Hélène Verheyden

Hyalomma marginatum occurs across a wide range of environments, from Mediterranean lowlands in North Africa to continental steppes in Eastern Europe and has recently colonized diverse habitats in southern France. This ecological breadth raises key questions regarding its capacity to complete its life cycle under contrasting environmental conditions and whether its current distribution in France reflects ecological limits. In this study, we investigated the phenotypic plasticity and developmental performance of H. marginatum through a population transplant experiment. Engorged females and nymphs reared under controlled conditions were released at two climatically distinct sites: Pompignan (Gard), where the species is highly abundant and characterized by a Mediterranean climate, and Gardouch (Haute-Garonne), where the species is not yet established and experiences a southwestern basin climate. At each site, individuals were placed in two contrasting microhabitats (open versus forested environments) to assess the influence of local and microclimatic conditions on life cycle timing and survival. Our results show that H. marginatum is able to complete its entire life cycle at both sites with broadly similar phenology. In contrast, microhabitat had a stronger influence on development and survival, indicating substantial phenotypic plasticity in response to local environmental conditions. The absence of established populations in Gardouch area therefore cannot be readily explained by climatic constraints alone, and may instead reflect dispersal limitations, unsuitable timing of introductions, or other environmental factors such as humidity or vegetation structure. Our findings suggest that H. marginatum has the potential to further expand its range in France in the coming years.

The risk of viral transmissions from domestic/wild animals to humans is a major public health concern. Humans can also transmit viruses back to domestic and wild animals, acting as reservoir for virus maintenance and sources of epidemic diseases re-emergence. The SARS-CoV-2, causing COVID-19, likely originated from wildlife and has been evidenced to transmit from humans to captive, domestic and wild animals. White-tailed deer ( Odocoileus virginianus ) show high SARS-CoV-2 prevalence following human contamination, suggesting they could act as an emerging virus reservoir. We completed recent research on European cervid species by investigating whether SARS-CoV-2 had emerged in longitudinally-monitored European roe deer ( Capreolus capreolus ) populations in direct contact with humans in France. We performed indirect tests (serological ELISAs and seroneutralization) on sera collected before and after the virus emergence in humans, and direct RT-PCR tests on nasal swabs collected in 2022. We also investigated the virus exposure and prevalence in three other cervid species. ELISA tests were positive for 2.20% of sera, pre- and post-pandemic, but seroneutralization and PCR tests were negative. Although one population showed increased seroprevalence post-2020, results suggest that SARS-CoV-2 has not emerged in those populations, and that ELISA cross-reaction with one or several unidentified circulating coronaviruses is possible.

Summary: Background: Lyme borreliosis and tick-borne encephalitis (TBE) are tick-borne zoonoses of public health importance, transmitted in Western Europe by the same tick vector Ixodes ricinus . Roe deer ( Capreolus capreolus ) is a widespread feeding host for I. ricinus stages. Here, we explored the spatial extent of both pathogens in the wild in Northern France by assessing serological status in roe deer. Methods: We collected 3,147 blood samples (2019-2022), and searched antibodies against both pathogens using ELISA tests. We fitted logistic models to investigate exposure to both or either pathogens, and explored spatial patterns using Kulldorf clustering scan tests. Results: The overall ELISA IgG prevalence was 23.24% (95%CI [21.77-34.72]) for Borrelia and 8.64% (95%CI [7.45-9.86]) for TBEV. The median Borrelia seroprevalence varied from 4% to 42% between departments, with two high-risk clusters of exposure identified. TBEV seroprevalence did not show any spatial pattern, and positives by seroneutralisation tests were reported in eight departments. Discussion: Our findings suggest the presence of both pathogens over large areas. Wider spatial sampling including areas at lower risk and improved serological methods would be beneficial to further validate those estimates.

Toll‐like receptors (TLR) play a central role in recognition and host frontline defence against a wide range of pathogens. A number of recent studies have shown that TLR genes (Tlrs) often exhibit large polymorphism in natural populations. Yet, there is little knowledge on how this polymorphism is maintained and how it influences disease susceptibility in the wild. In previous work, we showed that some Tlrs exhibit similarly high levels of genetic diversity as genes of the Major Histocompatibility Complex (MHC), and signatures of contemporary balancing selection in roe deer (Capreolus capreolus), the most abundant cervid species in Europe. Here, we investigated the evolutionary mechanisms by which pathogen‐mediated selection could shape this innate immunity genetic diversity by examining the relationships between Tlr (Tlr2, Tlr4 and Tlr5) genotypes (heterozygosity status and presence of specific alleles) and infections with Toxoplasma and Chlamydia, two widespread intracellular pathogens known to cause reproductive failure in ungulates. We showed that Toxoplasma and Chlamydia exposures vary significantly across years and landscape features with few co‐infection events detected and that the two pathogens exert antagonistic selection on Tlr2 polymorphism. By contrast, we found limited support for Tlr heterozygote advantage. Our study confirmed the importance of looking beyond Mhc genes in wildlife immunogenetic studies. It also emphasized the necessity to consider multiple pathogen challenges and their spatiotemporal variation to improve our understanding of vertebrate defence evolution against pathogens.

Because of their continuing expansion, wildlife ruminant species that prosper in rural landscapes may be increasingly affected by and/or contribute to the circulation of certain generalist pathogens also infecting domestic ruminants, when they share common spaces or resources. In this study, we aimed to test the hypothesis that parasitism with gastrointestinal nematodes (GINs) of wild roe deer inhabiting different rural landscapes is correlated with livestock density. We used faecal egg counts of GINs and spatial data of 74 GPS-collared roe deer, inhabiting various landscapes from closed forests to open fields, together with weekly records of livestock abundances on pasture. We tested whether the excretion of GIN eggs in roe deer was influenced by the density of livestock in their home range over the grazing season. Our results showed that all of the roe deer home ranges, except four, contained pastures occupied by livestock. Excretion of GIN eggs occurred in 77% of the roe deer. The excretion of GIN eggs in roe deer tended to increase with livestock density in their home range. This result suggests, but does not prove, a higher risk of ingesting GIN larvae originating from livestock dung. In the context of increasing overlap between roe deer and livestock ranges, the exchange of pathogens between both hosts is plausible, although species identity of the parasites present was not determined. Assessing which GIN species are shared between wild and domestic ruminants, and how this may affect the health of both hosts, is a central question for future research in the context of interspecific pathogen circulation.

Dans ce chapitre, nous étudions le risque pour l’homme ou les animaux domestiques d’être infectés par un agent infectieux (bactérie, parasite ou virus) transmis par les tiques, appelé par la suite « risque de maladies à tiques ». Nous examinons les différentes composantes de ce risque, les facteurs qui l’influencent et comment la modélisation peut permettre de mieux le caractériser et le prédire. Nous nous intéressons principalement aux risques liés aux tiques dures, qui ont pour caractéristique de se poster sur la végétation à l’affût de leur hôte (cf. chap. 2). L’état de nos connaissances sur les tiques molles, le plus souvent endophiles, rend difficile la formalisation mathématique en vue de modéliser la transmission d’agents pathogènes par ces espèces de tiques et donc la modélisation du risque. Le risque pour l’homme ou pour un animal de développer une maladie due à un agent infectieux transmis par les tiques dépend 1) de la probabil...

Dans ce chapitre, nous décrirons la dynamique générale des populations de tiques, c’est-à-dire les variations dans le temps et dans l’espace des densités de tiques. Ces variations de densité vont conditionner l’impact de ces ectoparasites sur les hôtes, ainsi que le risque de transmission des maladies à tiques. Comprendre les facteurs qui affectent ces variations de densités est donc essentiel pour prédire la distribution des maladies à tiques et ses changements au cours du temps. Nous commençons tout d’abord en considérant ces dynamiques séparément dans le temps et dans l’espace. Les variations de densités de tiques dans le temps dépendent de trois facteurs : le taux de natalité (fécondité), la durée du cycle biologique (conditionnée en partie par le temps d’attente des hôtes successifs et pour le reste par les conditions environnementales) et le taux de mortalité (longévité). Ces trois processus conduisent à des densités de tiques qui suivent une certaine saisonnalité, qu’il faut c...