Borrelia miyamotoi and Borrelia mayonii are two emerging pathogens within the genus Borrelia that have garnered increasing attention due to their ecological significance and potential as human disease agents. Both species are transmitted by hard ticks from the Ixodes genus, which are also vectors for other tick-borne pathogens, including Borrelia burgdorferi, the primary causative agent of Lyme disease. While B. miyamotoi is more closely related to relapsing fever spirochetes, B. mayonii belongs to the B. burgdorferi sensu lato (Bbsl) complex. Understanding their transmission, genetic diversity, and disease manifestation is critical for assessing their public health impact, particularly in the Northern Hemisphere.
Borrelia miyamotoi: Discovery, Genetic Diversity, and Transmission
First isolated in 1994 in Japan, Borrelia miyamotoi belongs to the relapsing fever (RF) group of spirochetes. It was named after Kenji Miyamoto, a Japanese entomologist. Since its discovery, B. miyamotoi has been documented across the Northern Hemisphere, including North America, Europe, and Asia. In contrast to the Lyme disease-causing species from the Bbsl complex, B. miyamotoi shares closer genetic ties with relapsing fever species associated with soft ticks. However, it is transmitted by Ixodes ticks, the same vectors responsible for the transmission of Lyme disease spirochetes.
Genetic Diversity and Clades
Miyamotoi displays significant genetic diversity, divided into three major clades: Asian, European, and North American. Initial classifications distinguished two groups: B. miyamotoi sensu stricto, found in Japan, and B. miyamotoi sensu lato, encompassing strains found globally. Further molecular studies identified additional intra-regional variations within each clade, with North America showing four distinct clades distributed across different geographical regions.
Transmission and Vectors
In North America, B. miyamotoi is primarily transmitted by Ixodes scapularis in the Northeastern and Midwestern United States and by Ixodes pacificus on the West Coast. The prevalence of B. miyamotoi in I. pacificus populations on the West Coast is often higher than that of B. burgdorferi, the agent responsible for Lyme disease.
A unique feature of B. miyamotoi is its ability to undergo transovarial transmission, in which the bacterium is passed from infected female ticks to their offspring. This allows for maintenance of the bacterium within tick populations independent of a reservoir host. In addition, transstadial transmission (from one developmental stage to another) also occurs, further facilitating its persistence within tick populations.
Reservoir Hosts and Human Risk
In North America, the white-footed mouse (Peromyscus leucopus) serves as the primary reservoir for B. miyamotoi. Other small mammals, including squirrels, voles, and chipmunks, also play a role in maintaining the bacterium in the environment. Birds have also been found to carry B. miyamotoi, though their role as amplifying hosts is still unclear.
Human infection with B. miyamotoi is relatively rare but increasing in recognition. Seroprevalence studies in endemic regions of the United States have identified baseline rates ranging from 0.12% to 3.9%, with higher rates among individuals frequently exposed to ticks. In some regions, such as Napa County, California, the infection rates in ticks exceed 15%.
Borrelia mayonii: A Novel Lyme Disease Agent
Borrelia mayonii is a relatively new species within the B. burgdorferi sensu lato complex, first identified in 2016 in six patients from Minnesota and Wisconsin. This discovery was notable for its unexpected detection through molecular diagnostics in a region where Lyme disease is already endemic.
Geographic Distribution and Genetic Characteristics
Unlike other species within the Bbsl complex, B. mayonii is geographically restricted to the Upper Midwestern United States. It has not been found in clinical samples from other regions of the United States or globally. Tick surveillance studies have confirmed its presence in Ixodes scapularis ticks in Minnesota, Wisconsin, and parts of Michigan. The prevalence of B. mayonii in ticks is low, ranging from 0.3% to 2.9%, but stable within these regions.
Transmission and Reservoir Hosts
Like B. burgdorferi, B. mayonii is transmitted during the tick’s blood meal, and it does not undergo transovarial transmission. Small mammals, including white-footed mice and eastern chipmunks, serve as natural reservoirs. Interestingly, the prevalence of B. mayonii in chipmunks (23.5%) is higher than in mice (1.19%), suggesting that chipmunks may play a more significant role in maintaining the bacterium in the environment.
Borrelia miyamotoi in Europe: Ecology and Human Health Concerns
In Europe, Borrelia miyamotoi is an emerging tick-borne pathogen. It co-circulates with B. burgdorferi sensu lato, as both are transmitted by Ixodes ricinus ticks. The first symptomatic human cases of B. miyamotoi disease (BMD) were reported in Russia in 2011, with subsequent cases identified in North America, Europe, and Asia.
Epidemiology and Prevalence
In Europe, Ixodes ricinus and Ixodes persulcatus are the primary vectors of B. miyamotoi. The pathogen has been detected in all life stages of ticks—larvae, nymphs, and adults—with prevalence rates ranging from 0.2% to 10%. Notably, the pathogen’s presence in larvae highlights the role of transovarial transmission in its ecology. Reservoirs of B. miyamotoi in Europe include small mammals such as Apodemus spp., the bank vole (Myodes glareolus), and the white-footed mouse.
Borrelia miyamotoi Disease (BMD): Clinical Manifestations and Diagnosis
The clinical presentation of BMD is often nonspecific, with fever and flu-like symptoms, such as headache, muscle aches, and fatigue, being the most common. In some cases, more severe manifestations, such as meningoencephalitis, have been reported, particularly in immunocompromised individuals. Human cases of BMD have been documented across Europe, including in the Netherlands, Sweden, Germany, and Poland.
Differences in clinical symptoms have been noted between infections caused by the Asian, European, and North American clades of B. miyamotoi. For instance, cytopenia (especially thrombocytopenia) is more frequently observed in American cases, whereas erythema migrans, typically associated with Lyme disease, has been reported in both European and Asian cases. Meningoencephalitis, a potentially life-threatening complication, has been recorded across all three clades.
Diagnosis and Treatment
Diagnosis of BMD is often challenging due to its overlapping symptoms with Lyme disease and other tick-borne infections. Molecular methods, such as polymerase chain reaction (PCR) testing, and serological tests for specific antibodies against B. miyamotoi (such as GlpQ) are commonly used for diagnosis. However, the lack of a specific serological marker necessitates the combination of molecular and serological techniques for accurate identification.
Treatment for BMD generally follows the same guidelines as those for Lyme disease, with antibiotics like doxycycline and ceftriaxone proving effective. However, the lack of a vaccine for B. miyamotoi or other Borrelia spirochetes underscores the importance of preventive measures, including tick bite avoidance and environmental management to reduce tick populations.
Conclusions
Borrelia miyamotoi and Borrelia mayonii represent emerging tick-borne pathogens with distinct ecological and clinical profiles. B. miyamotoi is globally distributed across the Northern Hemisphere, with transmission facilitated by transovarial and transstadial mechanisms. In contrast, B. mayonii appears to be geographically limited to the Upper Midwestern United States. Both pathogens pose significant public health challenges, particularly as their clinical manifestations overlap with other tick-borne diseases, complicating diagnosis. Continued surveillance, research into genetic diversity, and improved diagnostic methods are essential for managing the rising incidence of these infections.
Prevention of tick-borne diseases remains crucial, with personal protective measures and public health interventions to reduce tick exposure playing a critical role in limiting the impact of these emerging pathogens.
Reference: Borrelia miyamotoi—An Emerging Human Tick-Borne Pathogen in Europe
by Katarzyna Kubiak 1ORCID,Magdalena Szczotko 2ORCID andMałgorzata Dmitryjuk, 2021