Borrelia afzelii
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| Borrelia afzelii | |
|---|---|
| Name | Borrelia afzelii |
| Domain | Bacteria |
| Phylum | Spirochaetota |
| Class | Spirochaetia |
| Order | Spirochaetales |
| Family | Borreliaceae |
| Genus | Borrelia |
| Species | B. afzelii |
| Binomial | Borrelia afzelii |
| Binomial authority | Canica et al., 1994 |
Borrelia afzelii is a species of spirochete bacteria belonging to the Borrelia burgdorferi sensu lato complex, which is responsible for causing Lyme borreliosis. It is one of the primary pathogenic genospecies in Eurasia, particularly associated with a distinct cutaneous manifestation known as acrodermatitis chronica atrophicans. The bacterium is maintained in an enzootic cycle involving specific Ixodes tick vectors and various reservoir hosts, primarily small rodents.
Description and classification
Borrelia afzelii is a helical, Gram-negative bacterium characterized by its unique periplasmic flagella which confer a corkscrew-like motility. It was first validly described and named in 1994 by researchers including M. M. Canica, distinguishing it from other members of the Borrelia burgdorferi sensu lato group through molecular techniques like multilocus sequence typing. Phylogenetic analyses, including studies of the 16S ribosomal RNA gene and the ospC gene, place it within the broader Borreliaceae family. Its genome is composed of a linear chromosome and numerous linear and circular plasmids, which harbor genes critical for antigenic variation and host adaptation, such as the vlsE locus.
Transmission and life cycle
The primary vectors for Borrelia afzelii are hard ticks of the genus Ixodes, most notably Ixodes ricinus in Europe and Ixodes persulcatus in Asia. The enzootic cycle involves larval and nymphal ticks acquiring the spirochete from infected reservoir hosts during a blood meal. Key reservoir hosts include small mammals like the bank vole (Myodes glareolus) and the yellow-necked mouse (Apodemus flavicollis). The bacterium persists in the tick's midgut and migrates to the salivary glands during a subsequent feeding on a new host, such as a human, facilitating transmission. Transovarial transmission in ticks is considered inefficient, making these rodent hosts essential for maintaining the bacterium in nature.
Clinical significance
Borrelia afzelii is a principal causative agent of Lyme disease in Eurasia. It is particularly associated with the late-stage skin manifestation acrodermatitis chronica atrophicans, which presents as a bluish-red discoloration and atrophy, typically on the extremities. Earlier manifestations can include the characteristic erythema migrans rash, though the specific presentation may differ from that caused by Borrelia burgdorferi sensu stricto. If untreated, disseminated infection can potentially lead to Lyme arthritis and neurological complications, though its association with neuroborreliosis is less pronounced compared to Borrelia garinii.
Diagnosis and detection
Diagnosis of infection relies on a combination of clinical evaluation and laboratory testing. Serological methods like enzyme-linked immunosorbent assay and Western blot, supported by guidelines from the Centers for Disease Control and Prevention, are commonly used to detect antibodies. Direct detection can be achieved through polymerase chain reaction targeting specific genomic regions, such as the flagellin gene or the 5S-23S rRNA intergenic spacer. Culture from skin biopsies, particularly from acrodermatitis chronica atrophicans lesions, is possible but requires specialized Barbour-Stoenner-Kelly medium and is of low sensitivity.
Epidemiology and distribution
Borrelia afzelii has a predominantly Palaearctic distribution, being endemic across much of continental Europe and temperate regions of Asia, including Russia, China, and Japan. Its prevalence correlates with the geographical range of its tick vectors, Ixodes ricinus and Ixodes persulcatus. Epidemiological studies, such as those conducted by the European Centre for Disease Prevention and Control, indicate it is often the most frequently detected genospecies in questing ticks in Central Europe. Infection rates in ticks can vary significantly by region, influenced by factors like climate, host population density, and vegetation.
Prevention and control
Primary prevention focuses on avoiding tick bites in endemic areas through personal protective measures such as using DEET-based repellents, wearing protective clothing, and performing thorough tick checks after outdoor activities. Environmental management, including landscape modification and acaricide application in high-risk areas, can reduce tick populations. Public health initiatives by organizations like the World Health Organization promote awareness of Lyme borreliosis risks. Post-exposure prophylaxis with doxycycline may be considered following a tick bite in certain high-risk scenarios. Control of rodent reservoir populations is generally not considered a practical large-scale strategy.