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Crowding can result in greater disease transmission, yet crowded hosts may also remove infectious propagules from the environment, thereby lowering the encounter rate and infectious dose received by conspecifics. We combined experimental and modelling work to examine the impact of crowding of butterfly larvae on the per-capita risk of infection by a protozoan that is transmitted via the larval food plant, and the resulting infection load in adult butterflies. We reared larvae at different densities and exposed them to low and high doses of parasites. We modified an existing model to include effects of conspecific density on food (and thus parasite) consumption rate and infected adult mortality rate. Experimental work indicated that the proportion of infected hosts on plants with ten caterpillars were reduced by at least 50% compared with single caterpillars. High density reduced per-capita infection risk and parasite load and extended lifespan of all hosts, as crowded hosts removed parasites from the environment. Modelling suggested that the lower consumption rate due to crowding can lower infection prevalence by as much as 20%, although the number of new cases increases with larger population size. Our results highlight that the expected positive relationship between host density and infection prevalence breaks down when crowding results in removal of infectious propagules from the environment. 

ABSTRACT Rapid warming could drastically alter host–parasite relationships, which is especially important for fisheries crucial to human nutrition and economic livelihoods, yet we lack a synthetic understanding of how warming influences parasite‐induced mortality in these systems. We conducted a meta‐analysis using 266 effect sizes from 52 empirical papers on harvested aquatic species and determined the relationship between parasite‐induced host mortality and temperature and how this relationship was altered by host, parasite, and study design traits. Overall, higher temperatures increased parasite‐induced host mortality; however, the magnitude of this relationship varied. Hosts from the order Salmoniformes experienced a greater increase in parasite‐induced mortality with temperature than the average response to temperature across fish orders. Opportunistic parasites were associated with a greater increase in infected host mortality with temperature than the average across parasite strategies, while bacterial parasites were associated with lower infected host mortality as temperature increased than the average across parasite types. Thus, parasites will generally increase host mortality as the environment warms; however, this effect will vary among systems. 

Abstract The Oyashio Extension (OE) frontal zone in the northwest Pacific Ocean is associated with strong gradients of sea surface temperature (SST) and salinity. The OE front enhances baroclinicity and anchors the storm tracks; changes in its position and strength may impact atmospheric variability. North–south shifts in the OE front are often defined using the leading principal component for the latitude of the absolute maximum SST gradient in the northwest Pacific (145°–170°E), the so-called Oyashio Extension index (OEI). We show that the OEI is sensitive to the choice of SST dataset used in its construction, and that the significance of regressions of atmospheric fields onto the OEI also depends on the choice of SST datasets, leading to nonrobust results. This sensitivity primarily stems from the longitudinal domain used to define the OEI including a region with parallel or indistinct frontal zones in its central section (155°–164°E), leading to divergent results across datasets. We introduce a new index that considers the extent to which the SST front across this central section departs from climatology, the frontal disturbance index (FDI). For the months considered and over short time lags, the FDI produces more consistent results on air–sea interactions and associated high-frequency storm-track metrics than the conventional OEI, with a southward shift of the storm track for a more positive FDI. The FDI appears to be related to oceanic mesoscale eddy activity in the central OE region. There are significant asymmetric associations between the FDI and storm-track metrics dependent on the sign of the FDI. Significance StatementIn this study, we aim to understand how the choice of dataset may influence the interpretation of interactions between the ocean and the overlying atmosphere near sea surface temperature (SST) fronts. We find that using different SST datasets affects the results, due to slight differences in the representation of the location of the maximum SST gradient. To understand this, we develop a new index which relates to the degree of disturbance of the SST front. The new index produces regression results that are more consistent across the different datasets. We also identify some possible links between the frontal disturbance and the presence of ocean eddies. We advise that the sensitivity to dataset choice is given due consideration in regions near SST fronts. 

Abstract The Oyashio Extension (OE) frontal zone in the northwest Pacific Ocean is associated with strong gradients of sea surface temperature (SST) and salinity. The OE front enhances baroclinicity and anchors the storm tracks; changes in its position and strength may impact atmospheric variability. North–south shifts in the OE front are often defined using the leading principal component for the latitude of the absolute maximum SST gradient in the northwest Pacific (145°–170°E), the so-called Oyashio Extension index (OEI). We show that the OEI is sensitive to the choice of SST dataset used in its construction, and that the significance of regressions of atmospheric fields onto the OEI also depends on the choice of SST datasets, leading to nonrobust results. This sensitivity primarily stems from the longitudinal domain used to define the OEI including a region with parallel or indistinct frontal zones in its central section (155°–164°E), leading to divergent results across datasets. We introduce a new index that considers the extent to which the SST front across this central section departs from climatology, the frontal disturbance index (FDI). For the months considered and over short time lags, the FDI produces more consistent results on air–sea interactions and associated high-frequency storm-track metrics than the conventional OEI, with a southward shift of the storm track for a more positive FDI. The FDI appears to be related to oceanic mesoscale eddy activity in the central OE region. There are significant asymmetric associations between the FDI and storm-track metrics dependent on the sign of the FDI. Significance StatementIn this study, we aim to understand how the choice of dataset may influence the interpretation of interactions between the ocean and the overlying atmosphere near sea surface temperature (SST) fronts. We find that using different SST datasets affects the results, due to slight differences in the representation of the location of the maximum SST gradient. To understand this, we develop a new index which relates to the degree of disturbance of the SST front. The new index produces regression results that are more consistent across the different datasets. We also identify some possible links between the frontal disturbance and the presence of ocean eddies. We advise that the sensitivity to dataset choice is given due consideration in regions near SST fronts. 

Understanding why people adopt or ignore vector-borne disease (VBD) preventive measures is key to better risk assessment and control. However, little is known about why some households adopt preventive practices while others do not, which is particularly striking in this era of One Health’s attention to human, environmental, and animal health. We examined what household factors are linked to practices that may reduce exposure to disease vectors, with a focus on Chagas disease (CD) and American Cutaneous Leishmaniasis (ACL) vectors. We surveyed 204 households in 12 rural communities in Coclé Province, Republic of Panama, between March 2022 and December 2023. We used logistic regression models to explore the association between household contextual elements, such as respondents’ sex, wealth (consumer goods and agricultural), knowledge about ACL and CD, feelings about insects, and perceptions of control over health outcomes, and personal (use of repellents, and hand protection before touching a CD vector) and property-based behaviors (use of insecticides in the house, use of windows/doors screens, and cleaning peridomestic debris) that may reduce exposure to disease vectors. We found mixed associations between protective behaviors and the contextual household elements.Participants who reported feeling bothered by insects were more likely to use repellents (Odds Ratio [OR]: 2.97 95% Confidence Interval [95%CI]: 1.47-6.20), whereas those who reported being in control of their health were less likely to use protection before touching a CD vector (OR = 0.8, CI: 0.67-0.95). Window/door screens were associated with more household consumer goods wealth (OR: 1.44; CI: 1.23-1.71), while households with a history of ACL cases were more likely to have peridomestic debris accumulation (OR: 2.33; CI: 1.17-4.75). Additionally, householders bothered by insects were less likely to have peridomestic debris (OR: 0.42; CI: 0.20-0.84), as well as those who believe health outcomes happen by chance (OR: 0.89; CI: 0.82-0.98). Our findings emphasize the importance of considering a more comprehensive household background and attention to cultural values to foster context-sensitive strategies for vector-borne pathogen prevention and control. 

ABSTRACT Given the strong temperature dependence of ectothermic vector physiology, climate warming is expected to profoundly impact many vector‐borne diseases. Notably, endothermic hosts can also respond to warming by altering the timing of life history events like seasonal migration and reproduction, but relatively few predictive models of vector‐borne disease have considered both phenological and physiological responses to climate warming. Here, we extend the Ross‐MacDonald model for a vector‐borne disease to incorporate temperature effects on host and vector phenology and physiology. We use this model to understand how projections of moderate and severe warming influence the emergence of a hypothetical vector‐borne disease in a migratory bird. Modeled vector and host infection prevalence always increased under warming, and the increase was amplified when hosts failed to update their arrival phenology to keep pace with breeding site resources. While extreme warming scenarios yielded the highest infection prevalence, reduced vector survival in the hottest months caused late‐season declines in transmission, altering seasonal patterns of infection. By considering host, vector, and parasite responses to temperature together, our modeling framework could be employed to help decipher otherwise non‐intuitive wildlife infection outcomes under current and future climate conditions. 

Environmental temperature fundamentally shapes insect physiology, fitness and interactions with parasites. Differential climate warming effects on host versus parasite biology could exacerbate or inhibit parasite transmission, with far-reaching implications for pollination services, biocontrol and human health. Here, we experimentally test how controlled temperatures influence multiple components of host and parasite fitness in monarch butterflies (Danaus plexippus) and their protozoan parasitesOphryocystis elektroscirrha. Using five constant-temperature treatments spanning 18–34°C, we measured monarch development, survival, size, immune function and parasite infection status and intensity. Monarch size and survival declined sharply at the hottest temperature (34°C), as did infection probability, suggesting that extreme heat decreases both host and parasite performance. The lack of infection at 34°C was not due to greater host immunity or faster host development but could instead reflect the thermal limits of parasite invasion and within-host replication. In the context of ongoing climate change, temperature increases above current thermal maxima could reduce the fitness of both monarchs and their parasites, with lower infection rates potentially balancing negative impacts of extreme heat on future monarch abundance and distribution. 

ABSTRACT Anthropogenic food subsidies can have profound influences on wildlife behavior and health, including exposure to parasites. In many host–macroparasite systems, parasite exposure is tied to foraging behavior, but how different distributions of food subsidy shape macroparasite encounter and population‐level impacts is poorly understood. Here we modify a mathematical model of macroparasite transmission to explore how food subsidies could change parasite encounter rates and between‐host variation in parasite burdens, reflecting changes in host foraging and conspecific overlap. Hosts experience the highest average parasite abundance and associated reductions in population size when food subsidies increase and homogenize parasite encounter rates, for example when hosts center their home ranges on a point food source and overlap with many conspecifics. Conversely, hosts experience the lowest parasite abundance and impacts when subsidies result in lower and more heterogeneous parasite encounter rates, for example when multiple patchily distributed subsidies subdivide host populations and increase host commute times to food at the expense of time spent foraging. Even when resources affect other processes such as improving host immunity or fecundity, the overall effect of subsidies on infection is more strongly driven by changes in parasite encounter rates through altered foraging behavior. These patterns are robust to different effect sizes of resource subsidy on foraging and nonforaging parameters. Our findings demonstrate that resource‐driven shifts in host foraging behavior could play an integral role in determining infection dynamics for parasites with environmental (free‐living) infectious stages, with consequences for wildlife provisioning in recreational, conservation, and management contexts.