Dania Richter

Adaptive Introgression of Anticoagulant Rodent Poison Resistance by Hybridization between Old World Mice

Polymorphisms in the vitamin K 2,3-epoxide reductase subcomponent 1 (vkorc1) of house mice (Mus musculus domesticus) can cause resistance to anticoagulant rodenticides such as warfarin [1-3]. Here we show that resistant house mice can also originate from selection on vkorc1 polymorphisms acquired from the Algerian mouse (M. spretus) through introgressive hybridization. We report on a polymorphic introgressed genomic region in European M. m. domesticus that stems from M. spretus, spans >10 Mb on chromosome 7, and includes the molecular target of anticoagulants vkorc1 [1-4]. We show that in the laboratory, the homozygous complete vkorc1 allele of M. spretus confers resistance when introgressed into M. m. domesticus. Consistent with selection on the introgressed allele after the introduction of rodenticides in the 1950s, we found signatures of selection in patterns of variation in M. m. domesticus. Furthermore, we detected adaptive protein evolution of vkorc1 in M. spretus (Ka/Ks = 1.54-1.93) resulting in radical amino acid substitutions that apparently cause anticoagulant tolerance in M. spretus as a pleiotropic effect. Thus, positive selection produced an adaptive, divergent, and pleiotropic vkorc1 allele in the donor species, M. spretus, which crossed a species barrier and produced an adaptive polymorphic trait in the recipient species, M. m. domesticus.

Relationships of a Novel Lyme Disease Spirochete, Borrelia spielmani sp. nov., with Its Hosts in Central Europe

ABSTRACT To determine whether the pathogenic variant of Lyme disease spirochetes, isolate A14S, is perpetuated in a particular reservoir-vector relationship, we screened vector ticks in various Central European sites for a related spirochete and determined its host association. A14S-like spirochetes infect numerous questing ticks in the Petite Camargue Alsacienne (PC). They frequently infect dormice, but no mice or voles. Garden dormice appear to be better reservoir hosts for A14S-like spirochetes than for Borrelia afzelii, because these spirochetes are retained longer and infect ticks more readily. Spirochetes associated with garden dormice in the PC site form a homologous entity with those isolated from a human patient in The Netherlands. Its unique biological relationship together with previous genetic characterization justifies designating this dormouse-associated genospecies as a distinct entity. Garden dormice serve as the main reservoir hosts of a novel genospecies, Borrelia spielmani sp. nov., one of several that cause Lyme disease in people.

Relapsing Fever–Like Spirochetes Infecting European Vector Tick of Lyme Disease Agent

To determine whether relapsing fever–like spirochetes associated with hard ticks may infect Ixodes ricinus ticks in central Europe, we screened questing ticks for 16S rDNA similar to that of Asian and American relapsing fever–like spirochetes. We compared the prevalence of these spirochetes to that of Lyme disease spirochetes transmitted by the same vector. Relapsing fever-like spirochetes infect 3.5% of questing vector ticks in our three central European sites near the Rhein Valley. These spirochetes differ genetically from their American and Asian analogs while being relatively homogeneous in the region we sampled. The Lyme disease genospecies most commonly detected in central Europe are distributed broadly, whereas those that are less frequently found appear to be place-specific. The absence of co-infected ticks suggests that relapsing fever–like and Lyme disease spirochetes may not share hosts. Exposure risk for relapsing fever–like spirochetes is similar to that of certain Lyme disease genospecies. Although many persons may be bitten by ticks infected by relapsing fever–like spirochetes, health implications remain unknown.

Identification of novel rodent herpesviruses, including the first gammaherpesvirus of Mus musculus.

ABSTRACT Rodent herpesviruses such as murine cytomegalovirus (host, Mus musculus), rat cytomegalovirus (host, Rattus norvegicus), and murine gammaherpesvirus 68 (hosts, Apodemus species) are important tools for the experimental study of human herpesvirus diseases. However, alphaherpesviruses, roseoloviruses, and lymphocryptoviruses, as well as rhadinoviruses, that naturally infect Mus musculus (house mouse) and other Old World mice are unknown. To identify hitherto-unknown rodent-associated herpesviruses, we captured M. musculus, R. norvegicus, and 14 other rodent species in several locations in Germany, the United Kingdom, and Thailand. Samples of trigeminal ganglia, dorsal root ganglia, brains, spleens, and other organs, as well as blood, were analyzed with a degenerate panherpesvirus PCR targeting the DNA polymerase (DPOL) gene. Herpesvirus-positive samples were subjected to a second degenerate PCR targeting the glycoprotein B (gB) gene. The sequences located between the partial DPOL and gB sequences were amplified by long-distance PCR and sequenced, resulting in a contiguous sequence of approximately 3.5 kbp. By DPOL PCR, we detected 17 novel betaherpesviruses and 21 novel gammaherpesviruses but no alphaherpesvirus. Of these 38 novel herpesviruses, 14 were successfully analyzed by the complete bigenic approach. Most importantly, the first gammaherpesvirus of Mus musculus was discovered (Mus musculus rhadinovirus 1 [MmusRHV1]). This virus is a member of a novel group of rodent gammaherpesviruses, which is clearly distinct from murine herpesvirus 68-like rodent gammaherpesviruses. Multigenic phylogenetic analysis, using an 8-kbp locus, revealed that MmusRHV1 diverged from the other gammaherpesviruses soon after the evolutionary separation of Epstein-Barr virus-like lymphocryptoviruses from human herpesvirus 8-like rhadinoviruses and alcelaphine herpesvirus 1-like macaviruses.

Perpetuation of the Lyme Disease Spirochete Borrelia lusitaniae by Lizards

ABSTRACT To determine whether the Lyme disease spirochete Borrelia lusitaniae is associated with lizards, we compared the prevalence and genospecies of spirochetes present in rodent- and lizard-associated ticks at a site where this spirochete frequently infects questing ticks. Whereas questing nymphal Ixodes ricinus ticks were infected mainly by Borrelia afzelii, one-half of the infected adult ticks harbored B. lusitaniae at our study site. Lyme disease spirochetes were more prevalent in sand lizards (Lacerta agilis) and common wall lizards (Podarcis muralis) than in small rodents. Although subadult ticks feeding on rodents acquired mainly B. afzelii, subadult ticks feeding on lizards became infected by B. lusitaniae. Genetic analysis confirmed that the spirochetes isolated from ticks feeding on lizards are members of the B. lusitaniae genospecies and resemble type strain PotiB2. At our central European study site, lizards, which were previously considered zooprophylactic for the agent of Lyme disease, appear to perpetuate B. lusitaniae.

“Candidatus Neoehrlichia mikurensis,” Anaplasma phagocytophilum, and Lyme Disease Spirochetes in Questing European Vector Ticks and in Feeding Ticks Removed from People

ABSTRACT To estimate the likelihood of people coming into contact with the recently described tick-borne agent “Candidatus Neoehrlichia mikurensis,” we compared its prevalence to those of Lyme disease spirochetes and Anaplasma phagocytophilum in questing adult Ixodes ricinus ticks collected in various Central European sites and examined ticks, which had been removed from people, for the presence of these pathogens. Whereas spirochetes infected questing adult ticks most frequently (22.3%), fewer than a third as many ticks were infected by “Ca. Neoehrlichia mikurensis” (6.2%), and about a sixth harbored A. phagocytophilum (3.9%). On average, every twelfth encounter of a person with an I. ricinus tick (8.1%) may bear the risk of acquiring “Ca. Neoehrlichia mikurensis.” Although a fifth of the people (20%) had removed at least one tick infected by “Ca. Neoehrlichia mikurensis,” none displayed symptoms described for this pathogen, suggesting that its transmission may not be immediate and/or that immunocompetent individuals may not be affected. Because immunosuppressed patients may be at a particular risk of developing symptoms, it should be considered that “Ca. Neoehrlichia mikurensis” appears to be the second most common pathogen in I. ricinus ticks. In our survey, only Borrelia afzelii appears to infect Central European vector ticks more frequently.

Prevalence of Bartonella henselae and Borrelia burgdorferi Sensu Lato DNA in Ixodes ricinus Ticks in Europe

ABSTRACT Bartonella spp. can cause persistent bloodstream infections in humans and animals. To determine whether Bartonella henselae is present in questing Ixodes ricinus ticks, we analyzed the prevalence of B. henselae DNA among tick stages compared to the prevalence of DNA from Borrelia burgdorferi sensu lato, the pathogen most frequently transmitted by ticks. B. henselae DNA was present with a prevalence of up to ∼40% in tick populations sampled in four European sites (Eberdingen, Germany; Klasdorf, Germany; Lembach, France; and Madeira, Portugal). The odds of detecting B. henselae DNA in nymphal ticks was ∼14-fold higher than in adult ticks. No tick was found to be coinfected with B. henselae and B. burgdorferi sensu lato. Taken together, our data indicate that ticks might serve as a vector for the transmission of B. henselae to humans.

Absence of Lyme Disease Spirochetes in Larval Ixodes ricinus Ticks

To determine which kind of spirochete infects larval Ixodes ricinus, we examined questing larvae and larvae derived from engorged females for the presence of particular spirochetal DNA that permitted species differentiation. Borrelia miyamotoi was the sole spirochete detected in larval ticks sampled while questing on vegetation. Questing nymphal and adult ticks were infected mainly by Borrelia afzelii, whereas larval ticks resulting from engorged females of the same population were solely infected by B. miyamotoi. Since larvae acquire Lyme disease spirochetes within a few hours of attachment to an infected rodent, questing larvae in nature may have acquired Lyme disease spirochetes from an interrupted host contact. Even if transovarial transmission of Lyme disease spirochetes may occasionally occur, it seems to be an exceedingly rare event. No undisputable proof exists for vertical transmission of Lyme disease spirochetes, whereas B. miyamotoi appears to be readily passed between generations of vector ticks.

Co-feeding transmission and its contribution to the perpetuation of the lyme disease spirochete Borrelia afzelii.

To determine whether direct passage of spirochetes between co-feeding vector ticks contributes to the likelihood that the Lyme disease spirochete Borrelia afzelii will perpetuate in nature, we compared the effects of time and space on transmission efficiency between simultaneously feeding ticks. The likelihood of co-feeding transmission increases with duration of attachment of the infecting tick. Co-feeding transmission becomes less efficient as distance from the infecting tick increases. Approximately 6 times as many ticks acquire infection when feeding on infected mice than when co-feeding with infected ticks. Both subadult stages of the wood tick Ixodes ricinus infrequently co-infest mice and voles in nature; on approximately 1 in 20 small rodents, larvae co-feed with spirochete-infected nymphs. Because only 1 in 100 larvae in nature appear to acquire spirochetal infection when co-feeding with infected nymphs, perpetuation of B. afzelii depends largely on horizontal transmission of such pathogens from previously infected mice to noninfected larvae.

Adaptation of Diverse Lyme Disease Spirochetes in a Natural Rodent Reservoir Host

ABSTRACT We compared the relative reservoir competence of European wood mice for two genospecies of Lyme disease spirochetes by analyzing susceptibility, intrinsic incubation period, and degree and duration of infectivity. Borrelia afzelii, specializing in particular reservoir hosts, is better adapted to those hosts than is the more generalist genospecies B. burgdorferi sensu stricto.