A California man is the first person in the Western US to have a confirmed infection with a curious bacterium that has lurked in the region for over two decades—and researchers fear the pathogen may finally be emerging there.

The bacterium is Borrelia miyamotoi, a corkscrew-shaped spirochete that is spread by black-legged ticks and causes a rare disease called hard tick relapsing fever. The spiraled microbe is a relative of the more well-known Borrelia burgdorferi spirochete, the bacterium that causes Lyme disease. But B. miyamotoi has many notable differences from its cousin, including its inconspicuous spread.

While Lyme disease was first reported in 1975 in the US and B. burgdorferi first identified in 1982, B. miyamotoi was only identified in ticks in 1995 in Japan. But once discovered, it was soon found in many other places, including Europe and many parts of North America. Ticks collected in California as early as 2000 were found to carry the new spirochete, for example. Yet, the first cases of disease caused by B. miyamotoi in the US were only first confirmed in 2013 in the Northeast. Until now, no confirmed cases have been reported in the western part of the country, despite the bacterium's prevalence in adult black-legged ticks (Ixodes pacificus) being similar to that of B. burgdorferi, the Lyme disease spirochete.

Shadowy disease

B. miyamotoi disease, or hard tick relapsing fever, is a tricky infection to identify. It is often marked by fevers that come and go, along with nondescript fatigue, chills, and aches and pains. In severe cases, which often affect people with compromised immune systems, the disease can progress to meningoencephalitis, inflammation of the brain and surrounding tissue. But in many cases, the disease resolves on its own.

In clinical tests, people infected tend to show low levels of white blood cells and platelets (leukopenia and thrombocytopenia), as well as elevated liver enzymes and excess protein in their urine. But there are no simple tests to confirm the disease. Borrelia bacteria share many proteins, so probing for antibodies against the microbe will easily pick up other related bacteria, including the one that causes Lyme disease. The only method to definitively identify a B. miyamotoi infection is to directly probe for fragments of the bacterium's genetic sequence in a person's blood or cerebrospinal fluid, which few laboratories do.

If an infection is confirmed, there's no well-established treatment. With so few cases clearly identified, researchers have not been able to run clinical trials. But, so far, a two-week course of doxycycline or amoxicillin antibiotics seems to do the trick for most cases, with IV-antibiotic treatments being used in severe cases. A small number of people, however, may experience what's called "the Jarisch-Herxheimer reaction," which occurs in the first few hours or days of an antibiotic treatment for spirochete infection. That includes not only B. miyamotoi disease and Lyme disease, but also syphilis, leptospirosis, and others. The reaction is marked by fever, chills, nausea, vomiting, headache, rapid heart rate, low blood pressure, hyperventilation, flushing, and pain. It's unclear exactly what causes this, but researchers hypothesize that as the spirochetes begin to die, they release toxins that spur an acute inflammatory response. The reaction often resolves on its own within a day or so, but it can turn life-threatening for some, and patients need to be monitored closely.