Migratory monarchs that encounter resident monarchs show life-history differences and higher rates of parasite infection.

Environmental change induces some wildlife populations to shift from migratory to resident behaviours. Newly formed resident populations could influence the health and behaviour of remaining migrants. We investigated migrant-resident interactions among monarch butterflies and consequences for life history and parasitism. Eastern North American monarchs migrate annually to Mexico, but some now breed year-round on exotic milkweed in the southern US and experience high infection prevalence of protozoan parasites. Using stable isotopes (δ2 H, δ13 C) and cardenolide profiles to estimate natal origins, we show that migrant and resident monarchs overlap during fall and spring migration. Migrants at sites with residents were 13 times more likely to have infections and three times more likely to be reproductive (outside normal breeding season) compared to other migrants. Exotic milkweed might either attract migrants that are already infected or reproductive, or alternatively, induce these states. Increased migrant-resident interactions could affect monarch parasitism, migratory success and long-term conservation.

Responses of migratory species and their pathogens to supplemental feeding.

Migratory animals undergo seasonal and often spectacular movements and perform crucial ecosystem services. In response to anthropogenic changes, including food subsidies, some migratory animals are now migrating shorter distances or halting migration altogether and forming resident populations. Recent studies suggest that shifts in migratory behaviour can alter the risk of infection for wildlife. Although migration is commonly assumed to enhance pathogen spread, for many species, migration has the opposite effect of lowering infection risk, if animals escape from habitats where pathogen stages have accumulated or if strenuous journeys cull infected hosts. Here, we summarize responses of migratory species to supplemental feeding and review modelling and empirical work that provides support for mechanisms through which resource-induced changes in migration can alter pathogen transmission. In particular, we focus on the well-studied example of monarch butterflies and their protozoan parasites in North America. We also identify areas for future research, including combining new technologies for tracking animal movements with pathogen surveillance and exploring potential evolutionary responses of hosts and pathogens to changing movement patterns. Given that many migratory animals harbour pathogens of conservation concern and zoonotic potential, studies that document ongoing shifts in migratory behaviour and infection risk are vitally needed.This article is part of the theme issue 'Anthropogenic resource subsidies and host-parasite dynamics in wildlife'.

Food for contagion: synthesis and future directions for studying host-parasite responses to resource shifts in anthropogenic environments.

Human-provided resource subsidies for wildlife are diverse, common and have profound consequences for wildlife-pathogen interactions, as demonstrated by papers in this themed issue spanning empirical, theoretical and management perspectives from a range of study systems. Contributions cut across scales of organization, from the within-host dynamics of immune function, to population-level impacts on parasite transmission, to landscape- and regional-scale patterns of infection. In this concluding paper, we identify common threads and key findings from author contributions, including the consequences of resource subsidies for (i) host immunity; (ii) animal aggregation and contact rates; (iii) host movement and landscape-level infection patterns; and (iv) interspecific contacts and cross-species transmission. Exciting avenues for future work include studies that integrate mechanistic modelling and empirical approaches to better explore cross-scale processes, and experimental manipulations of food resources to quantify host and pathogen responses. Work is also needed to examine evolutionary responses to provisioning, and ask how diet-altered changes to the host microbiome influence infection processes. Given the massive public health and conservation implications of anthropogenic resource shifts, we end by underscoring the need for practical recommendations to manage supplemental feeding practices, limit human-wildlife conflicts over shared food resources and reduce cross-species transmission risks, including to humans.This article is part of the theme issue 'Anthropogenic resource subsidies and host-parasite dynamics in wildlife'.

Environmental Persistence Influences Infection Dynamics for a Butterfly Pathogen.

Many pathogens, including those infecting insects, are transmitted via dormant stages shed into the environment, where they must persist until encountering a susceptible host. Understanding how abiotic conditions influence environmental persistence and how these factors influence pathogen spread are crucial for predicting patterns of infection risk. Here, we explored the consequences of environmental transmission for infection dynamics of a debilitating protozoan parasite (Ophryocystis elektroscirrha) that infects monarch butterflies (Danaus plexippus). We first conducted an experiment to observe the persistence of protozoan spores exposed to natural conditions. Experimental results showed that, contrary to our expectations, pathogen doses maintained high infectivity even after 16 days in the environment, although pathogens did yield infections with lower parasite loads after environmental exposure. Because pathogen longevity exceeded the time span of our experiment, we developed a mechanistic model to better explore environmental persistence for this host-pathogen system. Model analysis showed that, in general, longer spore persistence led to higher infection prevalence and slightly smaller monarch population sizes. The model indicated that typical parasite doses shed onto milkweed plants must remain viable for a minimum of 3 weeks for prevalence to increase during the summer-breeding season, and for 11 weeks or longer to match levels of infection commonly reported from the wild, assuming moderate values for parasite shedding rate. Our findings showed that transmission stages of this butterfly pathogen are long-lived and indicated that this is a necessary condition for the protozoan to persist in local monarch populations. This study provides a modeling framework for future work examining the dynamics of an ecologically important pathogen in an iconic insect.

Host-pathogen evolutionary signatures reveal dynamics and future invasions of vampire bat rabies.

Anticipating how epidemics will spread across landscapes requires understanding host dispersal events that are notoriously difficult to measure. Here, we contrast host and virus genetic signatures to resolve the spatiotemporal dynamics underlying geographic expansions of vampire bat rabies virus (VBRV) in Peru. Phylogenetic analysis revealed recent viral spread between populations that, according to extreme geographic structure in maternally inherited host mitochondrial DNA, appeared completely isolated. In contrast, greater population connectivity in biparentally inherited nuclear microsatellites explained the historical limits of invasions, suggesting that dispersing male bats spread VBRV between genetically isolated female populations. Host nuclear DNA further indicated unanticipated gene flow through the Andes mountains connecting the VBRV-free Pacific coast to the VBRV-endemic Amazon rainforest. By combining Bayesian phylogeography with landscape resistance models, we projected invasion routes through northern Peru that were validated by real-time livestock rabies mortality data. The first outbreaks of VBRV on the Pacific coast of South America could occur by June 2020, which would have serious implications for agriculture, wildlife conservation, and human health. Our results show that combining host and pathogen genetic data can identify sex biases in pathogen spatial spread, which may be a widespread but underappreciated phenomenon, and demonstrate that genetic forecasting can aid preparedness for impending viral invasions.

Migratory monarchs wintering in California experience low infection risk compared to monarchs breeding year-round on non-native milkweed.

Long-distance migration can lower infection risk for animal populations by removing infected individuals during strenuous journeys, spatially separating susceptible age classes, or allowing migrants to periodically escape from contaminated habitats. Many seasonal migrations are changing due to human activities including climate change and habitat alteration. Moreover, for some migratory populations, sedentary behaviors are becoming more common as migrants abandon or shorten their journeys in response to supplemental feeding or warming temperatures. Exploring the consequences of reduced movement for host-parasite interactions is needed to predict future responses of animal pathogens to anthropogenic change. Monarch butterflies (Danaus plexippus) and their specialist protozoan parasite Ophryocystis elektroscirrha (OE) provide a model system for examining how long-distance migration affects infectious disease processes in a rapidly changing world. Annual monarch migration from eastern North America to Mexico is known to reduce protozoan infection prevalence, and more recent work suggests that monarchs that forego migration to breed year-round on non-native milkweeds in the southeastern and south central Unites States face extremely high risk of infection. Here, we examined the prevalence of OE infection from 2013 to 2016 in western North America, and compared monarchs exhibiting migratory behavior (overwintering annually along the California coast) with those that exhibit year-round breeding. Data from field collections and a joint citizen science program of Monarch Health and Monarch Alert showed that infection frequency was over nine times higher for monarchs sampled in gardens with year-round milkweed as compared to migratory monarchs sampled at overwintering sites. Results here underscore the importance of animal migrations for lowering infection risk and motivate future studies of pathogen transmission in migratory species affected by environmental change.

Loss of migratory behaviour increases infection risk for a butterfly host.

Long-distance animal migrations have important consequences for infectious disease dynamics. In some cases, migration lowers pathogen transmission by removing infected individuals during strenuous journeys and allowing animals to periodically escape contaminated habitats. Human activities are now causing some migratory animals to travel shorter distances or form sedentary (non-migratory) populations. We focused on North American monarch butterflies and a specialist protozoan parasite to investigate how the loss of migratory behaviours affects pathogen spread and evolution. Each autumn, monarchs migrate from breeding grounds in the eastern US and Canada to wintering sites in central Mexico. However, some monarchs have become non-migratory and breed year-round on exotic milkweed in the southern US. We used field sampling, citizen science data and experimental inoculations to quantify infection prevalence and parasite virulence among migratory and sedentary populations. Infection prevalence was markedly higher among sedentary monarchs compared with migratory monarchs, indicating that diminished migration increases infection risk. Virulence differed among parasite strains but was similar between migratory and sedentary populations, potentially owing to high gene flow or insufficient time for evolutionary divergence. More broadly, our findings suggest that human activities that alter animal migrations can influence pathogen dynamics, with implications for wildlife conservation and future disease risks.

Does skipping a meal matter to a butterfly's appearance? Effects of larval food stress on wing morphology and color in monarch butterflies.

In animals with complex life cycles, all resources needed to form adult tissues are procured at the larval stage. For butterflies, the proper development of wings involves synthesizing tissue during metamorphosis based on the raw materials obtained by larvae. Similarly, manufacture of pigment for wing scales also requires resources acquired by larvae. We conducted an experiment to test the effects of food deprivation in the larval stage on multiple measures of adult wing morphology and coloration of monarch butterflies (Danaus plexippus), a species in which long-distance migration makes flight efficiency critical. In a captive setting, we restricted food (milkweed) from late-stage larvae for either 24 hrs or 48 hrs, then after metamorphosis we used image analysis methods to measure forewing surface area and elongation (length/width), which are both important for migration. We also measured the brightness of orange pigment and the intensity of black on the wing. There were correlations between several wing features, including an unexpected association between wing elongation and melanism, which will require further study to fully understand. The clearest effect of food restriction was a reduction in adult wing size in the high stress group (by approximately 2%). Patterns observed for other wing traits were ambiguous: monarchs in the low stress group (but not the high) had less elongated and paler orange pigmentation. There was no effect on wing melanism. Although some patterns obtained in this study were unclear, our results concerning wing size have direct bearing on the monarch migration. We show that if milkweed is limited for monarch larvae, their wings become stunted, which could ultimately result in lower migration success.

sodC-based real-time PCR for detection of Neisseria meningitidis.

Real-time PCR (rt-PCR) is a widely used molecular method for detection of Neisseria meningitidis (Nm). Several rt-PCR assays for Nm target the capsule transport gene, ctrA. However, over 16% of meningococcal carriage isolates lack ctrA, rendering this target gene ineffective at identification of this sub-population of meningococcal isolates. The Cu-Zn superoxide dismutase gene, sodC, is found in Nm but not in other Neisseria species. To better identify Nm, regardless of capsule genotype or expression status, a sodC-based TaqMan rt-PCR assay was developed and validated. Standard curves revealed an average lower limit of detection of 73 genomes per reaction at cycle threshold (C(t)) value of 35, with 100% average reaction efficiency and an average R(2) of 0.9925. 99.7% (624/626) of Nm isolates tested were sodC-positive, with a range of average C(t) values from 13.0 to 29.5. The mean sodC C(t) value of these Nm isolates was 17.6±2.2 (±SD). Of the 626 Nm tested, 178 were nongroupable (NG) ctrA-negative Nm isolates, and 98.9% (176/178) of these were detected by sodC rt-PCR. The assay was 100% specific, with all 244 non-Nm isolates testing negative. Of 157 clinical specimens tested, sodC detected 25/157 Nm or 4 additional specimens compared to ctrA and 24 more than culture. Among 582 carriage specimens, sodC detected Nm in 1 more than ctrA and in 4 more than culture. This sodC rt-PCR assay is a highly sensitive and specific method for detection of Nm, especially in carriage studies where many meningococcal isolates lack capsule genes.