Borrelia garinii is a spirochete bacterium and one of the primary agents responsible for Lyme borreliosis (LB), particularly in Eurasia. Belonging to the Borrelia burgdorferi sensu lato (s.l.) complex, B. garinii is characterized by its close association with avian hosts, distinguishing it from other Lyme disease-causing species, such as Borrelia afzelii and Borrelia burgdorferi sensu stricto. In humans, B. garinii infections are often associated with neurological complications, especially Lyme neuroborreliosis. Although primarily a Eurasian pathogen, B. garinii has also been reported in North America, particularly in isolated regions, such as islands off the coast of Newfoundland and Labrador, and more recently in South Carolina, USA.
Morphology and Genome Characteristics
Like other spirochetes, B. garinii exhibits a spiral shape with a thin, elongated structure measuring approximately 0.2 μm in diameter and 8–30 μm in length. Its motility is facilitated by the presence of 7 to 20 periplasmic flagella, which are anchored at each pole of the bacterium, extending inward and overlapping at the center. This flagellar arrangement gives B. garinii its distinctive corkscrew movement, allowing it to burrow through host tissues.
The genome of B. garinii is similar to other members of the B. burgdorferi s.l. complex, consisting of a linear chromosome and numerous linear and circular plasmids. These plasmids play critical roles in the bacterium's ability to infect and evade the immune systems of its vertebrate hosts. Notably, B. garinii carries plasmids encoding outer surface proteins (Osps) such as OspA, OspB, OspC, and other proteins that contribute to antigenic variation and immune evasion. The genetic diversity of B. garinii is also shaped by variations in plasmid content, which may influence host range, pathogenicity, and transmission efficiency.
Epidemiology and Geographic Distribution
Borrelia garinii is predominantly found in Europe and Asia, where it is transmitted by Ixodes ricinus and Ixodes persulcatus ticks. Its primary reservoir hosts are birds, which serve as important vehicles for the dispersal of infected ticks. Migratory birds are especially crucial for the broad geographic distribution of B. garinii, enabling the spread of this pathogen across continents. The distribution of B. garinii overlaps with other Borrelia species, but it is distinguished by its affinity for avian hosts, as opposed to the rodent reservoirs favored by Borrelia afzelii.
In North America, B. garinii has historically been considered rare, with most cases of Lyme borreliosis in humans being attributed to Borrelia burgdorferi sensu stricto. However, reports of B. garinii from isolated regions, such as islands off the coast of Newfoundland and Labrador, have indicated that this species is present in North America, albeit in limited geographical areas. More recently, B. garinii has been isolated from rodents in South Carolina, USA. Phylogenetic studies indicate that these North American isolates are closely related to Eurasian strains, suggesting that B. garinii may have been introduced to North America via migratory birds or other vectors.
Transmission Dynamics
B. garinii is primarily transmitted by hard-bodied ticks of the genus Ixodes, specifically I. ricinus and I. persulcatus. These ticks acquire the bacterium by feeding on infected avian hosts, and they subsequently transmit it to new hosts, including humans. Unlike B. afzelii, which predominantly infects rodent hosts, B. garinii is highly adapted to birds, making them the key reservoirs in its life cycle. Migratory birds, in particular, play a vital role in the long-distance dissemination of B. garinii-infected ticks.
Co-feeding transmission, a process by which uninfected and infected ticks feed in close proximity on the same host, has also been documented as a transmission mechanism for B. garinii. This mode of transmission allows the bacterium to infect ticks without requiring a systemic infection in the host. While co-feeding transmission has been well-documented in competent rodent and bird hosts, studies on other vertebrates have suggested that this mode of transmission may allow B. garinii to bypass the immune defenses of hosts that are otherwise resistant to systemic infection.
Host Specificity and Immune Evasion
B. garinii demonstrates a clear preference for avian hosts, which are competent reservoirs for this pathogen. Birds such as the blackbird (Turdus merula), European robin (Erithacus rubecula), and great tit (Parus major) are frequently found to carry B. garinii-infected Ixodes larvae and nymphs. Experimental infection studies have confirmed that B. garinii is capable of establishing systemic infections in birds, facilitating efficient transmission to feeding ticks.
The host specificity of B. garinii is thought to be mediated by the complement system of the host. In vitro studies have shown that B. garinii is resistant to avian complement but is susceptible to the complement of other vertebrates, such as rodents. Conversely, rodent-adapted Borrelia species, such as B. afzelii, are resistant to rodent complement but are lysed by avian complement. This complement-mediated host specificity helps to explain the distinct host preferences exhibited by different Borrelia species within the B. burgdorferi sensu lato complex.
Clinical Manifestations in Humans
In humans, B. garinii is primarily associated with neurological complications, particularly Lyme neuroborreliosis (LNB). This form of Lyme disease is characterized by inflammation of the central nervous system, leading to symptoms such as meningitis, radiculitis, and cranial nerve palsies. While erythema migrans, the characteristic skin lesion of early Lyme disease, can occur following B. garinii infection, the progression to neuroborreliosis is more common than with other Borrelia species.
Lyme neuroborreliosis caused by B. garinii often presents as Bannwarth syndrome, a condition marked by severe radicular pain, motor weakness, and cranial nerve involvement. Early diagnosis and treatment with antibiotics such as ceftriaxone or doxycycline are crucial for preventing long-term neurological damage. However, even with appropriate treatment, some patients may experience residual neurological symptoms, highlighting the potential severity of B. garinii-associated infections.
Molecular Characteristics and Genetic Diversity
B. garinii exhibits significant genetic diversity across its various strains, which contributes to differences in host specificity, pathogenicity, and transmission dynamics. The genome of B. garinii includes a linear chromosome and multiple plasmids, many of which carry genes that encode outer surface proteins (Osps) involved in immune evasion and host adaptation. The genetic content of these plasmids varies between strains, and this variation may influence the bacterium’s ability to infect different hosts and evade the host immune response.
Phylogenetic studies using multilocus sequence typing (MLST) and whole-genome sequencing have revealed that B. garinii strains cluster into distinct clades, reflecting their geographic origins and evolutionary history. Notably, B. garinii strains from North America have been shown to be phylogenetically distinct from those found in Canada and Europe, suggesting multiple independent introductions of this species into North America. Recent studies have identified B. garinii isolates from rodents in South Carolina, USA, that are genetically related to Eurasian strains, indicating that this bacterium may have been introduced to North America via migratory birds.
The phylogenetic analysis of B. garinii isolates has demonstrated the presence of two major clades, with one clade primarily comprising isolates from Europe and Asia and another clade containing isolates from both Europe and North America. The North American isolates, while genetically related to European strains, form a distinct subclade, suggesting a separate introduction event. The presence of these isolates in rodents in the southeastern United States raises important questions about the potential for B. garinii to establish itself in new environments outside its traditional Eurasian range.
Transmission in North America
Although B. garinii is rare in North America, its presence in rodents and ticks in South Carolina indicates that it may be more widespread than previously thought. Two B. garinii isolates were recovered from rodents (a cotton mouse and an eastern woodrat) trapped in South Carolina in the 1990s. Phylogenetic analysis of these isolates revealed that they are closely related to Eurasian strains of B. garinii, rather than to strains found in Canada. This finding suggests that B. garinii may have been introduced to North America independently of the Canadian strains, possibly via migratory birds or other means of long-distance dispersal.
Despite the discovery of B. garinii in South Carolina, there have been no reports of Lyme disease outbreaks in the southeastern United States associated with this species. However, the potential for B. garinii to cause human infections in this region cannot be ruled out, and continued surveillance is warranted. The isolation of B. garinii from North American rodents also raises questions about the potential for this species to establish new enzootic cycles in regions where it was previously absent.
Borrelia garinii is a highly adaptable and genetically diverse spirochete that plays a significant role in Lyme borreliosis, particularly in Eurasia. Its association with avian hosts and its ability to cause Lyme neuroborreliosis in humans distinguish it from other members of the Borrelia burgdorferi sensu lato complex. While primarily found in Europe and Asia, the recent discovery of B. garinii in North America underscores the need for continued research into its epidemiology, transmission dynamics, and potential for human disease. Understanding the genetic diversity and host specificity of B. garinii is crucial for developing effective strategies to control its spread and mitigate the impact of Lyme borreliosis on human health.