Heterogeneities in the infection process drive ranavirus transmission

Caterpillar movement mediates spatially local interactions and determines the relationship between population density and contact

BACKGROUND

While interactions in nature are inherently local, ecological models often assume homogeneity across space, allowing for generalization across systems and greater mathematical tractability. Density-dependent disease models are a prominent example of models that assume homogeneous interactions, leading to the prediction that disease transmission will scale linearly with population density. In this study, we examined how the scale of larval butterfly movement interacts with the resource landscape to influence the relationship between larval contact and population density in the Baltimore checkerspot (Euphydryas phaeton). Our study was inspired by the recent discovery of a viral pathogen that is transmitted horizontally among Baltimore checkerspot larvae.

METHODS

We used multi-year larvae location data across six Baltimore checkerspot populations in the eastern U.S. to test whether larval nests are spatially clustered. We then integrated these spatial data with larval movement data in different resource contexts to investigate whether heterogeneity in spatially local interactions alters the assumed linear relationship between larval nest density and contact. We used Correlated Random Walk (CRW) models and field observations of larval movement behavior to construct Probability Distribution Functions (PDFs) of larval dispersal, and calculated the overlap in these PDFs to estimate conspecific contact within each population.

RESULTS

We found that all populations exhibited significant spatial clustering in their habitat use. Subsequent larval movement rates were influenced by encounters with host plants and larval age, and under many movement scenarios, the scale of predicted larval movement was not sufficient to allow for the "homogeneous mixing" assumed in density dependent disease models. Therefore, relationships between population density and larval contact were typically non-linear. We also found that observed use of available habitat patches led to significantly greater contact than would occur if habitat use were spatially random.

CONCLUSIONS

These findings strongly suggest that incorporating larval movement and spatial variation in larval interactions is critical to modeling disease outcomes in E. phaeton. Epidemiological models that assume a linear relationship between population density and larval contact have the potential to underestimate transmission rates, especially in small populations that are already vulnerable to extinction.

Interspecific host competition and parasite virulence evolution

Virulence, the harm to hosts caused by parasite infection, can be selected for by several ecological factors acting synergistically or antagonistically. Here, we focus on the potential for interspecific host competition to shape virulence through such a network of effects. We first summarize how host natural mortality, body mass changes, population density and community diversity affect virulence evolution. We then introduce an initial conceptual framework highlighting how these host factors, which change during host competition, may drive virulence evolution via impacts on life-history trade-offs. We argue that the multi-faceted nature of both interspecific host competition and virulence evolution still requires consideration and experimentation to disentangle contrasting mechanisms. It also necessitates a differential treatment for parasites with various transmission strategies. However, such a comprehensive approach focusing on the role of interspecific host competition is essential to understand the processes driving the evolution of virulence in a tangled bank.

First evidence of ranavirus in native and invasive amphibians in Colombia

Ranaviruses can cause mass mortality events in amphibians, thereby becoming a threat to populations that are already facing dramatic declines. Ranaviruses affect all life stages and persist in multiple amphibian hosts. The detrimental effects of ranavirus infections to amphibian populations have already been observed in the UK and in North America. In Central and South America, the virus has been reported in several countries, but the presence of the genus Ranavirus (Rv) in Colombia is unknown. To help fill this knowledge gap, we surveyed for Rv in 60 species of frogs (including one invasive species) in Colombia. We also tested for co-infection with Batrachochytrium dendrobatidis (Bd) in a subset of individuals. For Rv, we sampled 274 vouchered liver tissue samples collected between 2014 and 2019 from 41 localities covering lowlands to mountaintop páramo habitat across the country. Using quantitative polymerase chain reaction (qPCR) and end-point PCR, we detected Rv in 14 individuals from 8 localities, representing 6 species, including 5 native frogs of the genera Osornophryne, Pristimantis and Leptodactylus, and the invasive American bullfrog Rana catesbeiana. Bd was detected in 7 of 140 individuals, with 1 co-infection of Rv and Bd in an R. catesbeiana specimen collected in 2018. This constitutes the first report of ranavirus in Colombia and should set off alarms about this new emerging threat to amphibian populations in the country. Our findings provide some preliminary clues about how and when Rv may have spread and contribute to understanding how the pathogen is distributed globally.

Pathogens and predators: examining the separate and combined effects of natural enemies on assemblage structure

Natural enemy ecology strives to unify predator-prey and host-pathogen interactions under a common framework to gain insights into community- and ecosystem-level processes. To address this goal, ecologists need a greater emphasis on: (1) quantifying pathogen-mediated effects on community structure to enable comparisons with predator-mediated effects and (2) determining the interactive effects of combined natural enemies on communities. We conducted a mesocosm experiment to assess the individual and combined effects of predators (dragonfly larvae and adult water bugs) and a pathogen (ranavirus) on the abundance and composition of a larval amphibian assemblage. We found that our three natural enemies structured victim assemblages in unique ways, producing distinct assemblages. Additionally, we found that in combination treatments, predators mainly drove assemblage structure such that the assemblages most closely resembled their respective predator treatments. We also found that predators reduced infection prevalence in combination treatments, and that the magnitude of this effect was dependent on predator identity. Compared to virus-alone treatments, the presence of dragonflies and water bugs reduced infection prevalence by 79% and 63%, respectively. Additionally, the presence of dragonflies eliminated ranavirus infection in two species, which demonstrates the prominent role of predators in disease dynamics in this system. Overall, this work demonstrates the importance of considering natural enemies in community ecology, as each enemy can elicit a unique structural change. Additionally, this study provides a unique empirical test of the healthy herds hypothesis for multi-species assemblages and underscores the importance of advancing our understanding of multi-enemy interactions within communities.

Temperature and duration of exposure drive infection intensity with the amphibian pathogen Batrachochytrium dendrobatidis

The intensity of a pathogen infection plays a key role in determining how the host responds to infection. Hosts with high infections are more likely to transmit infection to others, and are may be more likely to experience progression from infection to disease symptoms, to being physiologically compromised by disease. Understanding how and why hosts exhibit variation in infection intensity therefore plays a major part in developing and implementing measures aimed at controlling infection spread, its effects, and its chance of persisting and circulating within a population of hosts. To track the relative importance of a number of variables in determining the level of infection intensity, we ran field-surveys at two breeding sites over a 12 month period using marked larvae of the common midwife toad (Alyes obstetricans) and their levels of infection with the amphibian pathogen Batrachochytrium dendrobatidis (Bd). At each sampling occasion we measured the density of larvae, the temperature of the water in the 48 h prior to sampling, the period of time the sampled individual had been in the water body, the developmental (Gosner) stage and the intensity of Bd infection of the individual. Overall our data suggest that the temperature and the duration of time spent in the water play a major role in determining the intensity of Bd infection within an individual host. However, although the duration of time spent in the water was clearly associated with infection intensity, the relationship was negative: larvae that had spent less than 3-6 months in the water had significantly higher infection intensities than those that had spent over 12 months, although this infection intensity peaked between 9 and 12 months. This could be due to animals with heavier infections developing more quickly, suffering increased mortality or, more likely, losing their mouthparts (the only part of anuran larvae that can be infected with Bd). Overall, our results identify drivers of infection intensity, and potentially transmissibility and spread, and we attribute these differences to both host and pathogen biology.

Frequency-dependent transmission of Batrachochytrium salamandrivorans in eastern newts

Transmission is the fundamental process whereby pathogens infect their hosts and spread through populations, and can be characterized using mathematical functions. The functional form of transmission for emerging pathogens can determine pathogen impacts on host populations and can inform the efficacy of disease management strategies. By directly measuring transmission between infected and susceptible adult eastern newts (Notophthalmus viridescens) in aquatic mesocosms, we identified the most plausible transmission function for the emerging amphibian fungal pathogen Batrachochytrium salamandrivorans (Bsal). Although we considered a range of possible transmission functions, we found that Bsal transmission was best explained by pure frequency dependence. We observed that >90% of susceptible newts became infected within 17 days post‐exposure to an infected newt across a range of host densities and initial infection prevalence treatments. Under these conditions, we estimated R₀ = 4.9 for Bsal in an eastern newt population. Our results suggest that Bsal has the capability of driving eastern newt populations to extinction and that managing host density may not be an effective management strategy. Intervention strategies that prevent Bsal introduction or increase host resistance or tolerance to infection may be more effective. Our results add to the growing empirical evidence that transmission of wildlife pathogens can saturate and be functionally frequency‐dependent.

Pathogen Dynamics in an Invasive Frog Compared to Native Species

Emerging infectious diseases threaten the survival of wildlife populations and species around the world. In particular, amphibians are experiencing population declines and species extinctions primarily in response to two pathogens, the fungus Batrachochytrium dendrobatidis (Bd) and the iridovirus Ranavirus (Rv). Here, we use field surveys and quantitative (q)PCR to compare infection intensity and prevalence of Bd and Rv across species and seasons on Jekyll Island, a barrier island off the coast of Georgia, USA. We collected oral and skin swabs for 1 year from four anuran species and three families, including two native hylids (Hyla cinerea and Hyla squirella), a native ranid (Rana sphenocephala), and the invasive rain frog Eleutherodactylus planirostris. Bd infection dynamics did not vary significantly over sampling months, but Rv prevalence and intensity were significantly higher in fall 2014 compared to spring 2015. Additionally, Rv prevalence and intensity were significantly higher in E. planirostris than in the other three species. Our study highlights the potential role of invasive amphibians as drivers of disease dynamics and demonstrates the importance of pathogen surveillance across multiple time periods and species to accurately capture the infectious disease landscape.

Water sports could contribute to the translocation of ranaviruses

Ranaviruses have been identified as the cause of explosive disease outbreaks in amphibians worldwide and can be transmitted between hosts both via direct and indirect contact, in which humans might contribute to the translocation of contaminated material. The aim of this study was to evaluate the possible role of water sports in the human translocation of ranavirus, Batrachochytrium dendrobatidis (Bd), and B. salamandrivorans (Bsal). A total of 234 boats were sampled during the spring Spanish Canoe Championship which took place in Pontillón de Castro, a reservoir with a history of ranavirosis, in May 2017. Boats were tested for the presence of ranavirus and Batrachochytrium spp. DNA, using quantitative real-time polymerase chain reaction techniques (qPCR). A total of 22 swabs (22/234, 9.40%) yielded qPCR-positive results for Ranavirus DNA while Bd or Bsal were not detected in any of the samples. We provide the first evidence that human-related water sports could be a source of ranavirus contamination, providing justification for public disinfecting stations in key areas where human traffic from water sports is high.

Functional variation at an expressed MHC class IIβ locus associates with Ranavirus infection intensity in larval anuran populations

Infectious diseases are causing catastrophic losses to global biodiversity. Iridoviruses in the genus Ranavirus are among the leading causes of amphibian disease-related mortality. Polymorphisms in major histocompatibility complex (MHC) genes are significantly associated with variation in amphibian pathogen susceptibility. MHC genes encode two classes of polymorphic cell-surface molecules that can recognize and bind to diverse pathogen peptides. While MHC class I genes are the classic mediators of viral-acquired immunity, larval amphibians do not express them. Consequently, MHC class II gene diversity may be an important predictor of Ranavirus susceptibility in larval amphibians, the life stage most susceptible to Ranavirus. We surveyed natural populations of larval wood frogs (Rana sylvatica), which are highly susceptible to Ranavirus, across 17 ponds and 2 years in Maryland, USA. We sequenced the peptide-binding region of an expressed MHC class IIβ locus and assessed allelic and genetic diversity. We converted alleles to functional supertypes and determined if supertypes or alleles influenced host responses to Ranavirus. Among 381 sampled individuals, 26% were infected with Ranavirus. We recovered 20 unique MHC class IIβ alleles that fell into two deeply diverged clades and seven supertypes. MHC genotypes were associated with Ranavirus infection intensity, but not prevalence. Specifically, MHC heterozygotes and supertype ST1/ST7 had significantly lower Ranavirus infection intensity compared to homozygotes and other supertypes. We conclude that MHC class IIβ functional genetic variation is an important component of Ranavirus susceptibility. Identifying immunogenetic signatures linked to variation in disease susceptibility can inform mitigation strategies for combatting global amphibian declines.

Effects of Emerging Infectious Diseases on Amphibians: A Review of Experimental Studies

Numerous factors are contributing to the loss of biodiversity. These include complex effects of multiple abiotic and biotic stressors that may drive population losses. These losses are especially illustrated by amphibians, whose populations are declining worldwide. The causes of amphibian population declines are multifaceted and context-dependent. One major factor affecting amphibian populations is emerging infectious disease. Several pathogens and their associated diseases are especially significant contributors to amphibian population declines. These include the fungi Batrachochytrium dendrobatidis and B. salamandrivorans, and ranaviruses. In this review, we assess the effects of these three pathogens on amphibian hosts as found through experimental studies. Such studies offer valuable insights to the causal factors underpinning broad patterns reported through observational studies. We summarize key findings from experimental studies in the laboratory, in mesocosms, and from the field. We also summarize experiments that explore the interactive effects of these pathogens with other contributors of amphibian population declines. Though well-designed experimental studies are critical for understanding the impacts of disease, inconsistencies in experimental methodologies limit our ability to form comparisons and conclusions. Studies of the three pathogens we focus on show that host susceptibility varies with such factors as species, host age, life history stage, population and biotic (e.g., presence of competitors, predators) and abiotic conditions (e.g., temperature, presence of contaminants), as well as the strain and dose of the pathogen, to which hosts are exposed. Our findings suggest the importance of implementing standard protocols and reporting for experimental studies of amphibian disease.

The human-snail transmission environment shapes long term schistosomiasis control outcomes: Implications for improving the accuracy of predictive modeling

INTRODUCTION

Schistosomiasis is a chronic parasitic trematode disease that affects over 240 million people worldwide. The Schistosoma lifecycle is complex, involving transmission via specific intermediate-host freshwater snails. Predictive mathematical models of Schistosoma transmission have often chosen to simplify or ignore the details of environmental human-snail interaction in their analyses. Schistosome transmission models now aim to provide better precision for policy planning of elimination of transmission. This heightens the importance of including the environmental complexity of vector-pathogen interaction in order to make more accurate projections.

METHODOLOGY AND PRINCIPAL FINDINGS

We propose a nonlinear snail force of infection (FOI) that takes into account an intermediate larval stage (miracidium) and snail biology. We focused, in particular, on the effects of snail force of infection (FOI) on the impact of mass drug administration (MDA) in human communities. The proposed (modified) model was compared to a conventional model in terms of their predictions. A longitudinal dataset generated in Kenya field studies was used for model calibration and validation. For each sample community, we calibrated modified and conventional model systems, then used them to model outcomes for a range of MDA regimens. In most cases, the modified model predicted more vigorous post-MDA rebound, with faster relapse to baseline levels of infection. The effect was pronounced in higher risk communities. When compared to observed data, only the modified system was able to successfully predict persistent rebound of Schistosoma infection.

CONCLUSION AND SIGNIFICANCE

The observed impact of varying location-specific snail inputs sheds light on the diverse MDA response patterns noted in operational research on schistosomiasis control, such as the recent SCORE project. Efficiency of human-to-snail transmission is likely to be much higher than predicted by standard models, which, in practice, will make local elimination by implementation of MDA alone highly unlikely, even over a multi-decade period.

Evaluating the Importance of Environmental Persistence for Ranavirus Transmission and Epidemiology

Viruses persist outside their hosts in a variety of forms, from naked virions to virus protected in sloughed tissues or carcasses, and for a range of times, all of which affect the likelihood and importance of transmission from the environment. This review synthesizes the literature on environmental persistence of viruses in the genus Ranavirus (family Iridoviridae), which are large double-stranded DNA viruses of ectothermic, often aquatic or semiaquatic vertebrates. Ranaviruses have been associated with mass mortality events in natural and captive settings around the world, and with population and community-wide declines in Europe. Early work suggested ranaviruses are environmentally robust and transmission from the environment should be common. More recent work has shown a large effect of temperature and microbial action on persistence times, although other aspects of the environment (e.g., water chemistry) and aquatic communities (e.g., zooplankton) may also be important. Ranaviruses may persist in the carcasses of animals that have died of infection, and so decomposing organisms and invertebrate scavengers may reduce these persistence times. The question is, do persistence times vary enough to promote or preclude substantial transmission from the environment. We built an epidemiological model with transmission from contacts, free virus in water, and carcasses, to explore the conditions in which environmental persistence could be important for ranavirus epidemiology. Based on prior work, we expected a substantial amount of transmission from the water and that longer persistence times would make this route of transmission dominant. However, neither water-borne nor transmission from carcasses played an important role in the simulated epidemics except under fairly restrictive conditions, such as when there were high rates of virus shedding or high rates of scavenging on highly infectious carcasses. While many aspects of environmental persistence of ranaviruses are being resolved by experiments, key parameters such as viral shedding rates are virtually unknown and will need to be empirically constrained if we are to determine whether environmental persistence and transmission from the environment are essential or insignificant features of Ranavirus epidemiology. We conclude by emphasizing the need to place environmental persistence research in an epidemiological framework.

Poor biosecurity could lead to disease outbreaks in animal populations

Human-mediated disease outbreaks due to poor biosecurity practices when processing animals in wild populations have been suspected. We tested whether not changing nitrile gloves between processing wood frog (Lithobates sylvaticus) tadpoles and co-housing individuals increased pathogen transmission and subsequent diseased-induced mortality caused by the emerging pathogen, ranavirus. We found that not changing gloves between processing infected and uninfected tadpoles resulted in transmission of ranavirus and increased the risk of mortality of uninfected tadpoles by 30X. Co-housing tadpoles for only 15 minutes with 10% of individuals infected resulted in ranavirus transmission and 50% mortality of uninfected tadpoles. More extreme mortality was observed when the co-housing infection prevalence was >10%. Our results illustrate that human-induced disease outbreaks due to poor biosecurity practices are possible in wild animal populations.

Resistance, tolerance and environmental transmission dynamics determine host extinction risk in a load-dependent amphibian disease

While disease-induced extinction is generally considered rare, a number of recently emerging infectious diseases with load-dependent pathology have led to extinction in wildlife populations. Transmission is a critical factor affecting disease-induced extinction, but the relative importance of transmission compared to load-dependent host resistance and tolerance is currently unknown. Using a combination of models and experiments on an amphibian species suffering extirpations from the fungal pathogen Batrachochytrium dendrobatidis (Bd), we show that while transmission from an environmental Bd reservoir increased the ability of Bd to invade an amphibian population and the extinction risk of that population, Bd-induced extinction dynamics were far more sensitive to host resistance and tolerance than to Bd transmission. We demonstrate that this is a general result for load-dependent pathogens, where non-linear resistance and tolerance functions can interact such that small changes in these functions lead to drastic changes in extinction dynamics.