Borrelia burgdorferi sensu lato

Borrelia burgdorferi sensu lato
Name	Borrelia burgdorferi sensu lato
Taxon	Species complex
Authority	Johnson et al., 1984 emend. Barbour & Benach, 2019
Subdivision ranks	Genospecies
Subdivision	See text.

Borrelia burgdorferi sensu lato. It is a species complex of spirochetal bacteria responsible for causing Lyme disease, the most common vector-borne illness in the Northern Hemisphere. The complex comprises over 20 validated genospecies, which are transmitted primarily by hard ticks of the genus Ixodes. These bacteria exhibit a complex enzootic life cycle involving small mammals and birds as reservoir hosts.

Microbiology and Classification

The organisms are Gram-negative, microaerophilic spirochetes characterized by their helical shape and endoflagella. They belong to the family Spirochaetaceae within the phylum Spirochaetota. Classification within the complex is based on genetic analyses, including multilocus sequence typing and whole-genome sequencing. Key pathogenic genospecies include Borrelia burgdorferi sensu stricto (predominant in North America), Borrelia afzelii, and Borrelia garinii (both predominant in Eurasia). Other members like Borrelia bavariensis and Borrelia spielmanii are also clinically relevant. The bacteria have a highly fragmented genome consisting of a linear chromosome and numerous linear and circular plasmids, which encode virulence factors like the outer surface proteins.

Epidemiology and Ecology

The geographic distribution is tied to the range of its primary tick vectors: Ixodes scapularis in the northeastern and north-central United States, Ixodes pacificus in the western United States, Ixodes ricinus in Europe, and Ixodes persulcatus in Asia. Transmission occurs through the bite of an infected nymph or adult tick during a blood meal. Reservoir hosts vary by region but commonly include the white-footed mouse (Peromyscus leucopus), various wood mice, and certain bird species like the American robin. The Centers for Disease Control and Prevention reports tens of thousands of confirmed cases annually in the United States, with high incidence in states like Connecticut and Wisconsin.

Pathogenesis and Clinical Significance

Following transmission, the spirochetes disseminate from the skin site, leading to the characteristic expanding rash known as erythema migrans. Without treatment, infection can progress to early disseminated disease, affecting the joints, heart, and nervous system, potentially causing Lyme arthritis, Lyme carditis, or neuroborreliosis. Different genospecies are associated with varying clinical manifestations; for instance, Borrelia afzelii is linked to chronic skin conditions like acrodermatitis chronica atrophicans, while Borrelia garinii is more frequently associated with neurological symptoms. The immune evasion strategies of the bacteria, including antigenic variation of VlsE, contribute to its persistence.

Diagnosis and Detection

Diagnosis is primarily based on clinical presentation, exposure history, and serological testing. The standard laboratory method is a two-tiered testing algorithm recommended by the Centers for Disease Control and Prevention, involving an initial enzyme immunoassay or immunofluorescence assay, followed by a confirmatory Western blot. Direct detection methods, such as polymerase chain reaction on samples from synovial fluid or skin biopsies, can be used but have variable sensitivity. Culture from patient samples, while definitive, is difficult and performed only in specialized research laboratories like those at the National Institutes of Health.

Treatment and Prevention

First-line treatment for early localized disease involves oral antibiotics such as doxycycline, amoxicillin, or cefuroxime axetil. For more disseminated manifestations, particularly neuroborreliosis, intravenous regimens with ceftriaxone or penicillin G may be required. Prevention focuses on avoiding tick bites through the use of DEET repellents, wearing protective clothing, and performing thorough tick checks after outdoor activities in endemic areas. Environmental strategies include landscaping practices to reduce tick habitats, and in some regions, the application of acaricides. A vaccine for humans, LYMErix, was available in the late 1990s but was withdrawn from the United States market.

History and Discovery

The connection between the arthritic condition in Lyme, Connecticut, and a tick-borne agent was investigated in the 1970s by researchers including Allen Steere and Stephen Malawista. The causative spirochete was first isolated in 1981 from Ixodes dammini ticks by Willy Burgdorfer at the Rocky Mountain Laboratories, leading to its naming. The original isolate, designated strain B31, became the type strain for Borrelia burgdorferi sensu stricto. Subsequent molecular studies by scientists like Alan Barbour and Klaus Kurtenbach revealed the genetic diversity within the complex, establishing the sensu lato grouping.

Category:Spirochaetes Category:Bacteria Category:Zoonoses