Borrelia miyamotoi disease (BMD), a hard tick-borne relapsing fever, is an emerging infection due to a relapsing fever Borrelia and transmitted by Ixodes tick species. While B miyamotoi was first identified in Japan in 1995 from ticks and field mice, human infections were not described until 2011 when 46 cases were reported from Russia. Ixodes ticks are hard-body ticks with worldwide distribution; they can also transmit Borrelia burgdorferi (Lyme disease), Babesia microti (babesiosis), Anaplasma phagocytophilum (anaplasmosis), and tick-borne encephalitis virus. Thus, coinfection with any of these pathogens can occur depending on the geographic location. To date, human cases of BMD have been reported from patients in Russia, Europe, Japan, and the United States.

Transmission of B miyamotoi to humans occurs following the attachment and bite of Ixodes ticks, primarily Ixodes ricinus and Ixodes persulcatus in Eurasia and Ixodes scapularis (blacklegged ticks, or deer ticks), Ixodes dentatus, and Ixodes pacificus (western blacklegged tick) in North America. Borrelia miyamotoi has been found to infect both nymphal and adult Ixodes ticks, with some studies suggesting approximately 1%-2% of ticks infected. Among patients who were aware of a preceding tick bite, the mean time to onset of symptoms was 15 days. Incidence of infection is increased in summer months when ticks are most actively seeking hosts.

The most commonly described symptoms of BMD are undifferentiated flu-like illness with acute onset of fever, chills, headache, arthralgias, myalgias, and fatigue. Headache is often severe. Other less commonly noted symptoms include nausea, abdominal pain, diarrhea, anorexia, dizziness, confusion, dyspnea, and vertigo. Relapsing fever may occur with time between relapses ranging from 2-14 days. Chronic meningoencephalitis has also been described in immunocompromised patients with hematologic malignancies presenting with symptoms including confusion, decreased hearing, and gait instability.

Laboratory findings include leukopenia, thrombocytopenia, and transaminitis without clinically apparent hepatitis. In cases of meningoencephalitis, cerebrospinal fluid (CSF) analysis may reveal pleocytosis (particularly lymphocytic), elevated protein, moderate hypoglycorrhachia, and spirochetes may be visible with Giemsa staining.