Borrelia is a genus of spirochete bacteria known for causing diseases such as Lyme disease (Borrelia burgdorferi) and relapsing fever (Borrelia recurrentis and other species). These bacteria are transmitted to humans through tick or lice bites, leading to systemic infections that can affect multiple organs, including the nervous system and joints.

Borrelia species are unique among bacteria because they can be directly observed on a peripheral blood smear, particularly in cases of relapsing fever. Their morphology is distinct; they appear as thin, helical (spiral-shaped) organisms that move with a characteristic corkscrew motion.

On a Giemsa- or Wright-stained peripheral blood smear, Borrelia spirochetes typically:

• Appear as long, thin, and wavy bacteria (approximately 10-30µm in length and 0.2-0.3µm in width).
• Stain a bluish to purple color, standing out against the background of red and white blood cells.
• Are seen free in the plasma, rather than inside blood cells.

Detection of Borrelia in blood smears is most common during febrile episodes of relapsing fever when spirochetemia peaks. While microscopy can be useful for diagnosis, confirmation is usually achieved through PCR or serological testing.

Recognising Borrelia in a blood film is a valuable diagnostic skill, as early detection and treatment with antibiotics can prevent severe complications.