptoms can linger for weeks or months, even after treatment.

Two recent papers published in the journals PLoS ONE and the American Journal of Pathology seem to provide support for why some people treated with antibiotics still have symptoms months or even years later.

Some experts caution, though, that because the study was done in monkeys, the results may not directly apply to people.

The papers come from a single study designed by Tulane University researchers.

Researchers looked for the presence of Borrelia burgdorferi, the bacteria that causes Lyme disease, in rhesus macaques before and after treatment with antibiotics.

Even after 28 days of antibiotic treatment, bacteria were present in ticks that fed on the monkeys and in the monkeys’ organs.

Researchers started treating the monkeys four months after infection. This is similar to the delay in diagnosis and treatment that many people go through after being infected by a deer tick carrying Lyme disease.

People often don’t realize that a tick has bitten them. Or they may not develop the characteristic bullseye-shaped rash that would lead them to seek medical care sooner.

So, weeks or months may have passed by the time people show up at the doctor’s office with more severe symptoms, such as heart problems, headaches, pain, or weakness.

Study author Monica Embers, PhD, assistant professor of microbiology and immunology at Tulane University School of Medicine, said in a press release that although antibiotics help many people with Lyme disease, standard treatment may not be enough when diagnosis is delayed.

Dr. Samuel Shor, president of the International Lyme and Associated Diseases Society, points to the study as evidence that a low-level persistent infection may be behind the long-term symptoms that some people treated for Lyme disease experience.

“The problem is that the organism has the potential capacity to continue to infect multiple areas of the body and ultimately result in what we characterize now as chronic Lyme disease,” Shor told Healthline.

As officials at the Centers for Disease Control and Prevention (CDC) point out, “chronic Lyme disease” is a term that is not used the same by everyone.

It’s often used to describe lingering symptoms after Lyme disease treatment. But some people also use it to describe those symptoms even when there is no evidence that a person has had a B. burgdorferi infection.

Many researchers instead refer to this collection of symptoms as post-treatment Lyme disease syndrome (PTLDS) or post Lyme disease syndrome (PLDS).

Whether Lyme disease is responsible for lingering symptoms is a contentious issue among doctors and patient advocate groups.

In the same press release, Wendy Adams, research grant director for the Bay Area Lyme Foundation, said the study shows a need to “move away from the one size fits all approach to Lyme treatment.”

For Lyme disease treatment, both the Infectious Diseases Society of America (IDSA) and the CDC recommend a single course of antibiotics no longer than 21 days.

Research shows that additional antibiotics don’t help people with lingering symptoms after treatment for Lyme disease.

“High-quality trials that looked at people who have had persistent symptoms after treatment for authentic Lyme disease have not shown that longer-term treatment with antibiotics — compared to placebo — really yield any significant or durable benefits,” said Dr. Paul Auwaerter, IDSA president and clinical director in the Division of Infectious Diseases and professor of medicine at Johns Hopkins University School of Medicine.

A study in the New England Journal of Medicine estimates that 10 to 20 percent of people treated with antibiotics for Lyme disease will continue to have fatigue, pain in their muscles or joints, and difficulty thinking for six months or longer after treatment.

Some people point to the new study in monkeys as evidence that the bacteria that cause Lyme disease can remain in the body after treatment with antibiotics.

“Am I saying that all of the people with chronic Lyme disease-like presentations have ongoing infection?” said Shor. “No. But I would argue that this study provides additional evidence to support that active infection may play a role in what we would characterize as post-treatment Lyme disease syndrome.”

Auwaerter said that persistent bacterial infections are “not a new concept.”

But he questions whether the results of the study can be directly applied to people.

“It is an animal model that doesn’t really resemble human disease,” Auwaerter told Healthline. “As far as we know, the animals aren’t terribly sick and they don’t develop arthritis, meningitis, or some of the other manifestations that afflict humans.”

Fatigue, in particular, is a difficult symptom to study in animals.

Auwaerter said that studies like the recent one are not the best way to look at the connection between treated Lyme disease and fatigue “because no one is asking these primates whether they’re tired and whether they’re getting better.”

So why do some people still suffer symptoms long after being treated for Lyme disease?

That is the big question — one that researchers are trying to solve.

In December, Shor spoke about the challenges of treating Lyme disease to the Tick-Borne Disease Working Group, which was established as part of the 21st Century Cures Act.

“It is the hope of the Lyme community that this working group will both identify and clarify some of the issues to which I’ve alluded,” said Shor.

He and others see a need for more accurate tests of Lyme disease infection and a different approach to treatment than the current one.

“It’s a perfect storm,” said Shor, “where you’ve got a condition that’s hard to identify, that’s a lot more common than people realize, and that’s being questioned by many in the medical community for various reasons.”

Auwaerter is not convinced that lingering infection is behind these long-term complications. He also stands behind the clinical studies that show limited benefits of longer antibiotic treatment.

He said that one possibility for long-term symptoms is that “debris” from the infection may be driving low-level immune responses in the body.

Understanding why some people still have symptoms months or years after treatment is also hampered by the fact that people respond differently to Lyme infection.

Some people who develop swelling in their joints as a result of Lyme disease may not have any flu-like symptoms after infection. They only show up at the doctor’s office when their knee bothers them.

But others who are seriously ill shortly after infection — and went to their doctor soon afterward — will continue to have fatigue even though they were treated early.

“There’s definitely a range,” said Auwaerter, “and I think it potentially has something to do with the organism and also with an individual’s own immune responses.”

Over the long run, though, most people get better.

“We know from longer-term studies that have been performed very carefully — even over 10 years — people generally feel well and return to what would be considered a baseline health for their age,” said Auwaerter.