borreliae

Borreliae are a group of motile, helical, spiral-shaped bacteria belonging to the genus Borrelia within the phylum spirochaetes. They are fastidious, Gram-negative or Gram-variable organisms with linear chromosomes and multiple plasmids. They are transmitted to humans primarily through arthropod vectors, notably ticks of the Ixodes and Ornithodoros genera, and, for some species, by body lice.

The genus is divided into two major groups with distinct disease spectra. The Borrelia burgdorferi sensu lato complex includes species that cause Lyme borreliosis, most notably B. burgdorferi sensu stricto in North America and B. afzelii and B. garinii in Europe and Asia. The relapsing fever group includes species such as B. recurrentis, B. miyamotoi, B. duttonii, and others; these cause relapsing fever, characterized by recurring febrile episodes associated with spirochetemia. Transmission cycles involve reservoir hosts such as small mammals and birds, and in the case of B. recurrentis, human body lice as vectors.

Clinical features vary by group. Lyme borreliosis commonly presents with erythema migrans, flu-like symptoms, and, if untreated, late manifestations such as intermittent arthritis, neurologic symptoms, or facial palsy. Relapsing fever presents with recurrent fevers and systemic symptoms during bacteremia. Diagnosis relies on serological tests for Lyme disease (ELISA followed by Western blot in many settings) and nucleic acid amplification or direct detection in certain cases; for relapsing fever, blood smears during febrile periods or molecular methods may be used.

Treatment typically involves antibiotics such as doxycycline, amoxicillin, or ceftriaxone for more severe disease. Prevention focuses on avoiding tick bites and controlling vectors in endemic areas. Borreliae have a global distribution with marked regional variation in species, vector associations, and clinical presentation. The genus has been studied extensively since the identification of B. burgdorferi in the 1980s, with ongoing research into pathogenesis, diagnostics, and vaccines.