Chapter 15: Spirochetes: Syphilis, Lyme & Relapsing Fever

Medically important spirochetes are categorized into three major genera: Treponema, Borrelia, and Leptospira. Treponema pallidum, the agent of syphilis, is a highly fragile organism transmitted primarily through sexual contact or transplacentally. It progresses through primary, secondary, latent, and tertiary stages, beginning with a painless chancre and potentially leading to systemic complications like cardiovascular damage or neurological decline. Because it cannot be grown in standard cultures, diagnosis often relies on dark-field microscopy or serological tests for specific and non-specific antibodies. The chapter also addresses non-venereal treponemal diseases such as yaws and bejel, which are common in specific tropical or arid regions. Borrelia species are highlighted for their complex genomes and roles in Lyme disease and relapsing fever. Lyme disease, spread by Ixodes ticks, typically manifests with a characteristic circular rash known as erythema migrans before potentially affecting the heart, joints, and nervous system. Relapsing fever is distinguished by the bacteria's ability to undergo antigenic variation, causing repeated bouts of illness. Lastly, Leptospira interrogans is discussed as a zoonotic pathogen spread through water contaminated with the urine of infected animals. This can lead to leptospirosis, a biphasic illness that may involve severe liver and kidney dysfunction, often referred to as Weil disease. The chapter emphasizes that while these pathogens vary in their transmission and clinical presentation, most remain susceptible to antibiotics like penicillin or doxycycline if treated appropriately.