A man in California has tested positive for a new tick-borne disease. Here's what to know about hard tick relapsing fever, and where it is in the US.

Hard tick relapsing fever, a disease caused by tick-borne bacteria, may cause symptoms similar to Lyme disease, but without the rash.

• A man in California had recurrent fevers that came and went during a three-month period.
• He had the state's first documented infection with the tick-borne bacteria Borrelia miyamotoi.
• These infections may become more common as people spend time outside and diagnostics improve.

It started with a fever in October 2021.

The patient, an adult man in the San Francisco area, suffered through night sweats and nausea for a day before his symptoms resolved. He felt better — until the fever came back around two weeks later. Again, it persisted for about 24 hours and stopped without explanation.

Recurring fevers are a hallmark symptom of infection with certain tick-borne bacteria, Dr. Luis Rubio, an assistant clinical professor at the University of California, San Francisco, told Insider.

Rubio treated the sick man and co-authored a report describing his rare case, which was published today in the journal Emerging Infectious Diseases.

Rubio took several blood tests to uncover clues about the man's condition. He screened for antibodies that would indicate an infection with Lyme disease, as well as markers related to other environmental infections.

The patient's immune system was suppressed by a medicine he was taking for multiple sclerosis, so he was at a higher risk for infections in general.  But these fevers were new, and they continued on and off through December 2021.

The first blood panel was unremarkable, so Rubio said the team next used a new type of DNA sequencing to inspect the man's blood on the molecular level. Eventually, finding the bacteria's DNA led them to the diagnosis and treatment of an extremely rare disease.

'Doctors can't test for things that they don't know about.'

The test revealed the man was infected with bacteria called Borrelia miyamotoi. The bacteria can cause an illness known as hard tick relapsing fever, and it's spread by the same blacklegged tick species, also sometimes called deer ticks, that can transmit Lyme disease. 

B. miyamotoi was first identified in ticks in Japan in 1995, according to the Centers for Disease Control and Prevention. It has since been found in blacklegged ticks on both coasts of the US, and the first human infection in North America was recorded in 2013.

Rubio said that people in California have tested positive for antibodies that could indicate exposure to B. miyamotoi in the past, but this case report describes the first time a human has been officially diagnosed and treated in the state.

"It's probably been around and we just haven't been able to identify it," Rubio said of the infectious agent, calling for more awareness of tick-borne pathogens. "Doctors can't test for things that they don't know about."

Symptoms include 'cyclical' fevers, chills, and headache

 Symptoms of infection with B. miyamotoi may appear non-specific: According to the CDC, the most commonly reported symptoms are recurrent fevers, chills, and headache.  

But the cyclical nature of the fevers should be a telltale sign for doctors to test for B. miyamotoi, Rubio said.  

Fortunately, most tick-borne infections, including this one, can be treated with a hefty dose of the common antibiotic doxycycline, Rubio said. After four weeks on the antibiotic, the California man made a full recovery, according to the report.

"I think a lot of people let their guards down in terms of thinking about tick borne illnesses on the West Coast," Rubio said. "Public awareness is important, given that it reinforces the preventative measures that people can take when hiking in these areas in California."

Anyone spending time outside during tick season, which spans from April through September in the US, should protect themselves with tick repellent, long sleeves, and diligent checks after walking or hiking in wooded areas, Rubio said.