Reliability of molecular host-identification methods for ticks: an experimental in vitro study with Ixodes ricinus

Background: Reliable information on host use by arthropod vectors is required to study pathogen transmission ecology and to predict disease risk. Direct observation of host use is often difficult or impossible and indirect methods are therefore necessary. However, the reliability of currently available methods to identify the last host of blood-feeding arthropods has not been evaluated, and may be particularly problematic for ticks because host blood has been digested at capture. Biases in host detection may lead to erroneous conclusions on both vector ecology and pathogen circulation. Methods: Here, we experimentally tested for biases in host detection using the generalist three-host tick Ixodes ricinus as a model system. We fed ticks using an artificial feeding system and amplified blood meal traces post-moult (i.e., in the succeeding unfed life stage) via both a quantitative real-time polymerase chain reaction assay and a reverse line blotting method. We then experimentally tested for three types of biases in host detection: 1) time post-moult, 2) tick life stage and 3) host type (non-nucleated mammal blood versus nucleated avian blood), and compared these biases between the two molecular methods. Results: Our results show that all three factors can influence host detection in ticks but not necessarily in the expected way. Although host detection rates decreased with time post-moult, mammal blood tended to be more readily detected than bird blood. Tick life stage was also an important factor; detection was higher in nymphs than in adults and, in some cases, remnants from both larval and nymphal blood meals could be detected in the adult stage. These biases were similar for the two detection techniques. Conclusions: We show that different factors associated with questing ticks may influence our ability to correctly infer previous host use and that these factors may bias inferences from field-based studies. As these biases may be common to other vector-borne disease systems, their implications for our understanding of vector ecology and disease transmission require more explicit consideration

Multiple independent transmission cycles of a tick-borne pathogen within a local host community

Many pathogens are maintained by multiple host species and involve multiple strains with potentially different phenotypic characteristics. Disentangling transmission patterns in such systems is often challenging, yet investigating how different host species contribute to transmission is crucial to properly assess and manage disease risk. We aim to reveal transmission cycles of bacteria within the Borrelia burgdorferi species complex, which include Lyme disease agents. We characterized Borrelia genotypes found in 488 infected Ixodes ricinus nymphs collected in the Sénart Forest located near Paris (France). These genotypes were compared to those observed in three sympatric species of small mammals and network analyses reveal four independent transmission cycles. Statistical modelling shows that two cycles involving chipmunks, an introduced species, and non-sampled host species such as birds, are responsible for the majority of tick infections. In contrast, the cycle involving native bank voles only accounts for a small proportion of infected ticks. Genotypes associated with the two primary transmission cycles were isolated from Lyme disease patients, confirming the epidemiological threat posed by these strains. Our work demonstrates that combining high-throughput sequence typing with networks tools and statistical modeling is a promising approach for characterizing transmission cycles of multi-host pathogens in complex ecological settings

Introduced Siberian chipmunks (Tamias sibiricus barberi) contribute more to lyme borreliosis risk than native reservoir rodents.

International audienceThe variation of the composition in species of host communities can modify the risk of disease transmission. In particular, the introduction of a new host species can increase health threats by adding a new reservoir and/or by amplifying the circulation of either exotic or native pathogens. Lyme borreliosis is a multi-host vector-borne disease caused by bacteria belonging to the Borrelia burgdorferi sensu lato complex. It is transmitted by the bite of hard ticks, especially Ixodes ricinus in Europe. Previous studies showed that the Siberian chipmunk, Tamias sibiricus barberi, an introduced ground squirrel in the Forest of Sénart (near Paris, France) was highly infested by I. ricinus, and consequently infected by B. burgdorferi sl. An index of the contribution of chipmunks to the density of infected questing nymphs on the vegetation (i.e., the acarological risk for humans) was compared to that of bank voles (Myodes glareolus) and of wood mice (Apodemus sylvaticus), two known native and sympatric competent reservoir hosts. Chipmunks produced nearly 8.5 times more infected questing nymphs than voles and mice. Furthermore, they contribute to a higher diversity of B. burgdorferi sl genospecies (B. afzelii, B. burgdorferi sensu stricto and B. garinii). The contribution of chipmunks varied between years and seasons, according to tick availability. As T. s. barberi must be a competent reservoir, it should amplify B. burgdorferi sl infection, hence increasing the risk of Lyme borreliosis in humans

Is the introduced Siberian chipmunk an amplifying host of the Lyme disease risk in a French periurban forest?

National audienc

Différenciation génétique entre les bactéries du complexe d'espèce <em>Borrelia burgdorferi</em> infectant trois espèces de petits mammifères en forêt péri-urbaine

National audienc

Temporal variation of <em>Borrelia burgdorferi</em> sl infection in introduced Siberian chipmunks (<em>Tamias sibiricus</em>) in a suburban forest

International audienc

Le paysage et la structure de la communauté des micromammifères agissent différemment sur la prévalence d'<i>Anaplasma phagocytophilum</i> et <i>Borrelia burgdorferi</i> sensu lato dans cette communauté

National audienceLes changements globaux, en modifiant les paysages et les écosystèmes, peuvent avoir des conséquences importantes sur l'émergence ou la réémergence de maladies vectorielles. Dans notre étude, au cours du projet OSCAR (Outil de simulation Cartographique à l'échelle du paysage Agricole du Risque acarologique), nous voulions identifier les effets du paysage sur la prévalence chez les micromammifères de deux agents infectieux transmis par les tiques, Anaplasma phagocytophilum et Borrelia burgdorferi s.l.. Nous avons cherché des relations à la fois avec des variables paysagères et avec la structure même de la communauté de micromammifères. Nous avons échantillonné dans 24 sites situés le long d'un gradient de fragmentation de l'habitat boisé et de linéaire de haies. Nous avons mesuré pour chaque site des variables structurelles du contexte paysager environnant relatives à sa composition et à sa configuration à différentes échelles, ainsi que des variables fonctionnelles de connectivité de l'habitat, pour les micromammifères, basées sur la théorie des graphes et des chemins de moindre coût. La communauté de micromammifères a été caractérisée par l'abondance relative de chaque espèce, et la richesse spécifique. Les agents infectieux ont été recherchés par PCR dans des échantillons de tissus des micromammifères. Nous avons trouvé que la prévalence d'A. phagocytophilum était plus forte dans les paysages boisés et plus faible dans les paysages avec beaucoup d'écotones boisés (interface bois / haie-prairie / culture). Elle était aussi liée positivement à l'abondance du Campagnol roussâtre (Myodes glareolus), qui sont de bons réservoirs par rapport au Mulot sylvestre (Apodemus sylvaticus), l'espèce dominante dans nos captures. La prévalence était aussi positivement reliée à la richesse spécifique en micromammifères, peut-être de par la présence d'espèces moins fréquentes, mais meilleurs réservoirs. Cependant, nous n'avons pas trouver de corrélation entre les variables paysagères et ces variables de communauté de micromammifères, ce qui suggère l'importance d'autres mécanismes de structuration des communautés de micromammifère comme la qualité de l'habitat ou la pression de prédation. Concernant B. burgdorferi s.l., aucune variable n'était significativement reliée à sa préférentielle par une espèce de tique plus généraliste, une gamme d'hôte réservoirs plus large, et la circulation de plusieurs génoespèces que nous n'avons paa pu identifier systématiquement. Cette étude montre l'intérêt de considérer les interactions entre les agents infectieux et leurs espèces hôtes, plus ou moins compétentes en tant que réservoirs, pour mieux comprendre les effets du paysage sur la transmission de maladie

Infection of Siberian chipmunks (Tamias sibiricus barberi) with Borrelia sp. reveals a low reservoir competence under experimental conditions.

International audienceReservoir competence is a key parameter in understanding the role of host species in the epidemiology of multi-host-especially vector-borne-pathogens. With this aim in view, we studied the reservoir competence of the Siberian chipmunk (Tamias sibiricus barberi) recently introduced into Europe, for the multi-host tick-borne bacteria, Borrelia burgdorferi sl, the agent of Lyme borreliosis. T. sibiricus were experimentally exposed to bites from Ixodes ricinus ticks infected with Borrelia burgdorferi sensu stricto and Borrelia afzelii, with subsequent assessment of bacteremia and antibody responses. Borrelia was detected in chipmunk blood samples, ear biopsies and organ necropsies, and in nymphs used for xenodiagnosis (at one and six months after the initial chipmunk infection) via both serological and molecular methods. In total, eight out of twelve chipmunks showed evidence of infection by Borrelia sp., either by ELISA or PCR. Five chipmunks developed an immune response against the bacteria one month after infection. Borrelia infection in at least one organ was observed in seven animals at 14, 38, 93 or 178 days post-infection. Xenodiagnosis was positive for one chipmunk at 38 days, but no longer at 178 days post-infection. Four chipmunks remained uninfected, despite similar infection pressures to those observed in the field. Taken together, these results suggest that chipmunks can be infected through Borrelia-infected tick bites, and can transmit Borrelia to nymphs, but do not remain persistently infected