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

Meta-analysis: A useful tool to assess infection prevalence and disease ecology of Borrelia burgdorferi sensu lato in nymphal ticks in North-Western Europe with recommendations for a standardised approach to future studies

Lyme borreliosis is a vector-borne disease of concern in Europe. While neuroborreliosis data are reportable at EU level, it can nevertheless be difficult to make comparisons of disease risk between neighbouring countries. This study used proportion meta-analyses to compare environmental markers of disease risk between woodland sites in two countries in North-Western Europe (Ireland, Scotland). 73 site-visits from 12 publications were analysed, resulting in a significantly higher pooled nymphal infection prevalence (NIP) in Ireland (8.2% (95% CI: 5.9–11.4%)) than Scotland (1.7%(95% CI 1.1–2.5%)). All other analysed parameters of disease risk were also higher in Ireland than Scotland. Subgroup-meta-analyses and meta-regressions were used to assess the influence of environmental variables on NIP. NIP increased significantly with increasing woodland size in Ireland, but not Scotland, which may be accounted for by Ireland's highly fragmented landscape.

Assuming the application of strict inclusion/exclusion criteria and control of variables, proportion meta-analysis can provide useful insights in disease ecology, as it allows for the achievement of high study powers incorporating samples collected across multiple sites, which is otherwise often a prohibitively difficult and resource-heavy feat in environmental studies in disease ecology. A standardised approach to data collection is recommended to achieve more robust meta-analyses in future in conjunction with additional research on environmental factors affecting Lyme borreliosis risk in Europe, particularly pertaining to the impact of host species on NIP.

Update on the presence of Ixodes ricinus at the western limit of its range and the prevalence of Borrelia burgdorferi sensu lato

It is often suggested that due to climate and environmental policy changes, the risk from tick-borne disease is increasing, particularly at the geographical limits of the vector distribution. Our project aimed to determine whether this was true for the risk of Lyme borreliosis in Ireland which is the western-most limit of Ixodes ricinus, the European vector of Borrelia burgdorferi sensu lato. The availability of a historical data set of tick infection rates compiled in the 1990s represented a unique opportunity as it provided a baseline against which current data could be compared. Following construction of a spatial predictive model for the presence and absence of I. ricinus based on data from 491 GPS locations visited between 2016 and 2019, 1404 questing nymphs from 27 sites were screened for the presence of Borrelia spp. using a TaqMan PCR aimed at the 23S rRNA gene sequence. All positive ticks were further analysed by nested PCR amplification and sequence analysis of the 5 S-23 S intergenic spacer. The model indicated that areas with the highest probability of tick presence were mostly located along the western seaboard and the Shannon and Erne river catchments, coinciding with historical high incidence areas of bovine babesiosis, while the infection rate of questing nymphs with B. burgdorferi s.l. and the prevalence of the various genospecies have remained surprisingly stable over the last 3 decades. Clear communication of the potential disease risk arising from a tick bite is essential in order to allay undue concerns over tick-borne diseases among the general public.

Landscape structure affects the prevalence and distribution of a tick-borne zoonotic pathogen

Background

Landscape structure can affect pathogen prevalence and persistence with consequences for human and animal health. Few studies have examined how reservoir host species traits may interact with landscape structure to alter pathogen communities and dynamics. Using a landscape of islands and mainland sites we investigated how natural landscape fragmentation affects the prevalence and persistence of the zoonotic tick-borne pathogen complex Borrelia burgdorferi (sensu lato), which causes Lyme borreliosis. We hypothesized that the prevalence of B. burgdorferi (s.l.) would be lower on islands compared to the mainland and B. afzelii, a small mammal specialist genospecies, would be more affected by isolation than bird-associated B. garinii and B. valaisiana and the generalist B. burgdorferi (sensu stricto).

Methods

Questing (host-seeking) nymphal I. ricinus ticks (n = 6567) were collected from 12 island and 6 mainland sites in 2011, 2013 and 2015 and tested for B. burgdorferi (s.l.). Deer abundance was estimated using dung transects.

Results

The prevalence of B. burgdorferi (s.l.) was significantly higher on the mainland (2.5%, 47/1891) compared to island sites (0.9%, 44/4673) (P < 0.01). While all four genospecies of B. burgdorferi (s.l.) were detected on the mainland, bird-associated species B. garinii and B. valaisiana and the generalist genospecies B. burgdorferi (s.s.) predominated on islands.

Conclusion

We found that landscape structure influenced the prevalence of a zoonotic pathogen, with a lower prevalence detected among island sites compared to the mainland. This was mainly due to the significantly lower prevalence of small mammal-associated B. afzelii. Deer abundance was not related to pathogen prevalence, suggesting that the structure and dynamics of the reservoir host community underpins the observed prevalence patterns, with the higher mobility of bird hosts compared to small mammal hosts leading to a relative predominance of the bird-associated genospecies B. garinii and generalist genospecies B. burgdorferi (s.s.) on islands. In contrast, the lower prevalence of B. afzelii on islands may be due to small mammal populations there exhibiting lower densities, less immigration and stronger population fluctuations. This study suggests that landscape fragmentation can influence the prevalence of a zoonotic pathogen, dependent on the biology of the reservoir host.

Electronic supplementary material

The online version of this article (10.1186/s13071-018-3200-2) contains supplementary material, which is available to authorized users.

Ticks and Tick-borne diseases in Ireland

Throughout Europe interest in tick-borne agents is increasing, particularly with regard to those that can cause human disease. The reason for this is the apparent rise in the incidence of many tick-borne diseases (TBD’s). While there has never been a national survey of ticks or TBD’s in Ireland, the trend here appears to be the reverse with a decline in the incidence of some agents seemingly associated with decreasing tick numbers particularly on agricultural land. In the absence of robust baseline data, however, this development cannot be confirmed.

This review collates the limited information available from several dated published records on tick species and a small number of studies focused on certain TBD’s. Some pilot data on tick density and TBD agents collected in 2016 are also presented. The aim is to explore the particular situation in Ireland with regard to ticks and TBD’s and to provide a reference for future workers in the field.

Risk of exposure to ticks (Ixodidae) and the prevalence of tick-borne encephalitis virus (TBEV) in ticks in Southern Poland

The article presents the results of the first study on seasonal activity of ticks and prevalence of tick-borne encephalitis virus (TBEV) in nymphs from the Silesian Province (Southern Poland). Previous studies on the prevalence of TBEV in ticks in Poland have been conducted mostly in northern and eastern regions, but none in the Silesian Province itself. The aims of this study were to analyse the seasonal variation in tick populations and compare TBEV prevalence in nymphs from different geographical locations in the Silesia. A total of 5160 questing Ixodes ricinus ticks were collected by the flagging method from 23 localities in southern Poland in 2010. Micro-climatic parameters (air temperature and humidity) were measured in order to estimate their influence on tick population. The highest tick activity was recorded in spring and was positively correlated with relative air humidity (RH). TBEV in the Silesian Province was analysed in 1750 nymphs and an overall prevalence was 0.11% (2 pools out of 175 analysed). The results of this study show that TBEV pool prevalence in nymphs is low in accordance with the low number of TBE cases reported within the region.

Host and pathogen DNA identification in blood meals of nymphal Ixodes ricinus ticks from forest parks and rural forests of Poland

DNA analysis of blood meals from unfed nymphal Ixodes ricinus allows for the identification of tick host and tick-borne pathogens in the host species. The recognition of host species for tick larvae and the reservoirs of Borrelia, Rickettsia and Anaplasma species were simultaneously carried out by analysis of the blood meals of 880 questing nymphal I. ricinus ticks collected in forest parks of Szczecin city and rural forests in northwestern Poland that are endemic areas for Lyme borreliosis. The results obtained from the study indicate that I. ricinus larvae feed not only on small or medium animals but also on large animals and they (i.e. roe deer, red deer and wild boars) were the most prevalent in all study areas as the essential hosts for larvae of I. ricinus. The composition of medium and small vertebrates (carnivores, rodents, birds and lizards) provided a more diverse picture depending on study site. The reservoir species that contain the most pathogens are the European roe deer Capreolus capreolus, in which two species of Rickettsia and two species of Borrelia were identified, and Sus scrofa, in which one Rickettsia and three Borrelia species were identified. Rickettsia helvetica was the most common pathogen detected, and other included species were the B. burgdorferi s.l. group and B. miyamotoi related to relapsing fever group. Our results confirmed a general association of B. garinii with birds but also suggested that such associations may be less common in the transmission cycle in natural habitats than what was thought previously.

Exploring gaps in our knowledge on Lyme borreliosis spirochaetes--updates on complex heterogeneity, ecology, and pathogenicity

The Lyme borreliosis complex is a heterogeneous group of tick-borne spirochaetes of the genus Borrelia (Spirochaetales: Spirochaetaceae) that are distributed all over the temperate zone of the northern hemisphere. Due to the usage of new methods for phylogenetic analysis, this group has expanded rapidly during the past 5 years. Along with this development, the number of Borrelia spp. regarded as pathogenic to humans also increased. Distribution areas as well as host and vector ranges of Lyme borreliosis agents turned out to be much wider than previously thought. Furthermore, there is evidence that ticks, reservoir hosts, and patients can be coinfected with multiple Borrelia spp. or other tick-borne pathogens, which indicates a need to establish new and well-defined diagnostic and therapeutic standards for Lyme borreliosis. This review gives a broad overview on the occurrence of Lyme borreliosis spirochaetes worldwide with particular emphasis on their vectors and vertebrate hosts as well as their pathogenic potential and resultant problems in diagnosis and treatment. Against the background that many issues regarding distribution, species identity, ecology, pathogenicity, and coinfections are still unsolved, the purpose of this article is to reveal directions for future research on the Lyme borreliosis complex.

Investigating the persistence of tick-borne pathogens via the R0model

In the epidemiology of infectious diseases, the basic reproduction number, R0, has a number of important applications, most notably it can be used to predict whether a pathogen is likely to become established, or persist, in a given area. We used the R0model to investigate the persistence of 3 tick-borne pathogens;Babesia microti, Anaplasma phagocytophilumandBorrelia burgdorferisensu lato in anApodemus sylvaticus-Ixodes ricinussystem. The persistence of these pathogens was also determined empirically by screening questing ticks and wood mice by PCR. All 3 pathogens behaved differently in response to changes in the proportion of transmission hosts on whichI. ricinusfed, the efficiency of transmission between the host and ticks and the abundance of larval and nymphal ticks found on small mammals. Empirical data supported theoretical predictions of the R0model. The transmission pathway employed and the duration of systemic infection were also identified as important factors responsible for establishment or persistence of tick-borne pathogens in a given tick-host system. The current study demonstrates how the R0model can be put to practical use to investigate factors affecting tick-borne pathogen persistence, which has important implications for animal and human health worldwide.

Molecular and serological evidence of Borrelia burgdorferi sensu lato in wild rodents in the Czech Republic

The aim of the present study was to determine the frequency and spatial distribution of the Borrelia species in wild rodents in the Czech Republic. In total, 293 muscle tissue samples and 106 sera from 293 wild rodents captured in North Bohemia and North-East and South Moravia were examined for the presence of Borrelia spp. and antibodies. Muscle samples were investigated with real-time polymerase chain reaction (PCR) with a recA primer set, with DNA quantification and melting curve analysis, and with restriction fragment length polymorphism (RFLP) analysis of the 5S-23S rDNA intergenic spacer. Infection with Borrelia burgdorferi sensu lato was found in 16.4% of the muscle samples. The most abundant genospecies was Borrelia afzelii (11.3%), followed by Borrelia burgdorferi sensu stricto (4.8%) and Borrelia garinii (0.7%). Borrelia infection was more frequently observed in Clethrionomys glareolus than in Apodemus spp. Sera were analyzed with an enzyme-linked immunosorbent assay (ELISA) test, yielding the total seropositivity rates of 24.5% for anti-Borrelia IgM antibodies and 25.5% for IgG antibodies. Total seroprevalence was higher in Apodemus spp. than in C. glareolus. In conclusion, our data indicate that in the Czech Republic small wild rodents can serve as hosts for B. burgdorferi s. s. as well as for B. afzelii.

SympatricIxodes triangulicepsandIxodes ricinusTicks Feeding on Field Voles (Microtus agrestis): Potential for Increased Risk ofAnaplasma phagocytophilumin the United Kingdom?

The importance of wild rodents as reservoirs of zoonotic tick-borne pathogens is considered low in the United Kingdom because, in studies to date, those parasitized by exophilic Ixodes ricinus ticks carry almost exclusively larvae and thus have a minor role in transmission cycles. In a cross-sectional study, 11 (6.7%) of 163 field voles (Microtus agrestis) captured at field sites in Northern England were PCR-positive for Anaplasma phagocytophilum. The voles were found to act as hosts for both larval and nymphal I. ricinus and all stages of the nidicolous tick I. trianguliceps, and eight individuals were infested with ticks of both species at the same time. Two of 158 larval and one of 13 nymphal I. ricinus, as well as one of 14 larval and one of 15 nymphal I. trianguliceps collected from the rodents were PCR-positive. These findings suggest that habitats where field voles are abundant in the United Kingdom may pose a risk of A. phagocytophilum infection because (i) field voles, the most abundant terrestrial mammal in the United Kingdom, may be a competent reservoir; (ii) the field voles are hosts for both nymphal and larval ixodid ticks so they could support endemic cycles of A. phagocytophilum; and (iii) they are hosts for nidicolous I. trianguliceps, which may alone maintain endemic cycles, and exophilic I. ricinus ticks, which could act as a bridge vector and transmit infections to humans and domesticated animals.

Prevalence of Borrelia burgdorferi sensu lato genospecies in Ixodes ricinus ticks in Europe: a metaanalysis

In Europe, Borrelia burgdorferi genospecies causing Lyme borreliosis are mainly transmitted by the tick Ixodes ricinus. Since its discovery, B. burgdorferi has been the subject of many epidemiological studies to determine its prevalence and the distribution of the different genospecies in ticks. In the current study we systematically reviewed the literature on epidemiological studies of I. ricinus ticks infected with B. burgdorferi sensu lato. A total of 1,186 abstracts in English published from 1984 to 2003 were identified by a PubMed keyword search and from the compiled article references. A multistep filter process was used to select relevant articles; 110 articles from 24 countries contained data on the rates of infection of I. ricinus with Borrelia in Europe (112,579 ticks), and 44 articles from 21 countries included species-specific analyses (3,273 positive ticks). These data were used to evaluate the overall rate of infection of I. ricinus with Borrelia genospecies, regional distributions within Europe, and changes over time, as well as the influence of different detection methods on the infection rate. While the infection rate was significantly higher in adults (18.6%) than in nymphs (10.1%), no effect of detection method, tick gender, or collection period (1986 to 1993 versus 1994 to 2002) was found. The highest rates of infection of I. ricinus were found in countries in central Europe. B. afzelii and B. garinii are the most common Borrelia species, but the distribution of genospecies seems to vary in different regions in Europe. The most frequent coinfection by Borrelia species was found for B. garinii and B. valaisiana.

Borrelia burgdorferi sensu stricto in yellow-necked mice and feeding Ixodes ricinus ticks in a forest habitat of west central Poland

Wild rodents and the subadult Ixodes ricinus (L.) ticks infesting them were examined for the presence of Borrelia burgdorferi Johnson, Schmid, Hyde, Steigerwalt & Brenner s.l. in a sylvatic habitat in west central Poland during May-September 2002. In total, 818 feeding ticks were recovered from 73 infested yellow-necked mice, Apodemus flavicollis Melchior; in addition, bank voles, Clethrionomys glareolus Schreber, were rarely captured and proved to be weakly parasitized. Only 2.7% of A. flavicollis and 2.2% of 320 engorging larvae were polymerase chain reaction (PCR) positive for the bacterium. All spirochete-PCR-positive samples yielded exclusively B. burgdorferi s.s. This genospecies was also the most prevalent in questing nymphs and accounted for 87.5% of the total number of Borrelia infections in nymphal ticks collected during May and June 2 yr later. The presence of the same genospecies both in naturally engorged larvae and blood-positive animals as well as the high predominance of B. burgdorferi s.s. in questing nymphs strongly differs from most study sites investigated in Europe. This unique pattern of Borrelia-diversity in both rodents and ticks seems to be determined by highly site-specific host vertebrate cenosis, and yellow-necked mice are involved in the maintenance of B. burgdorferi s.s. in the forest habitat. However, the transmission efficiency of this spirochete from the mice to the I. ricinus vector seems to be very low. The research provides additional information on the complexity of B. burgdorferi s.l. ecology in Europe, pointing to the importance of the local host community.

Borrelia burgdorferi sensu stricto in yellow-necked mice and feeding Ixodes ricinus ticks in a forest habitat of west central Poland

Wild rodents and the subadult Ixodes ricinus (L.) ticks infesting them were examined for the presence of Borrelia burgdorferi Johnson, Schmid, Hyde, Steigerwalt & Brenner s.l. in a sylvatic habitat in west central Poland during May-September 2002. In total, 818 feeding ticks were recovered from 73 infested yellow-necked mice, Apodemus flavicollis Melchior; in addition, bank voles, Clethrionomys glareolus Schreber, were rarely captured and proved to be weakly parasitized. Only 2.7% of A. flavicollis and 2.2% of 320 engorging larvae were polymerase chain reaction (PCR) positive for the bacterium. All spirochete-PCR-positive samples yielded exclusively B. burgdorferi s.s. This genospecies was also the most prevalent in questing nymphs and accounted for 87.5% of the total number of Borrelia infections in nymphal ticks collected during May and June 2 yr later. The presence of the same genospecies both in naturally engorged larvae and blood-positive animals as well as the high predominance of B. burgdorferi s.s. in questing nymphs strongly differs from most study sites investigated in Europe. This unique pattern of Borrelia-diversity in both rodents and ticks seems to be determined by highly site-specific host vertebrate cenosis, and yellow-necked mice are involved in the maintenance of B. burgdorferi s.s. in the forest habitat. However, the transmission efficiency of this spirochete from the mice to the I. ricinus vector seems to be very low. The research provides additional information on the complexity of B. burgdorferi s.l. ecology in Europe, pointing to the importance of the local host community.

Borrelia burgdorferi sensu lato infection in larval Ixodes ricinus (Acari: Ixodidae) feeding on blackbirds in northwestern Italy

Birds belonging to 59 species (n = 1,206) were live captured in Piemonte, northwestern Italy, in 2001. Ixodes ricinus (L.) larvae were collected from 59 birds belonging to nine species, and nymphs were recovered on 79 birds belonging to 10 species. Eurasian blackbirds, Turdus merula L., had significantly higher levels of infestation by ticks than other passerine species. Larval I. ricinus of blackbirds peaked in summer, when prevalence was 39% (95% confidence interval 24.2-55.5) and mean number of ticks per host was 3.3 (1.6-7.2), whereas nymphs peaked in spring, when prevalence was 72.2% (54.8-85.8) and mean number of ticks per host was 6.9 (4.4-10.7). Immature I. ricinus were coincidentally aggregated on blackbirds, with 15 blackbirds feeding 67.4% of nymphs and 40.3% of larvae, and coinfestation by both stages was relatively high in summer: Kappa = 0.64 (0.40-0.88). Borrelia burgdorferi sensu lato was identified by polymerase chain reaction (PCR) in 58.3% (35.9-78.5) of larvae with engorgement ratio > or = 3 that were collected from blackbirds. Larvae that were collected from other passerine species gave negative PCR results. Sixteen of 21 PCR-positive samples belonged to B. garinii (76.2%), and five (23.8%) were Borrelia valaisiana. Results of this study suggest that blackbirds play an important role as hosts for immature I. ricinus and as reservoir of Borrelia garinii in northwestern Italy.

The zoonotic reservoir of Borrelia burgdorferisensu lato in the Mazury Lakes district of North-Eastern Poland

The roles of two species of rodents: the bank vole (Clethrionomys glareolus) and the yellow-necked mouse (Apodemus flavicollis) in maintaining the vector Ixodes ricinus and Borrelia burgdorferi s.l. were studied. More particularly, seasonal patterns of rodent abundance and infestation rate with I. ricinus ticks as well as the prevalence of B. burgdorferi s.l. and the two species B. garinii and B. afzelii were determined. The studies were carried out in woodlands at Urwitałt near Mikołajki on the Mazury Lakes in 2001. A total of 196 rodents were trapped and 1349 I. ricinus ticks were collected. There was an increase of rodents in the autumn, with a maximum density of C. glareolus in October and in of A. flavicollis in September. The infestation rate of the investigated animals was above 87% and was dependent on the developmental stage of ticks and host species. Only two stages of I. ricinus were found on trapped rodents: larvae 80% and 84.5%, and nymphs 7.5% and 15.5% for C. glareolus and for A. flavicollis, respectively. The average level of infestation with ticks on rodents appears to be seasonally dependent, and decreases from spring to autumn. Both methods of B. burgdorferi s.l. detection, PCR and IFA, showed that larvae collected from both species of infested rodents were infected by these spirochetes. An increase in the infection rates with B. burgdorferi s.l. was observed from spring to autumn from 12.5% to 15.6% for larval ticks collected from C. glareolus, and from 5.6% to 8.8% for ticks collected from A. flavicollis. The infection rates of larvae examined by PCR were 5% in the spring and 8% in the autumn for ticks collected from C. glareolus, and 10% and 5% for ticks collected from A. flavicollis. A total number of 329 ticks were analysed for the detection of genospecies (184 for B. garinii and 145 for B. afzelii). In larvae collected from C. glareolus, B. garinii were found in 3% and B. afzelii in 1.1% of assessed ticks. Conversely, in larvae collected from A. flavicollis the percentage of infected ticks was 1.5% for B. garinii and 1.7% for B. afzelii. The examined ear biopsies were Borrelia positive in 2.5% and 4.3% of the 44 bank voles and 80 yellow-necked mice, respectively. Both species of rodents, highly abounded in the Mazury Lakes district od North-Eastern Poland, may represent significant, seasonally dependent, zoonotic reservoirs of B. burgdorferi s.l.

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.

First isolation and characterization of Borrelia burgdorferi sensu lato strains from Ixodes ricinus ticks in Turkey

In order to investigate the presence and prevalence of Lyme borreliosis (Lyme disease) Borrelia species, 312 unfed ticks were collected by flagging at a woodland area in Trakya, in the European side of Turkey, in May 2002. Twelve of 299 Ixodes ricinus ticks were infected with Borrelia spp., as determined by cultivation in BSK medium (prevalence rate 4.0 %). Ten pure cultures were subjected to further characterization by sequencing analysis of the 5S-23S rDNA intergenic spacer, 16S rDNA and flagellin gene. One isolate of Borrelia burgdorferi sensu stricto, two of Borrelia garinii (Eurasian type), two of Borrelia afzelii, four of Borrelia lusitaniae and one of Borrelia valaisiana were identified. However, no Asian-type B. garinii was found. Interestingly, all Borrelia species that are known to be carried by I. ricinus were discovered among the 10 isolates. These results provide the first evidence for the existence of the Lyme borreliosis agent in Turkey.

Visualization of Borrelia burgdorferi sensu lato by fluorescence in situ hybridization (FISH) on whole-body sections of Ixodes ricinus ticks and gerbil skin biopsies

The objective of this study was to visualize borreliae directly in whole-body sections of Ixodes ricinus by fluorescence in situ hybridization (FISH). Borrelia afzelii mono-infected or Borrelia burgdorferi sensu stricto (ss)/B. afzelii double-infected nymphs were fixed, embedded in cold polymerizing resin and sectioned. The same sample processing was applied to skin biopsies taken from a Mongolian gerbil after an infectious tick-bite. FISH was carried out using 16S-rRNA-directed, fluorescence-labelled oligonucleotide probes specific for the genus Borrelia and specific within the group of Lyme borreliosis-associated genospecies B. afzelii, B. burgdorferi ss, Borrelia garinii and Borrelia valaisiana. Sensitivity and specificity of the newly designed probes were evaluated using PCR, dot-blot hybridizations and FISH. Despite significant autofluorescence of certain tick tissues (which allowed good histological orientation within the sections), borreliae showing the typical spirochaetal morphotype were clearly visible in five out of six putatively infected ticks. These findings were confirmed by electron microscopy of ticks from the same infected batch as used for FISH. Attempts to produce ticks infected by two different Borrelia genospecies were not successful. FISH on whole-body sections of resin-embedded ticks offers the possibility of visualizing and identifying borreliae within tick tissues. This technique has great potential for the investigation of the transmission of bacteria or other micro-organisms by arthropod vectors. Furthermore, clear visualization of single spirochaetes distributed along subcutaneous fat cell membranes in gerbil skin biopsies suggests that FISH might also be suitable for the detection of borreliae in clinical tissue specimens.