Diversity of Ixodes-borne Borrelia species--clinical, pathogenetic, and diagnostic implications and impact on vaccine development

Laboratory Diagnosis of Lyme Borreliosis

Lyme borreliosis is caused by a growing list of related, yet distinct, spirochetes with complex biology and sophisticated immune evasion mechanisms. It may result in a range of clinical manifestations involving different organ systems, and can lead to persistent sequelae in a subset of cases. The pathogenesis of Lyme borreliosis is incompletely understood, and laboratory diagnosis, the focus of this review, requires considerable understanding to interpret the results correctly. Direct detection of the infectious agent is usually not possible or practical, necessitating a continued reliance on serologic testing. Still, some important advances have been made in the area of diagnostics, and there are many promising ideas for future assay development. This review summarizes the state of the art in laboratory diagnostics for Lyme borreliosis, provides guidance in test selection and interpretation, and highlights future directions.

Polymorphic factor H-binding activity of CspA protects Lyme borreliae from the host complement in feeding ticks to facilitate tick-to-host transmission

Borrelia burgdorferi sensu lato (Bbsl), the causative agent of Lyme disease, establishes an initial infection in the host's skin following a tick bite, and then disseminates to distant organs, leading to multisystem manifestations. Tick-to-vertebrate host transmission requires that Bbsl survives during blood feeding. Complement is an important innate host defense in blood and interstitial fluid. Bbsl produces a polymorphic surface protein, CspA, that binds to a complement regulator, Factor H (FH) to block complement activation in vitro. However, the role that CspA plays in the Bbsl enzootic cycle remains unclear. In this study, we demonstrated that different CspA variants promote spirochete binding to FH to inactivate complement and promote serum resistance in a host-specific manner. Utilizing a tick-to-mouse transmission model, we observed that a cspA-knockout B. burgdorferi is eliminated from nymphal ticks in the first 24 hours of feeding and is unable to be transmitted to naïve mice. Conversely, ectopically producing CspA derived from B. burgdorferi or B. afzelii, but not B. garinii in a cspA-knockout strain restored spirochete survival in fed nymphs and tick-to-mouse transmission. Furthermore, a CspA point mutant, CspA-L246D that was defective in FH-binding, failed to survive in fed nymphs and at the inoculation site or bloodstream in mice. We also allowed those spirochete-infected nymphs to feed on C3-/- mice that lacked functional complement. The cspA-knockout B. burgdorferi or this mutant strain complemented with cspA variants or cspA-L246D was found at similar levels as wild type B. burgdorferi in the fed nymphs and mouse tissues. These novel findings suggest that the FH-binding activity of CspA protects spirochetes from complement-mediated killing in fed nymphal ticks, which ultimately allows Bbsl transmission to mammalian hosts.

Identification of Lyme borreliae proteins promoting vertebrate host blood-specific spirochete survival in Ixodes scapularis nymphs using artificial feeding chambers

Lyme borreliosis, the most common vector-borne illness in Europe and the United States, is caused by spirochetes of the Borrelia burgdorferi sensu lato complex and transmitted by Ixodes ticks. In humans, the spirochetes disseminate from the tick bite site to multiple tissues, leading to serious clinical manifestations. The ability of spirochetes to survive in ticks during blood feeding is thought to be essential for Lyme borreliae to be transmitted to different vertebrate hosts. This ability is partly attributed to several B. burgdorferi proteins, including BBA52 and Lp6.6, which promote spirochete survival in nymphal ticks feeding on mice. One of the strategies to identify such proteins without using live animals is to feed B. burgdorferi-infected ticks on blood via artificial feeding chambers. In previous studies, ticks were only fed on bovine blood in the feeding chambers. In this study, we used this chamber model and showed that I. scapularis ticks will not only acquire bovine blood but human and quail blood as well. The latter two are the incidental host and an avian host of Lyme borreliae, respectively. We also investigated the roles that BBA52 and Lp6.6 play in promoting spirochete survival in nymphal ticks fed on human or quail blood. After feeding on human blood, spirochete burdens in ticks infected with an lp6.6-deficient B. burgdorferi were significantly reduced, while bba52-deficient spirochete burdens in ticks remained unchanged, similar to the wild-type strain. No strain showed a change in spirochete burdens in ticks fed on quail blood. These results indicate that Lp6.6 plays a role for B. burgdorferi in nymphs fed on human but not quail blood. Such information also demonstrates that the artificial feeding chamber is a powerful tool to identify B. burgdorferi proteins that promote vertebrate host blood-specific spirochete survival in I. scapularis ticks.

Explosive radiation of a bacterial species group

The current diversity of life on earth is the product of macroevolutionary processes that have shaped the dynamics of diversification. Although the tempo of diversification has been studied extensively in macroorganisms, much less is known about the rates of diversification in the exceedingly diverse and species-rich microbiota. Decreases in diversification rates over time, a signature of explosive radiations, are commonly observed in plant and animal lineages. However, the few existing analyses of microbial lineages suggest that the tempo of diversification in prokaryotes may be fundamentally different. Here, we use multilocus and genomic sequence data to test hypotheses about the rate of diversification in a well-studied pathogenic bacterial lineage, Borrelia burgdorferi sensu lato (sl). Our analyses support the hypothesis that an explosive radiation of lineages occurred near the origin of the clade, followed by a sharp decay in diversification rates. These results suggest that explosive radiations may be a general feature of evolutionary history across the tree of life.

Population genetics, taxonomy, phylogeny and evolution of Borrelia burgdorferi sensu lato

In order to understand the population structure and dynamics of bacterial microorganisms, typing systems that accurately reflect the phylogenetic and evolutionary relationship of the agents are required. Over the past 15 years multilocus sequence typing schemes have replaced single locus approaches, giving novel insights into phylogenetic and evolutionary relationships of many bacterial species and facilitating taxonomy. Since 2004, several schemes using multiple loci have been developed to better understand the taxonomy, phylogeny and evolution of Lyme borreliosis spirochetes and in this paper we have reviewed and summarized the progress that has been made for this important group of vector-borne zoonotic bacteria.

Established and emerging pathogens in Ixodes ricinus ticks collected from birds on a conservation island in the Baltic Sea

Tick-borne pathogens such as Lyme borreliosis spirochaetes, Anaplasma phagocytophilum, Rickettsia spp. and Babesia spp. cause a great variety of diseases in animals and humans. Although their importance with respect to emerging human diseases is increasing, many issues about their ecology are still unclear. In spring 2007, 191 Ixodes ricinus (Acari: Ixodidae) ticks were collected from 99 birds of 11 species on a bird conservation island in the Baltic Sea in order to test them for Borrelia spp., A. phagocytophilum, Rickettsia spp. and Babesia spp. infections. Identification of the pathogens was performed by polymerase chain reaction (PCR), restriction fragment length polymorphism and sequence analysis. The majority of birds with ticks testing positive were European robins and thrushes. Borrelia DNA was detected in 14.1%, A. phagocytophilum in 2.6%, rickettsiae in 7.3% and Babesia spp. in 4.7% of the ticks. Co-infections with different pathogens occurred in six ticks (3.1%). The fact that 11 ticks (five larvae, six nymphs) were infected with Borrelia afzelii suggests that birds may, contrary to current opinion, serve as reservoir hosts for this species. Among rickettsial infections, we identified Rickettsia monacensis and Rickettsia helvetica. As we detected five Rickettsia spp. positive larvae and two birds carried more than one infected tick, transmission of those pathogens from birds to ticks appears possible. Further characterization of Babesia infections revealed Babesia divergens and Babesia microti. The occurrence of Babesia spp. in a total of five larvae suggests that birds may be able to infect ticks, at least with Ba. microti, a species considered not to be transmitted transovarially in ticks.

A new Borrelia species defined by multilocus sequence analysis of housekeeping genes

Analysis of Lyme borreliosis (LB) spirochetes, using a novel multilocus sequence analysis scheme, revealed that OspA serotype 4 strains (a rodent-associated ecotype) of Borrelia garinii were sufficiently genetically distinct from bird-associated B. garinii strains to deserve species status. We suggest that OspA serotype 4 strains be raised to species status and named Borrelia bavariensis sp. nov. The rooted phylogenetic trees provide novel insights into the evolutionary history of LB spirochetes.

Lyme neuroborreliosis: manifestations of a rapidly emerging zoonosis

Lyme disease has a worldwide distribution and is the most common vector-borne disease in the United States. Incidence, clinical manifestations, and presentations vary by geography, season, and recreational habits. Lyme neuroborreliosis (LNB) is neurologic involvement secondary to systemic infection by the spirochete Borrelia burgdorferi in the United States and by Borrelia garinii or Borrelia afzelii species in Europe. Enhanced awareness of the clinical presentation of Lyme disease allows inclusion of LNB in the imaging differential diagnosis of facial neuritis, multiple enhancing cranial nerves, enhancing noncompressive radiculitis, and pediatric leptomeningitis with white matter hyperintensities on MR imaging. The MR imaging white matter appearance of successfully treated LNB and multiple sclerosis display sufficient similarity to suggest a common autoimmune pathogenesis for both. This review highlights differences in the epidemiology, clinical manifestations, diagnosis, and management of Lyme disease in the United States, Europe, and Asia, with an emphasis on neurologic manifestations and neuroimaging.

Lyme neuroborreliosis: infection, immunity, and inflammation

Lyme neuroborreliosis (LNB), the neurological manifestation of systemic infection with the complex spirochaete Borrelia burgdorferi, can pose a challenge for practising neurologists. This Review is a summary of clinical presentation, diagnosis, and therapy, as well as of recent advances in our understanding of LNB. Many new insights have been gained through work in experimental models of the disease. An appreciation of the genetic heterogeneity of the causative pathogen has helped clinicians in their understanding of the diverse presentations of LNB.

[Lyme disease: prophylaxis after tick bite]

Lyme disease is a bacterial infection caused by Borrelia burgdorferi, which is transmitted by infected ticks. The transmission depends on several factors, especially on the duration of the tick's presence in the host body (the nymph which is smaller than the adults and thus less visible, is in this case the most frequently involved) and on whether the tick is infected or not. The interpretation of results in the few available studies is made difficult by the lack of information obtained (due to difficulty to collect information and examination costs). The comparison is made even more difficult by the difference between Borrelia ticks species in various regions. Today, the best methods are preventive: protective clothing, tick repellents, checking and removal of ticks after a journey in an endemic zone, and in case of tick bite, regular examination of the bite site during the following weeks in order to initiate an early curative treatment if ECM is diagnosed. The currently available data seems to be insufficient to suggest systematic antimicrobial prophylaxis in case of tick bite.

Characterization of the humoral immune response in dogs after vaccination against the Lyme borreliosis agent A study with five commercial vaccines using two different vaccination schedules

Using five commercially available vaccines, groups of dogs were vaccinated against Lyme borreliosis and followed for 13 months. A modified vaccination schedule was included in the study in an attempt to induce higher and therefore longer-lasting protective antibody levels during the first year of immunization. Following vaccination antibodies were monitored using ELISA and Western blotting. Serum samples were examined either with antigen preparations derived from either Borrelia burgdorferi s. s., B. garinii or B. afzelii lysates or separately with recombinant OspA derived from the three species. Regardless of the vaccine used the third vaccination induced significantly higher antibody levels. Further analyses of the sera with homologue and heterologue detection systems showed in vitro only a minor cross-reactivity of vaccinal antibodies towards antigens derived from heterologous Borrelia species in vitro and consequently only minimal cross-protection can be expected.

Expression of complement factor H binding immunoevasion proteins in Borrelia garinii isolated from patients with neuroborreliosis

The Lyme disease-pathogen Borrelia burgdorferi binds the complement inhibitor factor H (FH) to its outer surface protein E- (OspE) and BbA68-families of lipoproteins. In earlier studies, only serum-resistant strains of the genospecies B. burgdorferi sensu stricto or B. afzelii, but not serum-sensitive B. garinii strains, have been shown to bind FH. Since B. garinii often causes neuroborreliosis in man, we have readdressed the interactions of B. garinii with FH. B. garinii 50/97 strain did not express FH-binding proteins. By transforming the B. garinii 50/97 strain with an OspE-encoding gene from complement-resistant B. burgdorferi (ospE-297), its resistance to serum killing could be increased. OspE genes were detected and cloned from the B. garinii BITS, Pistoia and 40/97 strains by PCR and sequencing. The deduced amino acid sequences differed in an N-terminal lysine-rich FH-binding region from OspE sequences of resistant strains. Recombinant B. garinii BITS OspE protein was found to have a considerably lower FH-binding activity than the B. burgdorferi sensu stricto 297 OspE protein P21 (P21-297). Unlike bacteria that had been kept in culture for a long time, neurovirulent B. garinii strains from neuroborreliosis patients were found to express approximately 27-kDa FH-binding proteins. These were not recognized by polyclonal anti-OspE or anti-BbA68 antibodies. We conclude that B. garinii strains carry ospE genes but have a decreased expression of OspE proteins and a reduced ability to bind FH, especially when grown for prolonged periods in vitro. Recently isolated neuroinvasive B. garinii strains, however, can express FH-binding proteins, which may contribute to the virulence of neuroborreliosis-causing B. garinii strains.

Tick–host interactions: saliva-activated transmission

The skin site at which ticks attach to their hosts to feed is the critical interface between the tick and its host, and tick-borne pathogens. This site is highly modified by the pharmacologically active molecules secreted in tick saliva. For pathogens, it is an ecologically privileged niche that many exploit. Such exploitation is referred to as saliva-activated transmission (SAT) – the indirect promotion of tick-borne pathogen transmission via the actions of bioactive tick saliva molecules on the vertebrate host. Here we review evidence for SAT and consider what are the most likely candidates for SAT factors among the tick pharmacopoeia of anti-haemostatic, anti-inflammatory and immunomodulatory molecules identified to date. SAT factors appear to differ for different pathogens and tick vector species, and possibly even depend on the vertebrate host species. Most likely we are searching for a suite of molecules that act together to overcome the redundancy in host response mechanisms. Whatever they turn out to be, the quest to identify the tick molecules that mediate SAT is an exciting one, and offers new insights to controlling ticks and tick-borne diseases.

Comparison of disseminated and nondisseminated strains of Borrelia burgdorferi sensu stricto in mice naturally infected by tick bite

Clinical isolates of Borrelia burgdorferi sensu stricto have been categorized into disseminated and nondisseminated groups based on distinct ribosomal spacer restriction fragment length polymorphism genotypes (RSTs). In order to determine whether transmission by tick bite would alter the dissemination dynamics and disease produced by distinct genotypes, disseminated isolates (RST1), nondisseminated isolates (RST3), and a standard laboratory strain (B-31) were established in a murine cycle utilizing infections transmitted by ticks. B-31 spirochetes circulated in the blood of inbred C3H/HeJ mice longer than in the blood of outbred mice. The majority of C3H mice exposed to RST1-infected ticks contained cultivable spirochetes in their blood for up to 17 days; in contrast, mice exposed to RST3 isolates demonstrated a precipitous decline in infection after day 7 postexposure. A quantitative PCR (q-PCR) assay demonstrated that the densities of spirochetes in blood were similar for the RST1 and RST3 isolates, except during the 2nd week postexposure, when the RST1 isolates displayed a markedly higher density in blood. Spirochete load in the heart and bladder of infected mice was measured by q-PCR at 8 weeks postexposure; the numbers of spirochetes in these tissues were similar for mice infected with either disseminated or nondisseminated strains. Similarly, histopathology samples of heart, bladder, and joint tissue obtained at 8 weeks postexposure did not reveal greater pathology in mice infected with the disseminated isolates. We conclude that although the spirochetemia induced by tick-transmitted disseminated isolates was more intense and of longer duration than that induced by nondisseminated isolates, the resultant pathologies produced by these strains were ultimately similar.

Genotype determines phenotype in experimental Lyme borreliosis

Borrelia burgdorferi sensu lato, the causative organism of Lyme borreliosis, is a heterogeneous group of spirochetes, consisting of at least three pathogenic species. To test the hypothesis that the genetic heterogeneity is the reason for the clinical differences, we investigated whether the experimental disease induced by European isolates is different from that induced by American isolates. Two American isolates of species B. burgdorferi sensu stricto were compared with three European isolates, two of species B. garinii, and one of species B. afzelii. The patterns of infection, immunity, and inflammation induced by the different species was distinctive. Inflammatory cells and levels of antibody in B. garinii- and B. afzelii-infected animals were lower than in B. burgdorferi s.s.-infected animals, whereas levels of spirochetal infection in the skin and nervous system were higher in the former group of animals. These data demonstrate that B. burgdorferi s.s. strains are more infective and inflammatory, whereas B. garinii and B. afzelii strains can survive the adaptive immune response to a greater degree and persist at greater numbers in the skin and nervous system. The results explain to a large extent the disparities between LNB in humans in the United States and Europe.