Evidence is mounting that certain bacteria spread by cat scratches and other animal exposures can sometimes have startling effects on our brain. In a new study, researchers detail a group of patients with psychiatric illness who simultaneously showed signs of infection from bacteria called Bartonella, including tell-tale skin lesions. However, a direct cause-and-effect link between the bacteria and worsening brain health still isn’t conclusive, nor is it clear how often this may happen.
Last year, researchers in North Carolina published a case report on a 14-year-old boy who suddenly developed violent psychosis and delusional episodes in 2015. Despite repeated hospitalizations and various treatments, the boy showed little improvement. Then in 2016, doctors noticed the boy had very characteristic lesions, resembling stretch marks, along his thighs and armpits. Eventually, tests revealed that he was carrying a chronic bloodstream infection caused by the bacteria Bartonella henselae, and he received intense antibiotic therapy. Once the infection was cleared, his schizophrenia-like symptoms disappeared as well, allowing him to resume a normal life and attend college.
The study’s lead author, Edward Breitschwerdt, a veterinary internist at North Carolina State University, told Gizmodo at the time that these bacteria could “potentially be causing a tremendous amount of undiagnosed disease worldwide.”
Breitschwerdt and his team have now published a follow-up to that report, released earlier this month in the journal Pathogens. The paper looks at 33 patients with similar neuropsychiatric symptoms and suspected exposure to Bartonella bacteria, not including the boy detailed in the 2019 report.
In 29 of the 33 patients studied, evidence of previous or current Bartonella infection was found, with tests finding antibodies to the bacteria or direct traces of the bacteria in their system. Twenty-four of these patients also had skin lesions linked to Bartonella infection that developed at around the same time their other symptoms began; these were usually vertical or horizontal reddish lesions that looked like stretch marks and were found along people’s limbs or trunk. The list of neuropsychiatric symptoms in these patients included sleep disorders, confusion, anger issues, anxiety, depression, and headaches/migraines.
The rod-shaped Bartonella bacteria are somewhat strange, even among their own kind. They can survive and replicate inside other cells, much like viruses—in people, they usually choose the cells that live inside or line our blood vessels. This disappearing act allows them to evade conventional tests for detecting infection and makes it harder for antibiotics to kill them.
Bartonella henselae is the most commonly seen infection in humans and causes cat scratch disease (also called cat scratch fever), named after its ability to spread from the bite or scratch of cats. Despite the name, B. henselae and other disease-causing Bartonella can be spread through dogs and other mammals, as well as through exposure to parasites like ticks, fleas, and lice.
Bartonella bacteria aren’t usually linked to serious illness in people. But certain groups, like those immunocompromised, might be more vulnerable generally. Other vulnerable groups, like those experiencing homelessness, can contract another disease caused by these bacteria, called trench fever, that can cause life-threatening heart problems if left untreated. And this is the latest bit of research to suggest that the harm from these bacteria might be greater than we know.
Right now, though, it’s important to emphasize this study alone doesn’t prove that Bartonella can cause mental illness, nor how often this is happening if the connection is real.
As the authors note, research on blood donors has shown that many people carry signs of infection from these bacteria but appear to be perfectly fine. It’s possible there’s a still-hidden factor that only makes some people susceptible to these more frightening psychiatric symptoms, like a person’s preexisting health or genetics. This study also can’t tell us how Bartonella could be causing brain problems, though the authors suggest that damage to blood vessels from these bacteria could trigger chronic inflammation that affects the brain and skin at the same time. Aside from the 2019 case, they don’t mention anyone else who seems to have recovered from their symptoms once their infection was cleared, which would be solid evidence for a causative link (that said, it’s possible that getting rid of these infections may only prevent the worsening of symptoms, not reverse them entirely).
According to Breitschwerdt, the study wasn’t meant to test whether treating Bartonella could alleviate people’s mental health problems. But anecdotally, several families have told him that treatment did seem to help their children.
All of these questions will take more research to answer, which Breitschwerdt and his team plan to keep pursuing. One type of research they hope to get funding for is a case-control study, where people with these symptoms are compared to a large group of people with similar demographics but without the symptoms, which should help us understand whether Bartonella exposure is a key risk factor. He also hopes future research will involve controlled treatment trials.
For now, they’re advising other doctors to keep an eye out for these sorts of cases in the future, where skin and brain problems seem to show up at the same time and could be traced back to these bacteria.
This article has been updated with comments from the study’s lead author.