Failure of Ixodes ticks to inherit Borrelia afzelii infection

Borrelia afzelii Infection in the Rodent Host Has Dramatic Effects on the Bacterial Microbiome of Ixodes ricinus Ticks

The microbiome of blood-sucking arthropods can shape their competence to acquire and maintain infections with vector-borne pathogens. We used a controlled study to investigate the interactions between Borrelia afzelii, which causes Lyme borreliosis in Europe, and the bacterial microbiome of Ixodes ricinus, its primary tick vector. We applied a surface sterilization treatment to I. ricinus eggs to produce dysbiosed tick larvae that had a low bacterial abundance and a changed bacterial microbiome compared to those of the control larvae. Dysbiosed and control larvae fed on B. afzelii-infected mice and uninfected control mice, and the engorged larvae were left to molt into nymphs. The nymphs were tested for B. afzelii infection, and their bacterial microbiome underwent 16S rRNA amplicon sequencing. Surprisingly, larval dysbiosis had no effect on the vector competence of I. ricinus for B. afzelii, as the nymphal infection prevalence and the nymphal spirochete load were the same between the dysbiosed group and the control group. The strong effect of egg surface sterilization on the tick bacterial microbiome largely disappeared once the larvae molted into nymphs. The most important determinant of the bacterial microbiome of I. ricinus nymphs was the B. afzelii infection status of the mouse on which the nymphs had fed as larvae. Nymphs that had taken their larval blood meal from an infected mouse had a less abundant but more diverse bacterial microbiome than the control nymphs. Our study demonstrates that vector-borne infections in the vertebrate host shape the microbiome of the arthropod vector. IMPORTANCE Many blood-sucking arthropods transmit pathogens that cause infectious disease. For example, Ixodes ricinus ticks transmit the bacterium Borrelia afzelii, which causes Lyme disease in humans. Ticks also have a microbiome, which can influence their ability to acquire and transmit tick-borne pathogens such as B. afzelii. We sterilized I. ricinus eggs with bleach, and the tick larvae that hatched from these eggs had a dramatically reduced and changed bacterial microbiome compared to that of control larvae. These larvae fed on B. afzelii-infected mice, and the resultant nymphs were tested for B. afzelii and for their bacterial microbiome. We found that our manipulation of the bacterial microbiome had no effect on the ability of the tick larvae to acquire and maintain populations of B. afzelii. In contrast, we found that B. afzelii infection had dramatic effects on the bacterial microbiome of I. ricinus nymphs. Our study demonstrates that infections in the vertebrate host can shape the tick microbiome.

Infection with Borrelia afzelii and manipulation of the egg surface microbiota have no effect on the fitness of immature Ixodes ricinus ticks

Arthropod vectors carry vector-borne pathogens that cause infectious disease in vertebrate hosts, and arthropod-associated microbiota, which consists of non-pathogenic microorganisms. Vector-borne pathogens and the microbiota can both influence the fitness of their arthropod vectors, and hence the epidemiology of vector-borne diseases. The bacterium Borrelia afzelii, which causes Lyme borreliosis in Europe, is transmitted among vertebrate reservoir hosts by Ixodes ricinus ticks, which also harbour a diverse microbiota of non-pathogenic bacteria. The purpose of this controlled study was to test whether B. afzelii and the tick-associated microbiota influence the fitness of I. ricinus. Eggs obtained from field-collected adult female ticks were surface sterilized (with bleach and ethanol), which reduced the abundance of the bacterial microbiota in the hatched I. ricinus larvae by 28-fold compared to larvae that hatched from control eggs washed with water. The dysbiosed and control larvae were subsequently fed on B. afzelii-infected or uninfected control mice, and the engorged larvae were left to moult into nymphs under laboratory conditions. I. ricinus larvae that fed on B. afzelii-infected mice had a significantly faster larva-to-nymph moulting time compared to larvae that fed on uninfected control mice, but the effect was small (2.4% reduction) and unlikely to be biologically significant. We found no evidence that B. afzelii infection or reduction of the larval microbiota influenced the four other life history traits of the immature I. ricinus ticks, which included engorged larval weight, unfed nymphal weight, larva-to-nymph moulting success, and immature tick survival. A retrospective power analysis found that our sampling effort had sufficient power (> 80%) to detect small effects (differences of 5% to 10%) of our treatments. Under the environmental conditions of this study, we conclude that B. afzelii and the egg surface microbiota had no meaningful effects on tick fitness and hence on the R₀ of Lyme borreliosis.

Masting by beech trees predicts the risk of Lyme disease

BACKGROUND

The incidence of Lyme borreliosis and other tick-borne diseases is increasing in Europe and North America. There is currently much interest in identifying the ecological factors that determine the density of infected ticks as this variable determines the risk of Lyme borreliosis to vertebrate hosts, including humans. Lyme borreliosis is caused by the bacterium Borrelia burgdorferi sensu lato (s.l.) and in western Europe, the hard tick Ixodes ricinus is the most important vector.

METHODS

Over a 15-year period (2004-2018), we monitored the monthly abundance of I. ricinus ticks (nymphs and adults) and their B. burgdorferi s.l. infection status at four different elevations on a mountain in western Switzerland. We collected climate variables in the field and from nearby weather stations. We obtained data on beech tree seed production (masting) from the literature, as the abundance of Ixodes nymphs can increase dramatically 2 years after a masting event. We used generalized linear mixed effects models and AIC-based model selection to identify the ecological factors that influence inter-annual variation in the nymphal infection prevalence (NIP) and the density of infected nymphs (DIN).

RESULTS

We found that the NIP decreased by 78% over the study period. Inter-annual variation in the NIP was explained by the mean precipitation in the present year, and the duration that the DNA extraction was stored in the freezer prior to pathogen detection. The DIN decreased over the study period at all four elevation sites, and the decrease was significant at the top elevation. Inter-annual variation in the DIN was best explained by elevation site, year, beech tree masting index 2 years prior and the mean relative humidity in the present year. This is the first study in Europe to demonstrate that seed production by deciduous trees influences the density of nymphs infected with B. burgdorferi s.l. and hence the risk of Lyme borreliosis.

CONCLUSIONS

Public health officials in Europe should be aware that masting by deciduous trees is an important predictor of the risk of Lyme borreliosis.

Incorporating tick feeding behaviour into R0 for tick-borne pathogens

Tick-borne pathogens pose a considerable disease burden in Europe and North America, where increasing numbers of human cases and the emergence of new tick-borne pathogens has renewed interest in resolving the mechanisms underpinning their geographical distribution and abundance. For Borrelia burgdorferi and tick-borne encephalitis (TBE) virus, transmission of infection from one generation of ticks to another occurs when older nymphal ticks infect younger larval ticks feeding on the same host, either indirectly via systemic infection of the vertebrate host or directly when feeding in close proximity. Here, expressions for the basic reproduction number, R₀, and the related tick type-reproduction number, T, are derived that account for the observation that larval and nymphal ticks tend to aggregate on the same minority of hosts, a tick feeding behaviour known as co-aggregation. The pattern of tick blood meals is represented as a directed, acyclic, bipartite contact network, with individual vertebrate hosts having in-degree, k_in, and out-degree, k_out, that respectively represent cumulative counts of nymphal and larval ticks fed over the lifetime of the host. The in- and out-degree are not independent when co-aggregation occurs such that [Formula: see text] where 〈.〉 indicates expected value. When systemic infection in the vertebrate host is the dominant transmission route R₀²=T, whereas when direct transmission between ticks co-feeding on the same host is dominant then R₀=T and the effect of co-aggregation on R₀ is more pronounced. Simulations of B. burgdorferi and TBE virus transmission on theoretical tick-mouse contact networks revealed that aggregation and co-aggregation have a synergistic effect on R₀ and T, that co-aggregation always increases R₀ and T, and that aggregation only increases R₀ and T when larvae and nymphs also co-aggregate. Co-aggregation has the greatest absolute effect on R₀ and T when the mean larval burden of hosts is high, and the largest relative effect on R₀ for pathogens sustained by co-feeding transmission, e.g. TBE virus in Europe, compared with those predominantly spread by systemic infection, e.g. B. burgdorferi. For both pathogens, though, co-aggregation increases the mean number of ticks infected per infectious tick, T, and so too the likelihood of pathogen persistence.

Modeling the effects of variable feeding patterns of larval ticks on the transmission of Borrelia lusitaniae and Borrelia afzelii

Spirochetes belonging to the Borrelia burgdoferi sensu lato (sl) group cause Lyme Borreliosis (LB), which is the most commonly reported vector-borne zoonosis in Europe. B. burgdorferi sl is maintained in nature in a complex cycle involving Ixodes ricinus ticks and several species of vertebrate hosts. The transmission dynamics of B. burgdorferi sl is complicated by the varying competence of animals for different genospecies of spirochetes that, in turn, vary in their capability of causing disease. In this study, a set of difference equations simplifying the complex interaction between vectors and their hosts (competent and not for Borrelia) is built to gain insights into conditions underlying the dominance of B. lusitaniae (transmitted by lizards to susceptible ticks) and the maintenance of B. afzelii (transmitted by wild rodents) observed in a study area in Tuscany, Italy. Findings, in agreement with field observations, highlight the existence of a threshold for the fraction of larvae feeding on rodents below which the persistence of B. afzelii is not possible. Furthermore, thresholds change as nonlinear functions of the expected number of nymph bites on mice, and the transmission and recovery probabilities. In conclusion, our model provided an insight into mechanisms underlying the relative frequency of different Borrelia genospecies, as observed in field studies.

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.

Lyme disease: Part I. Advances and perspectives

Lyme disease (LD) is an increasingly recognized multisystem, insect-borne zoonosis. Prevalent worldwide, it has a variety of presentations at different stages of infection. The characteristic rash with central clearing known as erythema chronicum migrans, or simply erythema migrans, appears in its first stage. Typical features may be absent, and important variations are evident among cases seen in different parts of the world. LD may be difficult to diagnose clinically; knowledge about its epidemiology and transmission may be of assistance when the diagnosis is unclear. Based upon our experience with LD and a comprehensive literature review, we provide an update of LD epidemiology, pathophysiology, and management. We also cover the three clinical stages of LD that parallel those of syphilis, another spirochetal disease.

Modeling the spread of vector-borne diseases on bipartite networks

Background

Vector-borne diseases for which transmission occurs exclusively between vectors and hosts can be modeled as spreading on a bipartite network.

Methodology/Principal Findings

In such models the spreading of the disease strongly depends on the degree distribution of the two classes of nodes. It is sufficient for one of the classes to have a scale-free degree distribution with a slow enough decay for the network to have asymptotically vanishing epidemic threshold. Data on the distribution of Ixodes ricinus ticks on mice and lizards from two independent studies are well described by a scale-free distribution compatible with an asymptotically vanishing epidemic threshold. The commonly used negative binomial, instead, cannot describe the right tail of the empirical distribution.

Conclusions/Significance

The extreme aggregation of vectors on hosts, described by the power-law decay of the degree distribution, makes the epidemic threshold decrease with the size of the network and vanish asymptotically.

Transmission dynamics of Borrelia lusitaniae and Borrelia afzelii among Ixodes ricinus, lizards, and mice in Tuscany, central Italy

To estimate the basic reproduction number (R(0)) of Borrelia lusitaniae and Borrelia afzelii, we formulated a mathematical model considering the interactions among the tick vector, vertebrate hosts, and pathogens in a 500-ha enclosed natural reserve on Le Cerbaie hills, Tuscany, central Italy. In the study area, Ixodes ricinus were abundant and were found infected by B. lusitaniae and B. afzelii. Lizards (Podarcis spp.) and mice (Apodemus spp.), respectively, are the reservoir hosts of these two Borrelia burgdorferi sensu lato (s.l.) genospecies and compete for immature ticks. B. lusitaniae R(0) estimation is in agreement with field observations, indicating the maintenance and diffusion of this genospecies in the study area, where lizards are abundant and highly infested by I. ricinus immature stages. In fact, B. lusitaniae shows a focal distribution in areas where the tick vector and the vertebrate reservoir coexist. Mouse population dynamics and their relatively low suitability as hosts for nymphs seem to determine, on the other hand, a less efficient transmission of B. afzelii, whose R(0) differs between scenarios in the study area. Considering host population dynamics, the proposed model suggests that, given a certain combination of the two host population sizes, both spirochete genospecies can coexist in our study area. Additional incompetent hosts for B. burgdorferi s.l. have a negative effect on B. afzelii maintenance, whose R(0) results > 1 only with high mouse population densities and/or low lizards abundance, but they do not seem to influence B. lusitaniae transmission cycle on Le Cerbaie. Secondly, our model confirms the importance of nymphs' infestation, of host population density and diversity, and spirochetes host association for the maintenance of the transmission cycle of B. burgdorferi s.l.

The basic reproduction number for complex disease systems: defining R(0) for tick-borne infections

Characterizing the basic reproduction number, R(0), for many wildlife disease systems can seem a complex problem because several species are involved, because there are different epidemiological reactions to the infectious agent at different life-history stages, or because there are multiple transmission routes. Tick-borne diseases are an important example where all these complexities are brought together as a result of the peculiarities of the tick life cycle and the multiple transmission routes that occur. We show here that one can overcome these complexities by separating the host population into epidemiologically different types of individuals and constructing a matrix of reproduction numbers, the so-called next-generation matrix. Each matrix element is an expected number of infectious individuals of one type produced by a single infectious individual of a second type. The largest eigenvalue of the matrix characterizes the initial exponential growth or decline in numbers of infected individuals. Values below 1 therefore imply that the infection cannot establish. The biological interpretation closely matches that of R(0) for disease systems with only one type of individual and where infection is directly transmitted. The parameters defining each matrix element have a clear biological meaning. We illustrate the usefulness and power of the approach with a detailed examination of tick-borne diseases, and we use field and experimental data to parameterize the next-generation matrix for Lyme disease and tick-borne encephalitis. Sensitivity and elasticity analyses of the matrices, at the element and individual parameter levels, allow direct comparison of the two etiological agents. This provides further support that transmission between cofeeding ticks is critically important for the establishment of tick-borne encephalitis.

Occurrence of multiple infections with different Borrelia burgdorferi genospecies in Danish Ixodes ricinus nymphs

The pathogen Borrelia burgdorferi causes Lyme Borreliosis in human and animals world-wide. In Europe the pathogen is transmitted to the host by the vector Ixodes ricinus. The nymph is the primary instar for transmission to humans. We here study the infection rate of five Borrelia genospecies: B. burgdorferi sensu stricto, B. afzelii, B. garinii, B. valaisiana, B. lusitaniae in nymphs, by IFA and PCR. 600 nymphs were collected in North Zealand of Denmark. Each nymph was first analysed by IFA. If positive for spirochaetal infection, the genospecies was determined by PCR. The infection rate of B. burgdorferi sensu lato was 15.5%, with the primary genospecies being B. afzelii (64.3%), B. garinii (57.1%), and B. lusitaniae (26.8%). It is the first time B. lusitaniae is documented in Denmark. Even though, the highest infection rate was discovered for B. afzelii and B. garinii, mixed infections are more common than single infections. Fifty-one percent (29/56) of these were infected with two genospecies, 7.1% (4/56) with three, and 5.3% (3/56) with four. We try to explain the high infection rate and the peculiar number of multiple infections, with a discussion of changes host abundance and occurrence of different transmission patterns.

Perpetuation of the Lyme disease spirochete Borrelia lusitaniae by lizards

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.

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.

Prevalence of Borrelia burgdorferi sensu lato in the nightingale (Luscinia megarhynchos) and other passerine birds

In order to explore the involvement of terrestrial birds in the ecology of Lyme disease spirochetes, we determined the presence of Borrelia burgdorferi s. I. DNA in tick larvae feeding on the hosts. Birds were caught at several study sites along the Rhine valley in SW Germany between August 1999 and March 2001. A total of 987 Ixodes ricinus larvae were collected from 225 birds belonging to 20 host species. The following four passerine species that have not yet been subject to detailed reservoir competence analyses were investigated: Nightingale (Luscinia megarhynchos), Dunnock (Prunella modularis), Chiffchaff (Phylloscopus collybita) and Reed warbler (Acrocephalus scirpaceus). Borrelia DNA was analysed in attached ticks and for one species, the Nightingale, blood samples were included. Borrelia DNA was amplified by means of the polymerase chain reaction (PCR) targeting the ospA gene of B. burgdorferi s. l. and 1-2 larvae per bird were analysed. On the basis of one larva per individual bird, Borrelia DNA could be detected in six out of nine larval ticks from the Nightingale, in one out of ten ticks from the Dunnock, in three of nine ticks from the Chiffchaff, and in two out of 21 larval ticks from Reed warblers. Five out of nine ticks removed from Robins Erithacus rubecula in winter were found to be Borrelia positive. Blood samples of Nightingales caught during the breeding period were ospA-gene-positive in 71 of 138 birds (51%).

Limited role of rodents as reservoirs of Borrelia burgdorferi sensu lato in Ireland

Despite the presence of Borrelia burgdorferi s.l.-infected Ixodes ricinus ticks in a west of Ireland location, very few small rodents from the same habitats were infected. Most of the infected ticks contained B. garinii or B. valaisiana, which implicates birds as the important reservoir hosts in this location.

Competence of American robins as reservoir hosts for Lyme disease spirochetes

To explore the competence of American robins as a reservoir for Lyme disease spirochetes, we determined the susceptibility of these birds to tickborne spirochetes and their subsequent infectivity for larval vector ticks. Robins acquired infection and became infectious to almost all xenodiagnostic ticks soon after exposure to infected nymphal ticks. Although infectivity waned after 2 months, the robins remained susceptible to reinfection, became infectious again, and permitted repeated feeding by vector ticks. In addition, spirochetes passaged through birds retained infectivity for mammalian hosts. American robins become as infectious for vector ticks as do reservoir mice, but infectivity in robins wanes more rapidly.