On the diverse and opposing effects of nutrition on pathogen virulence

Insect hosts are nutritional landscapes navigated by fungal pathogens

Nutrition can mediate host-pathogen interactions indirectly when specific deficiencies (e.g., iron or glutamine) constrain host immune performance. Nutrition can also directly govern these interactions as invading pathogens colonize finite landscapes of nutritionally variable host tissues that must be optimally foraged during pathogen development. We first used a conceptual framework of nutritional niches to show that insect-pathogenic Metarhizium fungi navigate host landscapes where different tissues vary widely in (protein [P] and carbohydrates [C]). We next tested whether host-specific Metarhizium species have narrower fundamental nutritional niches (FNNs) than host-generalists by measuring pathogen performance across an in vitro nutritional landscape simulating a within-host foraging environment. We then tested how developing pathogens navigate nutritional landscapes by developing a liquid-media approach to track pathogen intake of P and C over time. Host-specificity did not govern FNN dimensions, as the three tested Metarhizium species: (1) grew maximally across C treatments assuming P was present above a lower threshold, and (2) similarly initiated dispersal behaviors and sporulated when either C or P became depleted. However, specialist and generalist pathogens navigated nutritional landscapes differently. The host specialist (M. acridum) first prioritized C intake, but generalists (M. anisopliae, M. robertsii) prioritized P and C according to their availability. The numbers of known hosts may be insufficient to delimit pathogens as specialists or generalists as diverse hosts do not necessarily comprise diverse nutritional landscapes. Instead, the immune responses of hosts and nutritional niche breadth of pathogens are likely co-equal evolutionary drivers of host specificity.

Local and landscape factors differently influence health and pollination services in two important pollinator groups

Agricultural management significantly affects insects, especially pollinators, which are crucial for crop pollination and biodiversity. In agricultural landscapes, various factors spanning different spatial scales are known to affect pollinator health, which, in turn, can influence pollination services. However, the importance of these factors in driving the health and performance of different pollinator groups remains unclear. Using a long-term biodiversity research platform, the German Biodiversity Exploratories, we investigated links between local and landscape-level land-use, health and pollination services in common pollinators, the bumblebee Bombus lapidarius and the syrphid fly Episyrphus balteatus, by measuring various traits as proxies for pollinator health and pollination services. Because of their different life histories, we expected the territorial bumblebees to be more vulnerable to land-use intensification at both spatial levels, compared with the migratory syrphid flies. Both land-use and environmental factors (climate) across spatial scales affected pollinator health, mostly via changes in body size: High land-use intensity reduced bumblebee body size, whereas higher ambient air temperature decreased syrphid fly body size. Increasing proportions of intensively managed areas at the landscape level decreased viral infections in both species. Additionally, landscape-level land-use and climate changed the bumblebees cuticular chemical profile, which is essential for communication in these social insects. Increasing land-use intensity at the local level and higher proportions of intensive land-use at the landscape level both had an indirect negative effect on pollination services in bumblebees via local flower cover and body size. Pollination services in both species were linked to body size. Thus, land-use factors affect pollinator health differently: bumblebees are more vulnerable to local and landscape-level land-use intensification, while syrphid flies are more resilient potentially due to their higher mobility. As pollinator health affects pollination services, our results indicate that land-use intensification poses a high risk to crops pollinated by species with small home ranges.

Starvation increases susceptibility to bacterial infection and promotes systemic pathogen proliferation in Drosophila melanogaster females

Defense against pathogens and parasites requires substantial investment of energy and resources on part of the host. This makes the host immune function dependent on availability and accessibility of resources. A resource deprived host is therefore expected to be more susceptible to infections, although empirical results do not always align with this prediction. Limiting host access to resources can additionally impact within-host pathogen numbers, either directly by altering the amount of resources available to the pathogens for proliferation or indirectly by altering the efficiency of the host immune system. We tested for the effects of host starvation (complete deprivation of resources) on susceptibility to bacterial pathogens, and within-host pathogen proliferation, in Drosophila melanogaster females. Our results show that starvation increases post-infection mortality of the host, but in a pathogen-specific manner. This increase in mortality is always accompanied by increased within-host pathogen proliferation. We therefore propose that starvation compromises host resistance to bacterial infections in Drosophila melanogaster females thereby increasing susceptibility to infections.

Experimental adaptation to pathogenic infection ameliorates negative effects of mating on host post-infection survival in Drosophila melanogaster

Sexual activity (mating) negatively affects immune function in various insect species across both sexes. In Drosophila melanogaster females, mating increases susceptibility to pathogenic challenges and encourages within-host pathogen proliferation. This effect is pathogen and host genotype dependent. We tested if mating-induced increased susceptibility to infections is more, or less, severe in hosts experimentally adapted to pathogenic infection. We selected replicate D. melanogaster populations for increased post-infection survival following infection with a bacterial pathogen, Enterococcus faecalis. We found that females from the selected populations were better at surviving a pathogenic infection compared to the females from the control populations. This was true in the case of both the pathogen used for selection and other novel pathogens (i.e., pathogens the hosts have not encountered in recent history). Additionally, the negative effect of mating on post-infection survival was limited to only the females from control populations. Therefore, we have demonstrated that experimental selection for increased post-infection survival ameliorates negative effects of mating on host susceptibility to infections.

Order of arrival and nutrient supply alter outcomes of co-infection with two fungal pathogens

A pathogen arriving on a host typically encounters a diverse community of microbes that can shape priority effects, other within-host interactions and infection outcomes. In plants, environmental nutrients can drive trade-offs between host growth and defence and can mediate interactions between co-infecting pathogens. Nutrients may thus alter the outcome of pathogen priority effects for the host, but this possibility has received little experimental investigation. To disentangle the relationship between nutrient availability and co-infection dynamics, we factorially manipulated the nutrient availability and order of arrival of two foliar fungal pathogens (Rhizoctonia solani and Colletotrichum cereale) on the grass tall fescue (Lolium arundinaceum) and tracked disease outcomes. Nutrient addition did not influence infection rates, infection severity or plant biomass. Colletotrichum cereale facilitated R. solani, increasing its infection rate regardless of their order of inoculation. Additionally, simultaneous and C. cereale-first inoculations decreased plant growth and-in plants that did not receive nutrient addition-increased leaf nitrogen concentrations compared to uninoculated plants. These effects were partially, but not completely, explained by the duration and severity of pathogen infections. This study highlights the importance of understanding the intricate associations between the order of pathogen arrival, host nutrient availability and host defence to better predict infection outcomes.

Evaluating host diet effects on microparasites by measuring the stoichiometry of infrapopulations one cell at a time

Progress in the field of ecological stoichiometry has demonstrated that the outcome of ecological interactions can often be predicted a priori based on the nutrient ratios (e.g., carbon: nitrogen: phosphorus, C:N:P) of interacting organisms. However, the challenges of accurately measuring the nutrient content of active parasites within hosts has limited our ability to rigorously apply ecological stoichiometry to host-parasite systems. Traditional nutrient analyses require high parasite biomasses, often preventing individual-level analyses. This prevents researchers from estimating variation in the nutrient content of individual parasites within a single host infrapopulation, a critical factor that could define how the ecology of the parasite affects the host-parasite interaction. Here, we explain how energy dispersive technology, a technique currently used to measure the elemental content of free-living microbes, can be adapted for parasitic microbial infrapopulations. We demonstrate the power of accurately quantifying the biomass stoichiometry of individual microbial parasites sampled directly from individual hosts. Using this approach, we show that the stoichiometric composition of two microbial parasites capable of infecting the same host are stoichiometrically distinct and respond to host diet quality differently. We also demonstrate that characteristics of the stoichiometric trait distributions of these infrapopulations were important predictors of host fecundity, a proxy for virulence in this system, and better predictors of parasite load than the mean parasite stoichiometry or our parasite and diet treatments alone. EDS provides a rigorous tool for applying ecological stoichiometry to host-parasite systems and enables researchers to explore the nutritional physiology of host-parasite interactions at a scale that is more relevant to the ecology and evolution of the system than traditional nutrient analyses. Here we demonstrate that this level of resolution provides useful insights into the diet-dependent physiology of microbial parasites and their hosts. We anticipate that this improved level of resolution has the potential to elucidate a range of eco-evo interactions in host-parasite systems that were previously unobservable.

Decoding Early Childhood Caries: A Comprehensive Review Navigating the Impact of Evolving Dietary Trends in Preschoolers

This comprehensive review delves into the intricate relationship between evolving dietary trends in preschoolers and the prevalence of early childhood caries (ECC). The investigation meticulously analyzes ECC epidemiology, etiology, and preventive strategies. The review unveils the multifaceted nature of ECC, highlighting microbial, dietary, and environmental factors contributing to its development. Significantly, the study explores the global prevalence of ECC and its substantial implications for the overall health, nutrition, and development of preschool-aged children. The implications for public health and policy are deliberated, advocating for targeted interventions and collaborative efforts among healthcare professionals, policymakers, educators, and parents. The conclusion presents a compelling call to action, urging collective engagement to mitigate the impact of ECC and prioritize the well-being of preschoolers. This review offers valuable insights for healthcare professionals, policymakers, educators, and parents to inform evidence-based strategies for addressing ECC and promoting early childhood oral health.

Marine transmissible cancer navigates urbanized waters, threatening spillover

Inter-individual transmission of cancer cells represents a unique form of microparasites increasingly reported in marine bivalves. In this study, we sought to understand the ecology of the propagation of Mytilus trossulus Bivalve Transmissible Neoplasia 2 (MtrBTN2), a transmissible cancer affecting four Mytilus mussel species worldwide. We investigated the prevalence of MtrBTN2 in the mosaic hybrid zone of M. edulis and M. galloprovincialis along the French Atlantic coast, sampling contrasting natural and anthropogenic habitats. We observed a similar prevalence in both species, probably due to the spatial proximity of the two species in this region. Our results showed that ports had higher prevalence of MtrBTN2, with a possible hotspot observed at a shuttle landing dock. No cancer was found in natural beds except for two sites close to the hotspot, suggesting spillover. Ports may provide favourable conditions for the transmission of MtrBTN2, such as high mussel density, stressful conditions, sheltered and confined shores or buffered temperatures. Ships may also spread the disease through biofouling. Our results suggest ports may serve as epidemiological hubs, with maritime routes providing artificial gateways for MtrBTN2 propagation. This highlights the importance of preventing biofouling on docks and ship hulls to limit the spread of marine pathogens hosted by fouling species.

Naturally-associated bacteria modulate Orsay virus infection of Caenorhabditis elegans

Microbes associated with an organism can significantly modulate its susceptibility to viral infections, but our understanding of the influence of individual microbes remains limited. The nematode Caenorhabditis elegans is a model organism that in nature inhabits environments rich in bacteria. Here, we examine the impact of 71 naturally associated bacteria on C. elegans susceptibility to its only known natural virus, the Orsay virus. Our findings reveal that viral infection of C. elegans is significantly influenced by monobacterial environments. Compared to an Escherichia coli environmental reference, the majority of tested bacteria reduced C. elegans susceptibility to viral infection. This reduction is not caused by virion degradation or poor animal nutrition by the bacteria. The repression of viral infection by the bacterial strains Chryseobacterium JUb44 and Sphingobacterium BIGb0172 does not require the RIG-I homolog DRH-1, which is known to activate antiviral responses such as RNA interference and transcriptional regulation. Our research highlights the necessity of considering natural biotic environments in viral infection studies and opens the way future research on host-microbe-virus interactions.

The complex interactions between nutrition, immunity and infection in insects

Insects are the most diverse animal group on the planet. Their success is reflected by the diversity of habitats in which they live. However, these habitats have undergone great changes in recent decades; understanding how these changes affect insect health and fitness is an important challenge for insect conservation. In this Review, we focus on the research that links the nutritional environment with infection and immune status in insects. We first discuss the research from the field of nutritional immunology, and we then investigate how factors such as intracellular and extracellular symbionts, sociality and transgenerational effects may interact with the connection between nutrition and immunity. We show that the interactions between nutrition and resistance can be highly specific to insect species and/or infection type - this is almost certainly due to the diversity of insect social interactions and life cycles, and the varied environments in which insects live. Hence, these connections cannot be easily generalised across insects. We finally suggest that other environmental aspects - such as the use of agrochemicals and climatic factors - might also influence the interaction between nutrition and resistance, and highlight how research on these is essential.

Feeding the fever: Complex host-pathogen dynamics along continuous resource gradients

Food has long been known to perform dual functions of nutrition and medicine, but mounting evidence suggests that complex host-pathogen dynamics can emerge along continuous resource gradients. Empirical examples of nonmonotonic responses of infection with increasing host resources (e.g., low prevalence at low and high resource supply but high prevalence at intermediate resources) have been documented across the tree of life, but these dynamics, when observed, often are interpreted as nonintuitive, idiosyncratic features of pathogen and host biology. Here, by developing generalized versions of existing models of resource dependence for within- and among-host infection dynamics, we provide a synthetic view of nonmonotonic infection dynamics. We demonstrate that where resources jointly impact two (or more) processes (e.g., growth, defense, transmission, mortality, predation), nonmonotonic infection dynamics, including alternative states, can emerge across a continuous resource supply gradient. We review the few empirical examples that concurrently measured resource effects on multiple rates and pair this with a wide range of examples in which resource dependence of multiple rates could generate nonmonotonic infection outcomes under realistic conditions. This review and generalized framework highlight the likely generality of such resource effects in natural systems and point to opportunities ripe for future empirical and theoretical work.

Natural variation in host feeding behaviors impacts host disease and pathogen transmission potential

Animals ranging from mosquitoes to humans often vary their feeding behavior when infected or merely exposed to pathogens. These so-called "sickness behaviors" are part of the innate immune response with many consequences, including avoiding orally transmitted pathogens. Fully understanding the role of this ubiquitous behavior in host defense and pathogen evolution requires a quantitative account of its impact on host and pathogen fitness across environmentally relevant contexts. Here, we use a zooplankton host and fungal pathogen as a case study to ask if infection-mediated feeding behaviors vary across pathogen exposure levels and natural genetic variation in susceptibility to infection. Then, we connect these changes in behavior to pathogen transmission potential (spore yield) and fitness and growth costs to the host. Our results validate a protective effect of altered feeding behavior during pathogen exposure while also revealing significant variation in the magnitude of this response across host susceptibility and pathogen exposure levels. Across all four host genotypes, feeding rates were negatively correlated with susceptibility to infection and transmission potential. The most susceptible genotypes exhibited either strong anorexia, reducing food intake by 26%-42%, ("Standard") or pronounced hyperphagia, increasing food intake by 20%-54% ("A45"). Together, these results suggest that infection-mediated changes in host feeding behavior-which are traditionally interpreted as immunopathology- may in fact serve as crucial components of host defense strategies and warrant further investigation.

Increasing adult density compromises survival following bacterial infections in Drosophila melanogaster

The density-dependent prophylaxis hypothesis predicts that risk of pathogen transmission increases with increase in population density, and in response to this, organisms mount a prophylactic immune response when exposed to high density. This prophylactic response is expected to help organisms improve their chances of survival when exposed to pathogens. Alternatively, organisms living at high densities can exhibit compromised defense against pathogens due to lack of resources and density associated physiological stress; the crowding stress hypothesis. We housed adult Drosophila melanogaster flies at different densities and measured the effect this has on their post-infection survival and resistance to starvation. We find that flies housed at higher densities show greater mortality after being infected with bacterial pathogens, while also exhibiting increased resistance to starvation. Our results are more in line with the crowding stress hypothesis that postulates a compromised immune system when hosts are subjected to high densities.

Resource availability for the mosquito Aedes aegypti affects the transmission mode evolution of a microsporidian parasite

Ecological conditions may greatly affect the relative importance of vertical and horizontal transmission, in particular for parasites with a mixed mode of transmission. Resource availability is one important environmental factor, affecting host growth and fecundity, but also the parasite’s own development. The consequences for the potential of vertical and horizontal transmission and for the evolution of transmission mode are largely unknown. We let the mixed-mode microsporidian parasite Edhazardia aedis evolve on its mosquito host Aedes aegypti under high-food or low-food conditions, representing permissive and restricted conditions. These alter the timing of development of infected larvae and thereby the probabilities for the parasites to enter the vertical or horizontal transmission pathways. After 10 generations, evolved parasites were assayed under the two food levels. There was an ecological trade-off between transmission modes, mediated by nutrient effects on host development, resulting in a higher vertical transmission (VT) potential under high-food and a higher horizontal transmission (HT) potential under low-food test conditions. Evolution under high food increased the VT potential of the parasite, particularly if it was tested at low food. This involved higher probability of carrying binucleate spores for the emerging females, greater fecundity and a longer life compared to parasites that were tested in the same conditions but had evolved under low food. The changes are related to the developmental regulation and switch in the production of two spore types, affecting investment in VT or HT. In contrast, the HT potential remained relatively unaffected by the parasite’s evolutionary history, suggesting that, within our experiential design, the VT mode evolved independently of the HT mode. Our work illustrates the possible links between resource availability, within-host developmental processes and the evolution of parasite transmission investment. Future work, theoretical and experimental, should scale up from within-host to between-host levels, including eco-evolutionary and epidemiological dynamics.

Trade-offs in defence to pathogen species revealed in expanding nematode populations

Many host organisms live in polymicrobial environments and must respond to a diversity of pathogens. The degree to which host defences towards one pathogen species affect susceptibility to others is unclear. We used a panel of Caenorhabditis elegans nematode isolates to test for natural genetic variation in fitness costs of immune upregulation and pathogen damage, as well as for trade-offs in defence against two pathogen species, Staphylococcus aureus and Pseudomonas aeruginosa. We examined the fitness impacts of transient pathogen exposure (pathogen damage and immune upregulation) or exposure to heat-killed culture (immune upregulation only) by measuring host population sizes, which allowed us to simultaneously capture changes in reproductive output, developmental time and survival. We found significant decreases in population sizes for hosts exposed to live versus heat-killed S. aureus and found increased reproductive output after live P. aeruginosa exposure, compared with the corresponding heat-killed challenge. Nematode isolates with relatively higher population sizes after live P. aeruginosa infection produced fewer offspring after live S. aureus challenge. These findings reveal that wild C. elegans genotypes display a trade-off in defences against two distinct pathogen species that are evident in subsequent generations.

Resource limitation has a limited impact on the outcome of virus-fungus co-infection in an insect host

Infection by pathogens is strongly affected by the diet or condition of the prospective host. Studies that examine the impact of diet have mainly focused on single pathogens; however, co-infections within a single host are thought to be common. Different pathogen groups might respond differently to resource availability and diverse infections could increase the costs of host defense, meaning the outcome of mixed infections under varying dietary regimes is likely to be hard to predict. We used the generalist cabbage looper, Trichoplusia ni and two of its pathogens, the DNA virus T. ni nucleopolyhedrovirus (TniSNPV) and the entomopathogenic fungus, Beauveria bassiana to examine how nutrient reduction affected the outcome of mixed pathogen infection. We challenged insects with a low or high effective dose of virus, alone or combined with a single dose of fungus. We manipulated food availability after pathogen challenge by diluting artificial diet with cellulose, a non-nutritious bulking agent, and examined its impact on host and pathogen fitness. Reducing diet quantity did not alter overall or pathogen-specific mortality. In all cases, TniSNPV-induced mortality was negatively affected by fungus challenge. Similarly, B. bassiana-induced mortality was negatively affected by TniSNPV challenge, but only at the higher virus dose. Dietary dilution mainly affected B. bassiana speed of kill when mixed with a high dose of TniSNPV, with an increase in the duration of fungal infection when cellulose was low (high quantity). One pathogen dominated the production of transmission stages in the cadavers and co-infection did not affect the yield of either pathogen. There was no evidence that co-infections were more costly to the survivors of pathogen challenge. In conclusion, dietary dilution did not determine the outcome of mixed pathogen infection, but it had more subtle effects, that differed between the two pathogens and could potentially alter pathogen recycling and host-pathogen dynamics.

Sunflower pollen induces rapid excretion in bumble bees: Implications for host-pathogen interactions

Host diet can have a profound effect on host-pathogen interactions, including indirect effects on pathogens mediated through host physiology. In bumble bees (Bombus impatiens), the consumption of sunflower (Helianthus annuus) pollen dramatically reduces infection by the gut protozoan pathogen Crithidia bombi. One hypothesis for the medicinal effect of sunflower pollen is that consumption changes host gut physiological function, causing rapid excretion that flushes C. bombi from the system. We tested the effect of pollen diet and C. bombi infection on gut transit properties using a 2x2 factorial experiment in which bees were infected with C. bombi or not and fed sunflower or wildflower pollen diet. We measured several non-mutually exclusive physiological processes that underlie the insect excretory system, including gut transit time, bi-hourly excretion rate, the total number of excretion events and the total volume of excrement. Sunflower pollen significantly reduced gut transit time in uninfected bees, and increased the total number of excretion events and volume of excrement by 66 % and 68 %, respectively, in both infected and uninfected bees. Here we show that a sunflower pollen diet can affect host physiology gut function, causing more rapid and greater excretion. These results provide important insight into a mechanism that could underlie the medicinal effect of sunflower pollen for bumble bees.

Microbial evolution and transitions along the parasite-mutualist continuum

Virtually all plants and animals, including humans, are home to symbiotic microorganisms. Symbiotic interactions can be neutral, harmful or have beneficial effects on the host organism. However, growing evidence suggests that microbial symbionts can evolve rapidly, resulting in drastic transitions along the parasite-mutualist continuum. In this Review, we integrate theoretical and empirical findings to discuss the mechanisms underpinning these evolutionary shifts, as well as the ecological drivers and why some host-microorganism interactions may be stuck at the end of the continuum. In addition to having biomedical consequences, understanding the dynamic life of microorganisms reveals how symbioses can shape an organism's biology and the entire community, particularly in a changing world.

Viral susceptibility across host species is largely independent of dietary protein to carbohydrate ratios

The likelihood of a successful host shift of a parasite to a novel host species can be influenced by environmental factors that can act on both the host and parasite. Changes in nutritional resource availability have been shown to alter pathogen susceptibility and the outcome of infection in a range of systems. Here, we examined how dietary protein to carbohydrate altered susceptibility in a large cross-infection experiment. We infected 27 species of Drosophilidae with an RNA virus on three food types of differing protein to carbohydrate ratios. We then measured how viral load and mortality across species was affected by changes in diet. We found that changes in the protein:carbohydrate in the diet did not alter the outcomes of infection, with strong positive inter-species correlations in both viral load and mortality across diets, suggesting no species-by-diet interaction. Mortality and viral load were strongly positively correlated, and this association was consistent across diets. This suggests changes in diet may give consistent outcomes across host species, and may not be universally important in determining host susceptibility to pathogens.

The cross-kingdom roles of mineral nutrient transporters in plant-microbe relations

The regulation of plant physiology by plant mineral nutrient transporter (MNT) is well understood. Recently, the extensive characterization of beneficial and pathogenic plant-microbe interactions has defined the roles for MNTs in such relationships. In this review, we summarize the roles of diverse nutrient transporters in the symbiotic or pathogenic relationships between plants and microorganisms. In doing so, we highlight how MNTs of plants and microbes can act in a coordinated manner. In symbiotic relationships, MNTs play key roles in the establishment of the interaction between the host plant and rhizobium or mycorrhizae as well in the subsequent coordinated transport of nutrients. Additionally, MNTs may also regulate the colonization or degeneration of symbiotic microorganisms by reflecting the nutrient status of the plant and soil. This allows the host plant obtain nutrients from the soil in the most optimal manner. With pathogenic-interactions, MNTs influence pathogen proliferation, the efficacy of the host's biochemical defense and related signal transduction mechanisms. We classify the MNT effects in plant-pathogen interactions as either indirect by influencing the nutrient status and fitness of the pathogen, or direct by initiating host defense mechanisms. While such observations indicate the fundamental importance of MNTs in governing the interactions with a range of microorganisms, further work is needed to develop an integrative understanding of their functions.

Partitioning the environmental drivers of immunocompetence

By determining susceptibility to disease, environment-driven variation in immune responses can affect the health, productivity and fitness of vertebrates. Yet how the different components of the total environment control this immune variation is remarkably poorly understood. Here, through combining field observation, experimentation and modelling, we are able to quantitatively partition the key environmental drivers of constitutive immune allocation in a model wild vertebrate (three-spined stickleback, Gasterosteus aculeatus). We demonstrate that, in natural populations, thermal conditions and diet alone are sufficient (and necessary) to explain a dominant (seasonal) axis of variation in immune allocation. This dominant axis contributes to both infection resistance and tolerance and, in turn, to the vital rates of infectious agents and the progression of the disease they cause. Our results illuminate the environmental regulation of vertebrate immunity (given the evolutionary conservation of the molecular pathways involved) and they identify mechanisms through which immunocompetence and host-parasite dynamics might be impacted by changing environments. In particular, we predict a dominant sensitivity of immunocompetence and immunocompetence-driven host-pathogen dynamics to host diet shifts.

OneHealth implications of infectious diseases of wild and managed bees

The OneHealth approach aims to further our understanding of the drivers of human, animal and environmental health, and, ultimately, to improve them by combining approaches and knowledge from medicine, biology and fields beyond. Wild and managed bees are essential pollinators of crops and wild flowers. Their health thus directly impacts on human and environmental health. At the same time, these bee species represent highly amenable and relevant model organisms for a OneHealth approach that aims to study fundamental epidemiological questions. In this review, we focus on how infectious diseases of wild and managed bees can be used as a OneHealth model system, informing fundamental questions on ecological immunology and disease transmission, while addressing how this knowledge can be used to tackle the issues facing pollinator health.

Disease in Invasive Plant Populations

Non-native invasive plants can establish in natural areas, where they can be ecologically damaging and costly to manage. Like cultivated plants, invasive plants can experience a relatively disease-free period upon introduction and accumulate pathogens over time. Diseases of invasive plant populations are infrequently studied compared to diseases of agriculture, forestry, and even native plant populations. We evaluated similarities and differences in the processes that are likely to affect pathogen accumulation and disease in invasive plants compared to cultivated plants, which are the dominant focus of the field of plant pathology. Invasive plants experience more genetic, biotic, and abiotic variation across space and over time than cultivated plants, which is expected to stabilize the ecological and evolutionary dynamics of interactions with pathogens and possibly weaken the efficacy of infectious disease in their control. Although disease is expected to be context dependent, the widespread distribution of invasive plants makes them important pathogen reservoirs. Research on invasive plant diseases can both protect crops and help manage invasive plant populations.

From the host's point of view: Effects of variation in burying beetle brood care and brood size on the interaction with parasitic mites

The fitness and virulence of parasites is often determined by how many resources they can wrangle out of their hosts. Host defenses that help to keep resources from the parasites will then reduce virulence and parasite fitness. Here, we study whether host brood care and brood size regulation can protect host fitness and harm a parasite. We use the biparental brood-caring burying beetle Nicrophorus vespilloides and its phoretic Poecilochirus carabi mites as a model. Since paternal brood care does not seem to benefit the offspring in a clean laboratory setting, the male presence has been suggested to strengthen the defense against parasites. We manipulated male presence and found no effect on the fitness of the parasitic mites or the beetle offspring. We further manipulated beetle brood size and found larger broods to reduce parasite fitness. The specific pattern we observed suggests that beetle larvae are strong competitors and consume the carrion resource before all parasites develop. They thus starve the parasites. These results shed new light on the observation that the parasites appear to reduce host brood size early on–potentially to avert later competition their offspring might have to face.

Host density drives viral, but not trypanosome, transmission in a key pollinator

Supplemental feeding of wildlife populations can locally increase the density of individuals, which may in turn impact disease dynamics. Flower strips are a widely used intervention in intensive agricultural systems to nutritionally support pollinators such as bees. Using a controlled experimental semi-field design, we asked how density impacts transmission of a virus and a trypanosome parasite in bumblebees. We manipulated bumblebee density by using different numbers of colonies within the same area of floral resource. In high-density compartments, slow bee paralysis virus was transmitted more quickly, resulting in higher prevalence and level of infection in bumblebee hosts. By contrast, there was no impact of density on the transmission of the trypanosome Crithidia bombi, which may reflect the ease with which this parasite is transmitted. These results suggest that agri-environment schemes such as flower strips, which are known to enhance the nutrition and survival of bumblebees, may also have negative impacts on pollinators through enhanced disease transmission. Future studies should assess how changing the design of these schemes could minimize disease transmission and thus maximise their health benefits to wild pollinators.

Starving the Enemy? Feeding Behavior Shapes Host-Parasite Interactions

The loss of appetite that typically accompanies infection or mere exposure to parasites is traditionally considered a negative byproduct of infection, benefitting neither the host nor the parasite. Numerous medical and veterinary practices directly or indirectly subvert this 'illness-mediated anorexia'. However, the ecological factors that influence it, its effects on disease outcomes, and why it evolved remain poorly resolved. We explore how hosts use anorexia to defend against infection and how parasites manipulate anorexia to enhance transmission. Then, we use a coevolutionary model to illustrate how shifts in the magnitude of anorexia (e.g., via drugs) affect disease dynamics and virulence evolution. Anorexia could be exploited to improve disease management; we propose an interdisciplinary approach to minimize unintended consequences.

On the diverse and opposing effects of nutrition on pathogen virulence

Climate change and anthropogenic activity are currently driving large changes in nutritional availability across ecosystems, with consequences for infectious disease. An increase in host nutrition could lead to more resources for hosts to expend on the immune system or for pathogens to exploit. In this paper, we report a meta-analysis of studies on host-pathogen systems across the tree of life, to examine the impact of host nutritional quality and quantity on pathogen virulence. We did not find broad support across studies for a one-way effect of nutrient availability on pathogen virulence. We thus discuss a hypothesis that there is a balance between the effect of host nutrition on the immune system and on pathogen resources, with the pivot point of the balance differing for vertebrate and invertebrate hosts. Our results suggest that variation in nutrition, caused by natural or anthropogenic factors, can have diverse effects on infectious disease outcomes across species.