Host Population Research Articles

How do host population dynamics impact Lyme disease risk dynamics in theoretical models?

Lyme disease is the most common wildlife-to-human transmitted disease reported in North America. The study of this disease requires an understanding of the ecology of the complex communities of ticks and host species involved in harboring and transmitting this disease. Much of the ecology of this system is well understood, such as the life cycle of ticks, and how hosts are able to support tick populations and serve as disease reservoirs, but there is much to be explored about how the population dynamics of different host species and communities impact disease risk to humans. In this study, we construct a stage-structured, empirically-informed model with host dynamics to investigate how host population dynamics can affect disease risk to humans. The model describes a tick population and a simplified community of three host species, where primary nymph host populations are made to fluctuate on an annual basis, as commonly observed in host populations. We tested the model under different environmental conditions to examine the effect of environment on the interactions of host dynamics and disease risk. Results show that allowing for host dynamics in the model reduces mean nymphal infection prevalence and increases the maximum annual prevalence of nymphal infection and the density of infected nymphs. Effects of host dynamics on disease measures of nymphal infection prevalence were nonlinear and patterns in the effect of dynamics on amplitude in nymphal infection prevalence varied across environmental conditions. These results highlight the importance of further study of the effect of community dynamics on disease risk. This will involve the construction of further theoretical models and collection of robust field data to inform these models. With a more complete understanding of disease dynamics we can begin to better determine how to predict and manage disease risk using these models.

Internally Displaced Persons and Socio-territorial Integration in the Commune of Kaya in Burkina Faso: A Sustainable Solution Study

Burkina Faso, previously spared from destabilization of all kinds, suffered its first terrorist acts in April 2015 in Samorogouan. These first terrorist acts were followed by several others that led to the forced internal displacement of nearly 2,000,000 people. These IDPs have chosen 301 municipalities spread across the 13 regions of the country as destinations for refuge. The main objective of this study is to analyse the living conditions and socio-territorial integration of IDPs and durable solutions to address the crisis of forced internal displacement in the commune of Kaya. The research methodology consisted of conducting a literature review on the study topic, collecting primary data through field surveys, interviews and direct observations in the study area. The results of this study show that IDPs in Kaya commune face precarious living conditions. On the other hand, their conditions for socio-territorial integration are good in view of the cordial relations that the IDPs maintain with the host populations of the commune. In addition, durable solutions to address the forced internal displacement crisis include the resettlement of IDPs in other parts of the country, return to areas of origin and integration in IDP-hosting areas. These solutions could address forced internal displacement and address those affected. This would facilitate the reintegration or socio-territorial integration of IDPs either in their areas of origin or in their reception areas.

Abstract PO5-10-11: BREAST CANCER DIAGNOSIS AND TREATMENT AMONG DISPLACED AND HOST POPULATIONS IN NORTH-WEST SYRIA

Abstract INTRODUCTION: The protracted Syrian conflict has forcibly displaced more than half the pre-conflict population of 22 million people with 6.7 million IDPs (internally displaced people.) 80% of IDPs are women and children. Given a lack of humanitarian funding for oncology services, NGOs like the Syrian-American Medical Society (SAMS), have used private funding to establish oncology services. We sought to explore differences in breast cancer diagnosis and treatment among IDPs and host populations in SAMS oncology centre in Idlib governorate. METHODS: Charts of women seen diagnosed with breast cancer at the SAMS Oncology Centre in Idlib governorate between March 2018 and February 2021 were reviewed. Clinicopathologic data was extracted, and non-parametric statistical analyses were performed to compare citizens vs. those who were displaced. RESULTS: Data for 188 patients were available; for 108/188 (57%) details of IDP versus host status was available. 61/108 (56%) were IDPs, 47 (44%) were host. The median age at diagnosis was similar between citizens and those who were displaced. Further, the clinicopathologic features and treatment between these two groups were similar. The time between onset of symptoms and diagnosis was imilar between the host population and IDPs (median 125.5 days vs. 77 days, respectfully, p=0.255), although there was a trend towards a shorter time from diagnosis to initiation of treatment for IDPs (median 19 days vs. 14 days, p=0.051). CONCLUSION: From this cohort study, it is notable that there were no significant differences in the tumor size, node positivity or grade at diagnosis. Further, although IDPs are usually considered to have more challenging access to healthcare services compared to host populations, it was notable that there was no significant difference between the time of onset of symptoms to diagnosis between the two groups, and a trend towards a shorter time to treatment. Although this may be related to the relatively small sample size, it may also reflect other commitments of host populations (e.g., employment) that may delay care, or the work of humanitarian organizations in reaching displaced populations. Further, given that in northwest Syria, more than 65% of the 4-4.6 million people in the area are IDPs, access to care between the two groups may be similar. Further work is required to explore this. Table. Citation Format: Anees Chagpar, Aula Abbara, Alaa Alshemali, Fares Alahdab, Moahmed Hamze, Bassel Attassi, Jude Alawa, Sydney Cech, Lawrence Tang, Kaveh Khoshnood. BREAST CANCER DIAGNOSIS AND TREATMENT AMONG DISPLACED AND HOST POPULATIONS IN NORTH-WEST SYRIA [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO5-10-11.

Proportion of Life Spent in Canada and the Incidence of Multiple Sclerosis in Permanent Immigrants.

While immigrants to high-income countries have a lower risk of multiple sclerosis (MS) compared with host populations, it is unknown whether this lower risk among immigrants increases over time. Our objective was to evaluate the association between proportion of life spent in Canada and the hazard of incident MS in Canadian immigrants. We conducted a population-based retrospective cohort study in Ontario, using linked health administrative databases. We followed immigrants, who arrived in Ontario between 1985 and 2003, from January 1, 2003, to December 31, 2016, to record incident MS using a validated algorithm based on hospital admission or outpatient visits. We derived proportion of life spent in Canada based on age at arrival and time since immigration obtained from linked immigration records. We used multivariable proportional hazard models, adjusting for demographics and comorbidities, to evaluate the association between proportion of life in Canada and the incidence of MS, where proportion of life was modelled using restricted cubic spline terms. We further evaluated the role of age at migration (15 or younger vs older than 15 years), sex, and immigration class in sensitivity analyses. We included 1.5 million immigrants (49.9% female, mean age 35.9 [SD 14.2] years) who had spent a median of 20% (Q1-Q3 10%-30%) of their life in Canada. During a mean follow-up of 13.9 years (SD 1.0), 934 (0.44/100,000 person-years) were diagnosed with MS. Compared with the median, a higher risk of MS was observed at higher values of proportion of life spent (e.g., hazard ratio [70% vs 20% proportion of life] 1.38; 1.07-1.78). This association did not vary by sex (p(sex × proportion of life) = 0.70) or immigration class (p(immigration class × proportion of life) = 0.13). The results did not vary by age at migration but were statistically significant only at higher values of proportion of life for immigrants aged 15 years or younger at arrival. The risk of incident MS in immigrants varied with the proportion of life spent in Canada, suggesting an acculturation effect on MS risk. Further work is required to understand environmental and sociocultural factors driving the observed association.

Deep population structure linked to host vernalization requirement in the barley net blotch fungal pathogen.

Invasive fungal pathogens pose a substantial threat to widely cultivated crop species, owing to their capacity to adapt to new hosts and new environmental conditions. Gaining insights into the demographic history of these pathogens and unravelling the mechanisms driving coevolutionary processes are crucial for developing durably effective disease management programmes. Pyrenophora teres is a significant fungal pathogen of barley, consisting of two lineages, Ptt and Ptm, with global distributions and demographic histories reflecting barley domestication and spread. However, the factors influencing the population structure of P. teres remain poorly understood, despite the varietal and environmental heterogeneity of barley agrosystems. Here, we report on the population genomic structure of P. teres in France and globally. We used genotyping-by-sequencing to show that Ptt and Ptm can coexist in the same area in France, with Ptt predominating. Furthermore, we showed that differences in the vernalization requirement of barley varieties were associated with population differentiation within Ptt in France and at a global scale, with one population cluster found on spring barley and another population cluster found on winter barley. Our results demonstrate how cultivation conditions, possibly associated with genetic differences between host populations, can be associated with the maintenance of divergent invasive pathogen populations coexisting over large geographic areas. This study not only advances our understanding of the coevolutionary dynamics of the Pt-barley pathosystem but also prompts further research on the relative contributions of adaptation to the host versus adaptation to abiotic conditions in shaping Ptt populations.

“CLIMBING IS MY WEAPON” – A PSYCHOSOCIAL BOULDERING INTERVENTION FOR ADOLESCENTS AND ITS IMPACT ON THE PSYCHOLOGICAL WELL-BEING OF ADOLESCENTS IN LEBANON

INTRODUCTION AND AIMS Previous research highlights the positive effects of physical activity, also climbing therapy, on mental health, including benefits on depression and self-esteem. Recognising the mental health challenges faced by young people in Lebanon, particularly in the context of multiple crises and forced displacement, this interventional study explores the benefits of a psychosocial climbing programme in eastern Lebanon. METHODS In collaboration with the non-profit organisation ClimbAID, this waitlist-controlled, partially randomised, mixed-methods study examines the effects of an eight-week psychosocial bouldering group intervention on overall mental well-being, levels of psychological distress, self-efficacy and social cohesion among a group of host and refugee youth in the Bekaa Valley. Qualitative interviews with 40 participants provide insights into their experiences, lessons learned and subjective outcomes. The intervention, delivered by 2 trained facilitators and up to 2 volunteers, took place weekly for 2 hours with groups of up to 12 young people. RESULTS With 233 participants, the trial had a dropout rate of approximately 33%. The intervention group showed significant improvements in overall mental well-being and psychological distress compared with the waitlist groups. Group allocation was the only significant predictor of improved mental well-being, in addition to baseline scores. While no significant differences in self-efficacy and social cohesion were observed, qualitative analysis revealed that participants highly valued the non-judgmental approach, highlighting significant benefits in interpersonal relationships and mental well-being. CONCLUSION In the challenging humanitarian context of forced displacement, the psychosocial bouldering intervention effectively enhanced well-being and reduced psychological distress among host and refugee youth in Bekaa, Lebanon. As a group intervention, bouldering emerges as a valuable option for supporting host and refugee populations in low-income countries.

Understanding the research and practical needs required to control toxocariasis in Iran.

Human toxocariasis (HT) is a widespread zoonotic infection globally, notably prevalent in tropical areas. Enhancing our understanding of toxocariasis can lead to increased attention towards the socioeconomic impact and control of this neglected zoonosis. We conducted a comprehensive review of all available articles and official documents on toxocariasis in Iran to identify research gaps and critical needs for its control. This review highlights that despite numerous studies exploring various aspects of toxocariasis in definitive and paratenic hosts, as well as humans and environmental contamination, significant data deficiencies and gaps persist across different regions in the country. These gaps involve investigating the worm burden and reinfection rates in definitive hosts, developing more sensitive methods to detect and differentiate of Toxocara species, and understanding the behavior of definitive host animals. Additionally, identifying potential paratenic hosts for HT and exploring the organ-specific affinity and survival duration of Toxocara larvae within these hosts are essential areas for exploration. It's also imperative to comprehend the sylvatic and domestic cycles of the parasite in paratenic hosts. Furthermore, assessing egg density in the environment, exploring potential new sources such as water, and identifying regions with optimal climatic conditions for the survival and development of Toxocara eggs are crucial for the formulation of effective prevention and control strategies. Identifying at-risk groups, developing early diagnosis techniques, employing imaging methods, and identifying long-term complications in humans are also crucial. Community health organizations should prioritize health education for the public and professionals. Furthermore, accurately estimating definitive host populations, monitoring and preventing their movements in public places, implementing regular deworming practices for pets and stray hosts, and recognizing the infection's significance as a health priority are critical. This comprehensive understanding advocates for a holistic "one health" approach to control of HT.

Genetic Tracking of a Rabid Coyote (Canis latrans) Detected beyond a Rabies Enzootic Area in West Virginia, US.

Wildlife translocation and cross-species transmission can impede control and elimination of emerging zoonotic diseases. Tracking the geographic origin of both host and virus (i.e., translocation versus local infection) may help determine the most effective response when high-risk cases of emerging pathogens are identified in wildlife. In May 2022, a coyote (Canis latrans) infected with the raccoon (Procyon lotor) rabies virus variant (RRV) was collected in Lewis County, West Virginia, US, an area free from RRV. We applied host population genomics and RRV phylogenetic analyses to determine the most likely geographic origin of the rabid coyote. Coyote genomic analyses included animals from multiple eastern states bordering West Virginia, with the probable origin of the rabid coyote being the county of collection. The RRV phylogenetic analyses included cases detected from West Virginia and neighboring states, with most similar RRV sequences collected in a county 80 km to the northeast, within the oral rabies vaccination zone. The combined results suggest that the coyote was infected in an RRV management area and carried the RRV to Lewis County, a pattern consistent with coyote local movement ecology. Distant cross-species transmission and subsequent host movement presents a low risk for onward transmission in raccoon populations. This information helped with emergency response decision-making, thereby saving time and resources.

Analysis of a diffusive two-strain malaria model with the carrying capacity of the environment for mosquitoes ⋆

We propose a malaria model involving the sensitive and resistant strains, which is described by reaction-diffusion equations. The model reflects the scenario that the vector and host populations disperse with distinct diffusion rates, susceptible individuals or vectors cannot be infected by both strains simultaneously, and the vector population satisfies the logistic growth. Our main purpose is to get a threshold type result on the model, especially the interaction effect of the two strains in the presence of spatial structure. To solve this issue, the basic reproduction number (BRN) and invasion reproduction number (IRN) of each strain (i = 1 and 2 are for the sensitive and resistant strains, respectively) are defined. Furthermore, we investigate the influence of the diffusion rates of populations and vectors on BRNs and IRNs.

Cellular dynamics of host − parasitoid interactions: Insights from the encapsulation process in a partially resistant host

Cotesia typhae is an eastern African endoparasitoid braconid wasp that targets the larval stage of the lepidopteran stem borer, Sesamia nonagrioides, a maize crop pest in Europe. The French host population is partially resistant to the Makindu strain of the wasp, allowing its development in only 40% of the cases. Resistant larvae can encapsulate the parasitoid and survive the infection. This interaction provides a very interesting frame for investigating the impact of parasitism on host cellular resistance. We characterized the parasitoid ovolarval development in a permissive host and studied the encapsulation process in a resistant host by dissection and histological sectioning compared to that of inert chromatography beads. We measured the total hemocyte count in parasitized and bead-injected larvae over time to monitor the magnitude of the immune reaction. Our results show that parasitism of resistant hosts delayed encapsulation but did not affect immune abilities towards inert beads. Moreover, while bead injection increased total hemocyte count, it remained constant in resistant and permissive larvae. We conclude that while Cotesia spp virulence factors are known to impair the host immune system, our results suggest that passive evasion could also occur.

International Migration and Cardiovascular Health: Unraveling the Disease Burden Among Migrants to North America and Europe.

Europe and North America are the 2largest recipients of international migrants from low-resource regions in the world. Here, large differences in cardiovascular disease (CVD) morbidity and death exist between migrants and the host populations. This review discusses the CVD burden and its most important contributors among the largest migrant groups in Europe and North America as well as the consequences of migration to high-income countries on CVD diagnosis and therapy. The available evidence indicates that migrants in Europe and North America generally have a higher CVD risk compared with the host populations. Cardiometabolic, behavioral, and psychosocial factors are important contributors to their increased CVD risk. However, despite these common denominators, there are important ethnic differences in the propensity to develop CVD that relate to pre- and postmigration factors, such as socioeconomic status, cultural factors, lifestyle, psychosocial stress, access to health care and health care usage. Some of these pre- and postmigration environmental factors may interact with genetic (epigenetics) and microbial factors, which further influence their CVD risk. The limited number of prospective cohorts and clinical trials in migrant populations remains an important culprit for better understanding pathophysiological mechanism driving health differences and for developing ethnic-specific CVD risk prediction and care. Only by improved understanding of the complex interaction among human biology, migration-related factors, and sociocultural determinants of health influencing CVD risk will we be able to mitigate these differences and truly make inclusive personalized treatment possible.

Association between the gut microbiome and carotenoid plumage phenotype in an avian hybrid zone.

Vertebrates host complex microbiomes that impact their physiology. In many taxa, including colourful wood-warblers, gut microbiome similarity decreases with evolutionary distance. This may suggest that as host populations diverge, so do their microbiomes, because of either tight coevolutionary dynamics, or differential environmental influences, or both. Hybridization is common in wood-warblers, but the effects of evolutionary divergence on the microbiome during secondary contact are unclear. Here, we analyse gut microbiomes in two geographically disjunct hybrid zones between blue-winged warblers (Vermivora cyanoptera) and golden-winged warblers (Vermivora chrysoptera). We performed 16S faecal metabarcoding to identify species-specific bacteria and test the hypothesis that host admixture is associated with gut microbiome disruption. Species identity explained a small amount of variation between microbiomes in only one hybrid zone. Co-occurrence of species-specific bacteria was rare for admixed individuals, yet microbiome richness was similar among admixed and parental individuals. Unexpectedly, we found several bacteria that were more abundant among admixed individuals with a broader deposition of carotenoid-based plumage pigments. These bacteria are predicted to encode carotenoid biosynthesis genes, suggesting birds may take advantage of pigments produced by their gut microbiomes. Thus, host admixture may facilitate beneficial symbiotic interactions which contribute to plumage ornaments that function in sexual selection.

Validation of deep amplicon sequencing of Dicrocoelium in small ruminants from Northern regions of Pakistan.

Dicrocoelium lancet flukes cause significant production loss in ruminant livestock. Although co-infection with multiple Dicrocoelium species within a host is common, techniques for studying the composition of these complex parasite communities are lacking. The pathogenicity, epidemiology, and therapeutic susceptibility of different helminth species vary, and little is known about the interactions that take place between co-infecting species and their hosts. Here, we describe the first applicationof metabarcoding deep amplicon sequencing method to studythe Dicrocoelium species in sheep and goats. First, rDNA ITS-2 sequences of four Dicrocoelium species (Dicrocoelium dendriticum, Dicrocoelium hospes, Dicrocoelium orientalis, and Dicrocoelium chinensis) were extracted from the NCBI public database. Phylogenetic analysis revealed separate clades of Dicrocoelium species; hence, molecular differentiation between each species is possible in co-infections. Second, 202 flukes belonging to seventeen host populations (morphologically verified as belonging to the Dicrocoelium genus) were evaluated to determine the deep amplicon sequencing read threshold of an individual fluke for each of the four species. The accuracy of the method in proportional quantification of samples collected from single hosts was further assessed. Overall, 198 (98.01%) flukes were confirmed as D. dendriticum and 1.98% produced no reads. The comparison of genetic distances between rDNA ITS-2 revealed 86% to 98% identity between the Dicrocoelium species. Phylogenetic analysis demonstrated a distinct clustering of species, apart from D. orientalis and D. chinensis, which sit very close to each other in a single large clade whereas D. hospes and D. dendriticum are separated into their own clade. In conclusion each sample was identified as D. dendriticum based on the proportion of MiSeq reads and validated the presence of this group of parasites in the Gilgit Baltistan and Khyber Pakhtunkhwa provinces of Pakistan. The metabarcoding deep amplicon sequencing technology and bioinformatics pathway have several potential applications, including species interactions during co-infections, identifying the host and geographical distribution of Dicrocoelium in livestock, drug therapy response evaluation and understanding of the emergence and spread of drug resistance.

Effects of predation risk on parasite-host interactions and wildlife diseases.

Landscapes of fear can determine the dynamics of entire ecosystems. In response to perceived predation risk, prey can show physiological, behavioral, or morphological trait changes to avoid predation. This in turn can indirectly affect other species by modifying species interactions (e.g., altered feeding), with knock-on effects, such as trophic cascades, on the wider ecosystem. While such indirect effects stemming from the fear of predation have received extensive attention for herbivore-plant and predator-prey interactions, much less is known about how they alter parasite-host interactions and wildlife diseases. In this synthesis, we present a conceptual framework for how predation risk-as perceived by organisms that serve as hosts-can affect parasite-host interactions, with implications for infectious disease dynamics. By basing our approach on recent conceptual advances with respect to predation risk effects, we aim to expand this general framework to include parasite-host interactions and diseases. We further identify pathways through which parasite-host interactions can be affected, for example, through altered parasite avoidance behavior or tolerance of hosts to infections, and discuss the wider relevance of predation risk for parasite and host populations, including heuristic projections to population-level dynamics. Finally, we highlight the current unknowns, specifically the quantitative links from individual-level processes to population dynamics and community structure, and emphasize approaches to address these knowledge gaps.

TRNA anticodon cleavage by target-activated CRISPR-Cas13a effector.

Type VI CRISPR-Cas systems are among the few CRISPR varieties that target exclusively RNA. The CRISPR RNA-guided, sequence-specific binding of target RNAs, such as phage transcripts, activates the type VI effector, Cas13. Once activated, Cas13 causes collateral RNA cleavage, which induces bacterial cell dormancy, thus protecting the host population from the phage spread. We show here that the principal form of collateral RNA degradation elicited by Leptotrichia shahii Cas13a expressed in Escherichia coli cells is the cleavage of anticodons in a subset of transfer RNAs (tRNAs) with uridine-rich anticodons. This tRNA cleavage is accompanied by inhibition of protein synthesis, thus providing defense from the phages. In addition, Cas13a-mediated tRNA cleavage indirectly activates the RNases of bacterial toxin-antitoxin modules cleaving messenger RNA, which could provide a backup defense. The mechanism of Cas13a-induced antiphage defense resembles that of bacterial anticodon nucleases, which is compatible with the hypothesis that type VI effectors evolved from an abortive infection module encompassing an anticodon nuclease.

Molecular characterization of the complete genome of a novel ormycovirus infecting the ectomycorrhizal fungus Hortiboletus rubellus.

Advancements in high-throughput sequencing and the development of new bioinformatics tools for large-scale data analysis play a crucial role in uncovering virus diversity and enhancing our understanding of virus evolution. The discovery of the ormycovirus clades, a group of RNA viruses that are phylogenetically distinct from all known Riboviria members and are found in fungi, highlights the value of these tools for the discovery of novel viruses. The aim of this study was to examine viral populations in fungal hosts to gain insights into the diversity, evolution, and classification of these viruses. Here, we report the molecular characterization of a newly discovered ormycovirus, which we have named "Hortiboletus rubellus ormycovirus 1" (HrOMV1), that was found in the ectomycorrhizal fungus Hortiboletus rubellus. The bipartite genome of HrOMV1, whose nucleotide sequence was determined by HTS and RLM-RACE, consists of two RNA segments (RNA1 and RNA2) that exhibit similarity to those of previously studied ormycoviruses in their organization and the proteins they encode. The presence of upstream, in-frame AUG triplets in the 5' termini of both RNA segments suggests that HrOMV1, like certain other ormycoviruses, employs a non-canonical translation initiation strategy. Phylogenetic analysis showed that HrOMV1 is positioned within the gammaormycovirus clade. Its putative RNA-dependent RNA polymerase (RdRp) exhibits sequence similarity to those of other gammaormycovirus members, the most similarity to that of Termitomyces ormycovirus 1, with 33.05% sequence identity. This protein was found to contain conserved motifs that are crucial for RNA replication, including the distinctive GDQ catalytic triad observed in gammaormycovirus RdRps. The results of this study underscore the significance of investigating the ecological role of mycoviruses in mycorrhizal fungi. This is the first report of an ormycovirus infecting a member of the ectomycorrhizal genus Hortiboletus.

‘To be human is to bury the enemy dead’: migrant deaths, posthumous citizenship, and the ‘soldier-migrant analogy’ in the refugee crisis

ABSTRACT While states increasingly govern migration through the administration and distribution of death, they occasionally invest considerable resources in recovering, identifying, and burying dead migrants. How to explain this paradox? A forceful argument in the critical scholarship on migration and citizenship maintains that death sometimes compels states to acknowledge the humanity of the migrants and grant them some degree of posthumous citizenship. In this article, we advance a view that is both different and complementary. We suggest that burying dead migrants is a way for the state and its host population to affirm and celebrate their own humanity. This perspective rests on three arguments: to be human is to bury; the ultimate act of humanity is not burying the loved ones but the enemies; burying dead migrants can be compared to burying dead enemy soldiers. The article sheds further light on the still underexplored phenomenon of ‘caring for dead migrants’ and assesses how these practices of care challenge but also reinforce dominant exclusionary understandings of citizenship.

CAN INFECTIOUS DISEASES THAT DO NOT CONFER IMMUNITY ERADICATE HOST SPECIES?

In SIS epidemic models with host population dynamics, two classes of incidence functions are identified for their potential to be linked to host extinction. Mildly upper density-dependent incidences are never linked to host extinction. Incidences that are positively homogeneous of degree one are linked to host extinction for certain parameter constellations and to mere host decline for others; a threshold parameter is identified that separates host decline from host extinction.

Parasite-mediated predation determines infection in a complex predator-prey-parasite system.

The interplay of host-parasite and predator-prey interactions is critical in ecological dynamics because both predators and parasites can regulate communities. But what is the prevalence of infected prey and predators when a parasite is transmitted through trophic interactions considering stochastic demographic changes? Here, we modelled and analysed a complex predator-prey-parasite system, where parasites are transmitted from prey to predators. We varied parasite virulence and infection probabilities to investigate how those evolutionary factors determine species' coexistence and populations' composition. Our results show that parasite species go extinct when the infection probabilities of either host are small and that success in infecting the final host is more critical for the survival of the parasite. While our stochastic simulations are consistent with deterministic predictions, stochasticity plays an important role in the border regions between coexistence and extinction. As expected, the proportion of infected individuals increases with the infection probabilities. Interestingly, the relative abundances of infected and uninfected individuals can have opposite orders in the intermediate and final host populations. This counterintuitive observation shows that the interplay of direct and indirect parasite effects is a common driver of the prevalence of infection in a complex system.

Local adaptation of a generalist hemiparasitic plant to one of its potential host plants

Coevolution is often found in parasite–host interactions, but has not yet been described for hemiparasitic plants and their hosts. Root hemiparasites like Rhinanthus alectorolophus perform photosynthesis but also parasitize other plant species, some of which (e.g. Plantago lanceolata) may defend themselves against parasite attack by blocking the haustoria of the parasites. We grew seedlings of the hemiparasite R. alectorolophus and the potential host P. lanceolata from seven grassland sites in a factorial design. To detect differences in host defence, we also included hosts from two ‘naïve' populations from regions where the parasite does not occur. R. alectorolophus grew consistently larger and had higher fitness with sympatric than with allopatric hosts, suggesting parasite adaptation to local host populations. Moreover, R. alectorolophus remained smallest with allopatric hosts from the same region and reached intermediate sizes with allopatric hosts from other regions or naïve hosts, suggesting host adaptation to parasites at the regional scale. Parasite presence reduced the size of the host plants already after four weeks, but only that of hosts with ‘experience' of the parasite, suggesting an early host response. Follow‐up experiments confirmed that parasites attach to hosts already after four weeks and hosts respond by changing belowground allocation patterns. However, parasite roots did not preferentially grow towards sympatric hosts. Our results suggest that local adaptation to hosts can occur even in generalist parasites and does not require specialization on individual hosts. We discuss the role of potential mechanisms, including variation in chemical signalling (early) and in host defence (late effects).