Gern, Lise

2013, Herrmann, Coralie, Gern, Lise

La tique Ixodes ricinus est très sensible à la dessiccation et elle est particulièrement exposée à des conditions desséchantes lorsqu’elle quête sur la végétation en attente d’un hôte. Nous avons donc étudié la survie de cette tique à la dessiccation en exposant des nymphes et des adultes à des conditions desséchantes créées en laboratoire à l’aide de solutions salines saturées produisant différentes conditions d’humidité relative à différentes températures. Nous avons observé que lorsque les tiques sont infectées par Borrelia burgdorferi sensu lato (s.l.), elles survivent mieux à la dessiccation que les tiques non infectées. Ces résultats suggèrent que les borrélies pourraient modifier la physiologie et/ou le métabolisme des organes impliqués dans l’absorption, le stockage, et/ou la perte d’eau résultant en une capacité à stocker l’eau améliorée chez les tiques infectées, modifiant leur besoin d’humidité. Nous avons vérifié cette hypothèse en exposant des nymphes I. ricinus récoltées dans la nature à un gradient d’humidité dans une arène proposant deux choix (sec ou humide), puis nous avons relevé leur position dans l’arène (indiquant leur mouvement général). Les nymphes infectées se sont moins déplacées le long du gradient d’humidité que les nymphes non infectées et sont restées préférentiellement dans un endroit modérément sec, comparé aux tiques non infectées qui, elles, se sont déplacées préférentiellement vers un endroit plus humide. Ceci laisse penser que les tiques infectées par B. burgdorferi s.l. tolèrent mieux des conditions desséchantes que celles qui ne sont pas infectées.
Dans la nature, quand l’atmosphère est très sèche, I. ricinus doit descendre au sol plus fréquemment pour se réhydrater (Randolph and Storey, 1999; Perret et al., 2004), ce qui augmente sa consommation d’énergie. La survie accrue à la dessiccation observée chez les tiques infectées pourrait être due à des réserves d’énergie (contenu en gras) plus grandes chez ces dernières. Nous avons donc quantifié le contenu en gras chez des nymphes récoltées dans la nature et nous avons observé que les nymphes infectées par B. burgdorferi s.l. possédaient de plus grandes réserves d’énergie que les nymphes non infectées. Ces résultats impliquent que les nymphes infectées ont la possibilité de quêter plus longtemps, car elles peuvent effectuer un plus grand nombre mouvements de haut en bas sur la végétation avant épuisement de leurs réserves d’énergie et donc survivre plus longtemps. Un temps de quête plus long augmente les chances de trouver des hôtes et ainsi de transmettre Borrelia. On peut donc parler d’une stratégie de manipulation de la part des spirochètes B. burgdorferi pour optimiser leur transmission à d’autres hôtes en modifiant les traits phénotypiques de leur vecteur.
La survie durant les mois d’hiver représente également un défi pour I. ricinus. C’est pourquoi nous avons testé la survie de nymphes I. ricinus à des températures froides et à des variations de température plus ou moins fréquentes. Nous avons observé que les nymphes plus jeunes (récoltées en automne) survivent mieux aux conditions hivernales que les nymphes plus âgées (récoltées au printemps). De plus, plus la fréquence de variation de température augmente, plus le taux de mortalité augmente chez les nymphes. Cette augmentation du taux de mortalité reflète probablement l’augmentation des coûts énergétiques des adaptations métaboliques que les variations de température génèrent à une fréquence croissante. En outre, les nymphes infectées par Borrelia survivent légèrement mieux aux conditions hivernales que les nymphes non infectées.
Nous avons combiné les données concernant l’infection par Borrelia des nymphes I. ricinus récoltées dans la nature et précédemment testées dans des tests comportementaux ou physiologiques. Cet important jeu de données nous a permis d’analyser statistiquement les associations entre les génoespèces de borrélies. Nous avons observé que les génoespèces de borrélies spécialisées pour les mêmes hôtes réservoirs co-infectaient fréquemment I. ricinus, alors que celles spécialisées pour des hôtes réservoirs différents étaient rarement détectées dans les mêmes tiques. De plus, les infections impliquant des génoespèces fréquemment en co-infection comprenaient un nombre élevé de spirochètes dans la tique, alors que les infections impliquant des génoespèces rarement en co-infection consistaient en des nombres faibles de spirochètes. De tels résultats laissent supposer que le système immunitaire des hôtes vertébrés joue un rôle dans la détermination à la fois de la fréquence des infections impliquant deux génoespèces de borrélies et du nombre de spirochètes dans la tique infectée par deux génoespèces.
Cette thèse contribue de manière significative à améliorer nos connaissances concernant la biologie des tiques I. ricinus infectées par Borrelia et non infectées dans un contexte de changement climatique, en révélant notamment que les tiques infectées par Borrelia risquent de mieux survivre et de trouver des hôtes plus aisément dans des conditions desséchantes au printemps et en été, et que les tiques I. ricinus sont susceptibles de mieux survivre en hiver si les écarts de température se font plus rares., Ixodes ricinus is known to display little resistance to desiccation. This tick is particularly exposed to desiccating conditions while questing for a host on the vegetation. Therefore, survival under hot and dry conditions in field-collected I. ricinus nymphs and adults was studied by exposing ticks to desiccating conditions created in the laboratory using saturated salt solutions producing various relative humidity conditions at different temperatures. We observed that I. ricinus nymphs and adults infected with Borrelia burgdorferi sensu lato (s.l.) survived better under hot and dry conditions than uninfected ticks. These findings suggest that Borrelia spirochetes might change the physiology and/or metabolism of organs involved in water sorption, storage, and/or loss, resulting in enhanced water storage in infected ticks, thereby modifying I. ricinus need for humidity. To test this hypothesis, field-collected I. ricinus nymphs were allowed to walk within a humidity gradient in a two-choice arena, and their position (indicating their overall movement) was recorded. I. ricinus nymphs harbouring spirochetes walked less within the humidity gradient than uninfected nymphs. Borrelia-infected individuals showed a lower need to move to an environment that was favourable for maintaining water balance since they preferentially stayed in a moderately dry area, compared to uninfected ticks, which preferentially moved towards a moister area. This suggests that B. burgdorferi-infected ticks might tolerate desiccating conditions differently from uninfected individuals. Since I. ricinus ticks go down the vegetation more frequently when desiccation increases in nature (Randolph and Storey, 1999; Perret et al., 2004), which increases energy consumption, this higher tolerance to desiccation in infected ticks might be due to higher energy reserves (fat content) in the latter. Fat content was quantified in field-collected I. ricinus nymphs and we observed that nymphs infected with B. burgdorferi s.l. possessed higher energy reserves than uninfected nymphs. These findings imply that infected I. ricinus nymphs with higher energy reserves may quest for a longer time than uninfected individuals. Higher energy reserves allow more movements up and down the vegetation before depletion of energy reserves, which allows longer survival. Prolonged questing time increases chances for infected ticks to find hosts and thereby to transmit Borrelia. This is thought to be a manipulating strategy of B. burgdorferi spirochetes to enhance their transmission to other hosts by modifying phenotypic traits of their vector.
Survival during winter months is also a challenge for I. ricinus. Therefore, we exposed field-collected I. ricinus nymphs to cold temperatures at different frequencies of exposition. Results showed that younger (autumn) nymphs were more likely to survive under cold conditions than older (spring) nymphs. Moreover, nymphs died faster when they were exposed to temperature variations at high frequency than low frequency. This probably reflected higher energetic costs due to metabolic adaptations that temperature variations generated when experienced at higher frequency. Besides, Borrelia-infected nymphs survived slightly better than uninfected nymphs.
Borrelia infections in field-collected I. ricinus nymphs used in behavioural or physiological tests were combined in one data set to analyse statistically the associations between Borrelia genospecies. We observed that Borrelia genospecies that were specialised for the same reservoir hosts frequently occurred together, while Borrelia genospecies that were specialised for different reservoir hosts rarely co-occurred. Furthermore, infections involving Borrelia genospecies frequently co-occurring consisted of high spirochete numbers, whereas infections involving Borrelia genospecies rarely co-occurring consisted of low spirochete numbers. We suggest that the vertebrate immune system plays a role in shaping both the frequency of infections involving two Borrelia genospecies and the spirochete load inside the tick harbouring two genospecies.
This thesis has contributed to improve significantly our knowledge on the biology of Borrelia-infected and uninfected I. ricinus ticks in the context of climate change, in particular by revealing that Borrelia-infected ticks are more likely to survive and find hosts under desiccating conditions in spring and summer, and that I. ricinus ticks are likely to survive better in winter if temperature variations are less frequent.

2009, Hügli, Delphine, Moret, Jacqueline, Rais, Olivier, Moosmann, Yves, Erard, Philippe, Malinverni, Raffaele, Gern, Lise

The duration of tick feeding is an important indicator to evaluate the risk of Borrelia burgdorferi sensu lato transmission, which increases considerably with the blood meal duration. This blood meal duration may be estimated from scutal index, the ratio between body length (idiosoma) and scutum width. For the estimation of blood meal duration in Ixodes ricinus, nymphal and adult female ticks were detached at predetermined intervals (24, 48, 72, and 96 h) from laboratory mice and rabbits and their scutal index calculated. From this, non-linear regression equations were developed to determine the duration of attachment for nymphal and adult female I. ricinus ticks. As part of an epidemiological study addressing the risk of subclinical (seroconversion) and clinical infections after a tick bite in the Neuchâtel area (Switzerland) over 3 years (2003–2005), duration of tick attachment and anatomical site of bites collected on participants as well as seasonal distribution of tick bites were studied. Tick attachment duration was estimated in all ticks collected during this study (n=261). Nymphs were attached for a mean (± standard error, SE) of 31.6 h (±2.6) and females for a mean (±SE) of 29.6 h (±3.2). Most nymphs were removed after 24 h of blood meal whereas most females were removed before 24 h. Legs were the major anatomical sites of bites for women (40.7%), men (44.4%), and almost all age classes. Only children <10 years old were bitten more frequently on the head (41.2%) and on the neck (38.5%) than participants >10 years. The majority of tick bites were recorded from May to July during the 3 years. Attachment sites can influence the discovery of ticks, hence the duration of the tick bite. A detailed body examination after each outing in forest and an early withdrawal of an attached tick is an effective way to prevent Lyme borreliosis.

2007, Daix, V., Schroeder, H., Praet, N., Georgin, J.-P., Chiappino, I., Gillet, L., de Fays, K., Decrem, Y., Leboulle, G., Godfroid, E., Bollen, A., Pastoret, P.-P., Gern, Lise, Sharp, P. M., Vanderplasschen, A.

The alternative pathway of complement is an important innate defence against pathogens including ticks. This component of the immune system has selected for pathogens that have evolved countermeasures. Recently, a salivary protein able to inhibit the alternative pathway was cloned from the American tick Ixodes scapularis (Valenzuela et al., 2000; J. Biol. Chem. 275, 18717–18723). Here, we isolated two different sequences, similar to Isac, from the transcriptome of I. ricinus salivary glands. Expression of these sequences revealed that they both encode secreted proteins able to inhibit the complement alternative pathway. These proteins, called I. ricinus anticomplement (IRAC) protein I and II, are coexpressed constitutively in I. ricinus salivary glands and are upregulated during blood feeding. Also, we demonstrated that they are the products of different genes and not of alleles of the same locus. Finally, phylogenetic analyses demonstrate that ticks belonging to the Ixodes ricinus complex encode a family of relatively small anticomplement molecules undergoing diversification by positive Darwinian selection.

2003, Perret, Jean-Luc, Guerin, Patrick, Diehl, Peter-Allan, Vlimant, Michèle, Gern, Lise

The behaviour of Ixodes ricinus nymphs was recorded in 10-day experiments using computer-assisted video-tracking, in the absence of any host stimuli. These ticks switch spontaneously from questing in a desiccating atmosphere to quiescence in a water-saturated atmosphere after dark. Quantification of both questing and quiescence duration demonstrates that questing duration is inversely related to saturation deficit whereas quiescence duration is not. Distance walked after quiescence increased with desiccating conditions, while the distance walked after questing remained unchanged. Almost all locomotor activities of I. ricinus occurred during darkness under either a 14 h:10 h L:D or a 8 h:4 h L:D cycle. We established that all life stages of I. ricinus are equipped to sense shifts in light intensity with bilaterally placed strings of photoreceptors. This permits I. ricinus to use onset of darkness to trigger mobility when desiccation risk is reduced in nature.

2012, Lommano, Elena, Gern, Lise

Dans les régions tempérées de l’hémisphère nord, les tiques représentent le premier vecteur d’agents infectieux d’importance médicale et vétérinaire. Parmi elles, Ixodes ricinus, abondante en Europe, transmet bon nombre de microorganismes aux Hommes et aux animaux, aussi bien des bactéries, des virus, que des protozoaires, pour la plupart responsables de zoonoses considérées comme émergentes. Ces microorganismes sont Borrelia spp., Anaplasma phagocytophilum, Rickettsia spp., Babesia spp. ou encore le virus de l’encéphalite à tique (TBEV). Depuis les années 80’, plus d’une dizaine d’agents pathogènes pour l’Homme ont été découverts dans les tiques en Europe. En Suisse, de nouvelles espèces comme Babesia venatorum, Rickettsia monacensis, Borrelia miyamotoi ou B. spielmanii ont fait leur apparition durant la dernière décennie. De plus, un cas de septicémie dû à Candidatus Neoehrlichia mikurensis, une bactérie transmise par I. ricinus, a récemment été signalé chez un homme résident en Suisse, sans que la bactérie n’ait jamais été décrite dans les tiques du pays. Face à l’émergence de ces pathogènes, nous avons évalué leur distribution géographique ainsi que leur prévalence dans les tiques dans l’Ouest de la Suisse afin d’identifier les zones à risque. Nous avons recherché Borrelia spp., Rickettsia spp., Babesia spp., A. phagocytophilum et Candidatus N. mikurensis dans les tiques en quête récoltées dans 11 sites. Globalement, 34.2% (505/1’476) des tiques étaient infectées par au moins un pathogène. Borrelia spp., Rickettsia spp. et Candidatus N. mikurensis étaient présents dans tous les sites investigués avec des prévalences de 22.5%, 10.2% et 6.4%, respectivement. A l’inverse, Babesia spp. et A. phagocytophilum ont démontré une répartition géographique plus restreinte et une prévalence plus faible (1.9% et 1.5%). Des co-infections, impliquant le plus souvent Borrelia spp. et Rickettsia helvetica, ont été détectées dans 19.6% des tiques infectées. Nous avons identifié Candidatus N. mikurensis pour la première fois dans les tiques sur le territoire helvétique ainsi que des espèces rarement signalées comme R. monacensis, B. lusitaniae et B. spielmanii.
En Suisse, ces dernières années, des cas humains d’encéphalite à tiques ont été déclarés hors du périmètre qui délimitait les foyers de TBEV jusque-là, dans l’Ouest du pays. Ainsi, à partir des années 2'000, de nouvelles zones endémiques au TBEV ont été répertoriées dans cette région. Nous avons confirmé la présence du TBEV dans les tiques libres dans l’une de ces nouvelles zones endémiques au virus, la Plaine de l’Orbe avec une prévalence globale de 0.1% (6/6’120). Parmi les cinq foyers identifiés, la prévalence du virus variait de 0.21 à 0.95. La diversité génétique des souches virales nous laisse supposer que les oiseaux pourraient être impliqués dans l’émergence de ces foyers, probablement par la dissémination de tiques infectées dans des milieux propices. Pour valider cette hypothèse et estimer l’éventail des pathogènes disséminés par les oiseaux, 1’205 tiques récoltées sur ces hôtes vertébrés ont été analysées. Cinq pathogènes de genres différents ont été détectés dans les tiques d’oiseaux. Le plus fréquent est Borrelia spp. (19.5%), suivi de Rickettsia spp. (12.3%), A. phagocytophilum (2%), Candidatus N. mikurensis (3.3%) et du TBEV (0.2%).
L’identification du TBEV dans deux larves et une nymphe fixées sur des oiseaux migrateurs (deux rouges-gorges et un merle noir) étaie notre hypothèse de l’implication des oiseaux dans l’émergence de foyers de TBEV dans l’Ouest de la Suisse. Par ailleurs, cette étude constitue un des premiers rapports de Candidatus N. mikurensis dans les tiques d’oiseaux et montre que plusieurs espèces de passereaux, dont le merle noir, sont impliquées dans les cycles de transmission de ces microorganismes. Nos résultats mettent en relief la circulation et co-circulation d’agents pathogènes d’importance médicale et vétérinaire dans les tiques en Suisse et l’implication des oiseaux dans le maintien de certains de ces pathogènes., In temperate regions of the northern hemisphere, ticks are considered as the primary vector of infectious agents of human and medical relevance. Among them, Ixodes ricinus is the most abundant in Europe. This tick species transmits to humans and animals many microorganisms that may cause zoonoses, including bacteria, viruses and protozoa like, for example, Borrelia spp., Anaplasma phagocytophilum, Rickettsia spp., Babesia spp. or the TBE virus (TBEV). In Europe since the 80s, more than 10 human pathogenic agents have been described in ticks. In Switzerland, new pathogen species like Babesia venatorum, Rickettsia monacensis, Borrelia miyamotoi or B. spielmanii appeared recently. Moreover, Candidatus Neoehrlichia mikurensis, transmitted by I. ricinus ticks, was detected in the blood of one man with signs of septicemia in Switzerland whereas this bacterium had never been described in ticks in the country before. In this context, our aim was to evaluate the geographic distribution and prevalence of tick-borne pathogens in order to identify risk areas in western Switzerland. Therefore, we prospected the presence of Borrelia spp., Rickettsia spp., Babesia spp., A. phagocytophilum and Candidatus N. mikurensis in ticks collected at 11 sites. Globally, 34.2% (505/1’476) of ticks were infected with at least one pathogen. Borrelia spp., Rickettsia spp. and Candidatus N. mikurensis were present at all investigated sites with prevalences of 22.5%, 10.2% and 6.4%, respectively. Conversely, Babesia spp. and A. phagocytophilum had smaller geographical ranges and lower prevalence rates (1.9% and 1.5%). Co-infections, involving mostly Borrelia spp. and Rickettsia helvetica, were detected in 19.6% of infected ticks. We identified Candidatus N. mikurensis for the first time in ticks in Switzerland as well as species rarely reported like R. monacensis, B. lusitaniae and B. spielmanii.
In Switzerland, over the last years, human TBE cases have been reported in the West of the country, outside a perimeter that included all TBEV foci until then. Thus, since the early 2000s, new TBE endemic areas were recognised in this region. Our results confirmed the presence of TBEV in ticks in one of these new endemic areas, the Plaine de l’Orbe, with a global prevalence of 0.1% (6/6’120). In this area, five foci were identified with TBEV prevalence values ranging from 0.21 to 0.95. The genetic diversity of the virus circulating in this endemic area led us suggest that birds were implicated in the emergence of these new TBEV foci, probably by disseminating infected ticks in environments favourable to the maintenance of TBEV foci. To test this hypothesis and evaluate the diversity of tick-borne pathogens that can be disseminated by birds, 1’205 bird-feeding ticks were analysed. Five pathogens of different genera were detected; Borrelia spp. was the most frequent (19.5%), followed by Rickettsia spp. (12.3%), A. phagocytophilum (2%), Candidatus N. mikurensis (3.3%) and TBEV (0.2%). The identification of TBEV in two larvae et one nymph feeding on migrating birds (two European robins and one blackbird) support our hypothesis on the implication of birds in the emergence of TBEV foci in western Switzerland. In addition, our study is one of the first reports on Candidatus N. mikurensis in bird-feeding ticks and shows that several passerines, including the blackbird, are implicated in the transmission cycles of these microorganisms. Our results highlight the circulation and co-circulation of tick-borne pathogens of medical and veterinary importance in ticks in Switzerland and the implication of birds in the maintenance of some of them in nature.

2008, Casati, Simona, Bernasconi, Marco V., Gern, Lise, Jean-claude Piffaretti

The Ixodes ricinus complex is composed of 14 species distributed worldwide. Some members of this complex are involved in the transmission of a number of diseases to animals and humans, in particular Lyme borreliosis, tick-borne encephalitis, ehrlichiosis and babesiosis. While the phylogenetic relationships between species of the I. ricinus complex have been investigated in the past, still little is known about the genetic structure within the species I. ricinus sensu stricto. We have investigated the intraspecific variability among 26 I. ricinus s.s. ticks collected in various European countries, including Switzerland, Italy, Austria, Denmark, Sweden, and Finland by using five mitochondrial gene fragments corresponding to the control region, 12S rDNA, cytb, COI, and COII. The five genes considered here showed a low genetic variability (1.6–5%). Our results based on both statistical parsimony (applied to the COI + COII + cytb + 12S + CR data set, for a total of 3423 bp) and maximum parsimony (applied to the COI + COII + cytb + 12S data set, for a total of 2980 bp) did not provide any evidence for a correlation between the identified haplotypes and their geographic origin. Thus, the European I. ricinus s.s. ticks do not seem to show any phylogeography structure.

2004, Jouda, Fatima, Perret, Jean-Luc, Gern, Lise

Lyme borreliosis, the most important vector-borne disease in the Northern hemisphere, causes health problem for populations in endemic areas. In the present study, the density of questing Ixodes ricinus ticks and their infection with Borrelia burgdorferi sensu lato (sl) was examined in 11 areas located on the Swiss Plateau and in an alpine valley. From 1999 to 2001, free-living I. ricinus ticks were collected on a monthly basis by flagging vegetation in these areas. Each tick was examined for the presence of B. burgdorferi sl using direct fluorescent antibody assay, and for isolation of the bacteria. Borreliae were characterized by PCR followed by RFLP. Density of questing ticks varied greatly between studied areas. Borreliae were observed in ticks collected in all investigated sites. However, the prevalence of infection differed significantly among areas. Infection prevalence varied from 9% to 40% in nymphs and from 22% to 47% in adults. Adult ticks were significantly more infected (129/366, 35%) than nymphs (109/552, 20%). There was no correlation between nymphal density and infection prevalence as well as between adult density and infection prevalence, but there was a correlation between density of ticks and density of infected ticks. During the spring peak of questing tick density, a range of 2-30.3 infected ticks per 100 m² was observed. B. burgdorferi sl isolates (n = 129) were obtained from ticks collected in 10/11 areas. Five Borrelia species were identified: B. garinii, B. burgdorferi sensu stricto, B. afzelii, B. valaisiana, B. lusitaniae, and six mixed infections were also obtained. Borrelia species were heterogeneously distributed in the different areas.

2012, Tonetti, Nicolas, Gern, Lise

La protéine de surface C (OspC) de Borrelia burgdorferi sensu lato (sl) est impliquée dans de nombreux mécanismes participant à la dissémination de l'agent pathogène chez son hôte. La grande diversité du gène ospC a permis une classification des différents allèles en groupes, laissant apparaître un lien entre l'invasivité de la bactérie chez l'Homme et les différents groupes ospC.
Sachant que le gène ospC est aussi impliqué dans la dissémination des spirochètes chez la tique, nous avons étudié la dynamique de transmission des spirochètes de la tique à l’hôte et vice-versa en fonction de leur appartenance à un groupe ospC. Nous avons donc évalué la dynamique de transmission de B. afzelii par Ixodes ricinus après infection expérimentale et naturelle pour des isolats appartenant aux groupes invasifs A1, A2, A3, A4, A5 et aux groupes indéfinis ME et YU. L'efficacité de transmission de la tique à l'hôte, de l'hôte à la tique, mais aussi de tique à tique lors du co-feeding a ainsi été évaluée pour chaque isolat.
Tous les groupes ospC ont été infectieux pour les souris et/ou les tiques après injection sous-cutanée et/ou via des tiques infectées. Par contre, aucune transmission par co-feeding n'a été observée pour les génotypes A3 et A4, ce qui laisse supposer une composante génétique propre à cette voie de transmission. Des différences importantes dans la dynamique de transmission ont été observées entre les infections expérimentales et naturelles. La voie d'inoculation des spirochètes est donc un facteur influençant fortement la dynamique de transmission.
Finalement, le temps de repas sanguin nécessaire pour qu'il y ait transmission de spirochètes de la l'hôte à la tique, et inversement, a aussi été évalué. Un repas sanguin de 6 heures seulement a été suffisant pour observer une transmission de spirochètes de l'hôte à la tique, avec une infectivité des souris pour les tiques augmentant avec la durée de repas sanguin. Pour la situation inverse, seules les souris infestées pendant 24 heures et plus ont déclaré des infections systémiques. Toutefois, placer des tiques 24 heures à 37°C avant infestation des souris a permis une transmission après seulement 12 heures de repas sanguin.
Des différences significatives dans la dynamique de transmission ont été observées autant entre génotypes qu'au sein d'un même génotype, laissant supposer que le gène ospC seul n'est pas responsable des différences d'efficacité de transmission observées au niveau intra spécifique. Il ne semble donc pas possible d'évaluer correctement les risques encourus par l'Homme de développer une maladie de Lyme invasive en se basant uniquement sur la distribution des différents ospC dans la nature., Borrelia burgdorferi sensu lato (sl) outer surface protein C (OspC) is involved in numerous crucial mechanisms during the life cycle of the pathogen, influencing its dissemination in the vector and in the host. The genetic diversity at the ospC locus has led to a classification into groups of alleles. While there is a linkage between invasiveness in human and the different ospC genotypes, many interrogations remain on how the host-vector-host dynamic of transmission is influenced by those genotypes, which could have an impact on their distribution in nature.
We therefore investigated the transmission dynamics of B. afzelii isolates belonging to invasive genotypes A1, A2, A3, A4, A5 and to undefined groups ME and YU, using a laboratory-based system including Balb/c mice and Ixodes ricinus ticks with experimental or tick infections. The efficiency of transmission from infected mice to ticks and from infected ticks to mice was evaluated for each isolate as well as the efficiency of co-feeding transmission between infected nymphs and uninfected larvae feeding on uninfected mice.
Each ospC group was infectious for mice and/or ticks after experimental and/or natural infections. On the other hand, no co-feeding transmission occurred for ospC groups A3 and A4, which suggests a genetic component unique to this transmission pathway. Important differences in the transmission efficiencies were observed between experimental and natural infections. Inoculation route therefore greatly influences the transmission dynamic of spirochetes.
Finally, delays in the transmission of B. afzelii from mice to tick and from tick to mice were also evaluated. Host to tick transmission was observed as soon as six hours after tick attachment. Transmission to ticks was enhanced with the duration of the blood meal. For the tick to host transmission delay, mice were infected after being challenged for 24 hours with infectious ticks. Incubating infectious ticks 24 hours at 37°C shortened the delay to only 12 hours of blood meal.
Significant differences in the tick to host, host to tick and co-feeding transmissions were observed among and within B. afzelii ospC groups, suggesting that B. afzelii ospC gene alone is not responsible for variations in the efficiency of host-vector-host dynamic of transmission at the intra specific level. Evaluating risks of infection with an invasive form of the Lyme disease agent based on the distribution of those genotypes in nature seems therefore not relevant.

2008, Morán Cadenas, Francisca, Burri, Caroline, Gern, Lise

In the context of climate change, the seasonal activity of questing Ixodes ricinus and their infection with Borrelia burgdorferi sensu lato (s.l.) were examined in relation to some climatic data along altitudinal gradients in Switzerland. The first study took place in an Alpine area (Valais) from 750 to 1020 m above sea level. The other gradient was located on a mountain in the foothills of the Jura chain (Neuchâtel) from 620 to 1070 m above sea level. In the Alpine area, the highest questing tick density was observed at the highest altitude. At the lowest altitudes (750 and 880 m), very high saturation deficits, >10 mmHg, were present during most of the tick activity season and they seem to have impaired the thriving of tick populations. The second study in Neuchâtel (2003–2005) was a follow-up of a previous study (1999–2001) in which it was observed that tick density decreased with increasing altitude. During the follow-up study, substantial differences in questing tick density and phenology of ticks were observed: At high elevations, questing tick densities were 2.25 and 3.5 times higher for nymphs and adults, respectively, than during 1999–2001. As observed during 1999–2001, questing tick density decreased with increasing altitude in this site in 2003–2005. Tick questing density remained higher at the lowest altitude. Increased temperatures during summer months, more favorable for ticks, reaching values similar to those registered in the first study at the lowest elevations are probably responsible for the higher tick questing density at high altitudes. B. burgdorferi s.l. infection prevalence in ticks decreased with increasing altitudes along both altitudinal gradients. Long-lasting high saturation deficit values may limit the development of tick populations as too high a moisture stress has a negative effect on tick survival. This factor may have a permanent impact, as it is probably the case at the lowest altitudes in the Alpine area or a more transient effect like in the Neuchâtel gradient.

2004, Casati, Simona, Bernasconi, Marco V., Gern, Lise, Piffaretti, Jean-Claude

A total of 874 Ixodes ricinus ticks were collected in Switzerland to investigate the genetic diversity of the Borrelia population. We integrated to the RT-PCR method the DNA sequence analysis of a 162-bp fragment of the recA gene. Five genospecies were detected: Borrelia afzelii, Borrelia burgdorferi s.s., Borrelia garinii, Borrelia valaisiana, and Borrelia lusitaniae. A heterogeneous distribution was observed within the B. burgdorferi s.l. genospecies. The most prevalent and diverse genospecies found in Switzerland was Borrelia afzelii, which might suggest a rapid evolution of this genospecies.