Understanding the transmission of bacterial agents of sapronotic diseases using an ecosystem-based approach: A first spatially realistic metacommunity model

Pathogens such as bacteria, fungi and viruses are important components of soil and aquatic communities, where they can benefit from decaying and living organic matter, and may opportunistically infect human and animal hosts. One-third of human infectious diseases is constituted by sapronotic disease agents that are natural inhabitants of soil or aquatic ecosystems. They are capable of existing and reproducing in the environment outside of the host for extended periods of time. However, as ecological research on sapronosis is infrequent and epidemiological models are even rarer, very little information is currently available. Their importance is overlooked in medical and veterinary research, as well as the relationships between free environmental forms and those that are pathogenic. Here, using dynamical models in realistic aquatic metacommunity systems, we analyze sapronosis transmission, using the human pathogen Mycobacterium ulcerans that is responsible for Buruli ulcer. We show that the persistence of bacilli in aquatic ecosystems is driven by a seasonal upstream supply, and that the attachment and development of cells to aquatic living forms is essential for such pathogen persistence and population dynamics. Our work constitutes the first set of metacommunity models of sapronotic disease transmission, and is highly flexible for adaptation to other types of sapronosis. The importance of sapronotic agents on animal and human disease burden needs better understanding and new models of sapronosis disease ecology to guide the management and prevention of this important group of pathogens.

Beyond the swab: ecosystem sampling to understand the persistence of an amphibian pathogen

Understanding the ecosystem-level persistence of pathogens is essential for predicting and measuring host-pathogen dynamics. However, this process is often masked, in part due to a reliance on host-based pathogen detection methods. The amphibian pathogens Batrachochytrium dendrobatidis (Bd) and B. salamandrivorans (Bsal) are pathogens of global conservation concern. Despite having free-living life stages, little is known about the distribution and persistence of these pathogens outside of their amphibian hosts. We combine historic amphibian monitoring data with contemporary host- and environment-based pathogen detection data to obtain estimates of Bd occurrence independent of amphibian host distributions. We also evaluate differences in filter- and swab-based detection probability and assess inferential differences arising from using different decision criteria used to classify samples as positive or negative. Water filtration-based detection probabilities were lower than those from swabs but were > 10%, and swab-based detection probabilities varied seasonally, declining in the early fall. The decision criterion used to classify samples as positive or negative was important; using a more liberal criterion yielded higher estimates of Bd occurrence than when a conservative criterion was used. Different covariates were important when using the liberal or conservative criterion in modeling Bd detection. We found evidence of long-term Bd persistence for several years after an amphibian host species of conservation concern, the boreal toad (Anaxyrus boreas boreas), was last detected. Our work provides evidence of long-term Bd persistence in the ecosystem, and underscores the importance of environmental samples for understanding and mitigating disease-related threats to amphibian biodiversity.

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.

Environmental Variation Generates Environmental Opportunist Pathogen Outbreaks

Many socio-economically important pathogens persist and grow in the outside host environment and opportunistically invade host individuals. The environmental growth and opportunistic nature of these pathogens has received only little attention in epidemiology. Environmental reservoirs are, however, an important source of novel diseases. Thus, attempts to control these diseases require different approaches than in traditional epidemiology focusing on obligatory parasites. Conditions in the outside-host environment are prone to fluctuate over time. This variation is a potentially important driver of epidemiological dynamics and affect the evolution of novel diseases. Using a modelling approach combining the traditional SIRS models to environmental opportunist pathogens and environmental variability, we show that epidemiological dynamics of opportunist diseases are profoundly driven by the quality of environmental variability, such as the long-term predictability and magnitude of fluctuations. When comparing periodic and stochastic environmental factors, for a given variance, stochastic variation is more likely to cause outbreaks than periodic variation. This is due to the extreme values being further away from the mean. Moreover, the effects of variability depend on the underlying biology of the epidemiological system, and which part of the system is being affected. Variation in host susceptibility leads to more severe pathogen outbreaks than variation in pathogen growth rate in the environment. Positive correlation in variation on both targets can cancel the effect of variation altogether. Moreover, the severity of outbreaks is significantly reduced by increase in the duration of immunity. Uncovering these issues helps in understanding and controlling diseases caused by environmental pathogens.

Modeling the environmental growth of Pseudogymnoascus destructans and its impact on the white-nose syndrome epidemic

White-nose syndrome (WNS) has had a devastating effect on North American bat populations. The causal agent of WNS is the fungal pathogen, Pseudogymnoascus destructans (Pd), which has been shown to persist in caves after the eradication of host populations. As nonpathogenic Pseudogymnoascus spp. display saprophytic growth and are among the most commonly isolated fungi from caves, we examined whether Pd could grow in cave sediments and the contribution such growth could have to WNS disease progression. We inoculated a range of diverse cave sediments and demonstrated the growth of Pd in all sediments tested. These data indicate that environmental growth of Pd could lead to the accumulation of spores above the estimated infection threshold for WNS, allowing environment-to-bat infection. The obtained growth parameters were then used in a susceptible-infected-susceptible mathematic model to determine the possible contribution of environmental Pd growth to WNS disease progression in a colony of little brown bats (Myotis lucifugus). This model suggests that the environmental growth of Pd would increase WNS infection rates, particularly in colonies experiencing longer hibernation periods or in hibernacula with high levels of organic detritus. The model also suggests that once introduced, environmental Pd growth would allow the persistence of this pathogen within infected hibernacula for decades, greatly compromising the success of bat reintroduction strategies. Together these data suggest that Pd is not reliant on its host for survival and is capable of environmental growth and amplification that could contribute to the rapid progression and long-term persistence of WNS in the hibernacula of threatened North American bats.

Loss of competition in the outside host environment generates outbreaks of environmental opportunist pathogens

Environmentally transmitted pathogens face ecological interactions (e.g., competition, predation, parasitism) in the outside-host environment and host immune system during infection. Despite the ubiquitousness of environmental opportunist pathogens, traditional epidemiology focuses on obligatory pathogens incapable of environmental growth. Here we ask how competitive interactions in the outside-host environment affect the dynamics of an opportunist pathogen. We present a model coupling the classical SI and Lotka–Volterra competition models. In this model we compare a linear infectivity response and a sigmoidal infectivity response. An important assumption is that pathogen virulence is traded off with competitive ability in the environment. Removing this trade-off easily results in host extinction. The sigmoidal response is associated with catastrophic appearances of disease outbreaks when outside-host species richness, or overall competition pressure, decreases. This indicates that alleviating outside-host competition with antibacterial substances that also target the competitors can have unexpected outcomes by providing benefits for opportunist pathogens. These findings may help in developing alternative ways of controlling environmental opportunist pathogens.

Outside-host growth of pathogens attenuates epidemiological outbreaks

Opportunist saprotrophic pathogens differ from obligatory pathogens due to their capability in host-independent growth in environmental reservoirs. Thus, the outside-host environment potentially influences host-pathogen dynamics. Despite the socio-economical importance of these pathogens, theory on their dynamics is practically missing. We analyzed a novel epidemiological model that couples outside-host density-dependent growth to host-pathogen dynamics. Parameterization was based on columnaris disease, a major hazard in fresh water fish farms caused by saprotrophic Flavobacterium columnare. Stability analysis and numerical simulations revealed that the outside-host growth maintains high proportion of infected individuals, and under some conditions can drive host extinct. The model can show stable or cyclic dynamics, and the outside-host growth regulates the frequency and intensity of outbreaks. This result emerges because the density-dependence stabilizes dynamics. Our analysis demonstrates that coupling of outside-host growth and traditional host-pathogen dynamics has profound influence on disease prevalence and dynamics. This also has implications on the control of these diseases.

Multihost saprobes are facultative pathogens of bullfrog Lithobates catesbeianus eggs

Pathogens that infect more than one host species create complex linkages in ecological communities. We tested whether saprobes that grow on multiple host species in aquatic systems can be facultative pathogens of amphibian eggs. We isolated oomycetes from dead arthropods, vertebrates, plant leaves, and frog eggs that coexisted in a small pond. Analysis of internal transcribed spacer regions of rDNA (ITS1-5.8S-ITS2) indicated that several of the strains colonized more than one substrate, including bullfrog Lithobates catesbeianus eggs. In a controlled experiment, isolates from 7 different host species were pathogenic to L. catesbeianus eggs. These results demonstrate that dead organisms can serve as reservoirs for facultative pathogens.

Persistence of the emerging pathogen Batrachochytrium dendrobatidis outside the amphibian host greatly increases the probability of host extinction

Pathogens do not normally drive their hosts to extinction; however, Batrachochytrium dendrobatidis, which causes amphibian chytridiomycosis, has been able to do so. Theory predicts that extinction can be caused by long-lived or saprobic free-living stages. The hypothesis that such a stage occurs in B. dendrobatidis is supported by the recent discovery of an apparently encysted form of the pathogen. To investigate the effect of a free-living stage of B. dendrobatidis on host population dynamics, a mathematical model was developed to describe the introduction of chytridiomycosis into a breeding population of Bufo bufo, parametrized from laboratory infection and transmission experiments. The model predicted that the longer that B. dendrobatidis was able to persist in water, either due to an increased zoospore lifespan or saprobic reproduction, the more likely it was that it could cause local B. bufo extinction (defined as decrease below a threshold level). Establishment of endemic B. dendrobatidis infection in B. bufo, with severe host population depression, was also possible, in agreement with field observations. Although this model is able to predict clear trends, more precise predictions will only be possible when the life history of B. dendrobatidis, including free-living stages of the life cycle, is better understood.

The form of host density-dependence and the likelihood of host-pathogen cycles in forest-insect systems

Forest-insect systems frequently show cyclic dynamics which has been of considerable interest to both experimental and theoretical ecologists. One important issue has been the manner in which density-dependence acting on the host population through resource competition influences the likelihood of population cycles. Existing models make contradictory predictions. Here, we explore two models that allow different forms of density-dependence to be examined. We find that host density-dependence can influence the persistence of the host-pathogen interaction, the likelihood of population cycles and the stability of the host-pathogen interaction. In particular, over-compensatory density-dependence is likely to lead to host-pathogen cycles while under-compensatory density-dependence can promote stability. We discuss these differences with reference to the different forms of intraspecific competition and recent developments in insect population ecology.

The gut bacteria of insects: nonpathogenic interactions

The diversity of the Insecta is reflected in the large and varied microbial communities inhabiting the gut. Studies, particularly with termites and cockroaches, have focused on the nutritional contributions of gut bacteria in insects living on suboptimal diets. The indigenous gut bacteria, however, also play a role in withstanding the colonization of the gut by non-indigenous species including pathogens. Gut bacterial consortia adapt by the transfer of plasmids and transconjugation between bacterial strains, and some insect species provide ideal conditions for bacterial conjugation, which suggests that the gut is a "hot spot" for gene transfer. Genomic analysis provides new avenues for the study of the gut microbial community and will reveal the molecular foundations of the relationships between the insect and its microbiome. In this review the intestinal bacteria is discussed in the context of developing our understanding of symbiotic relationships, of multitrophic interactions between insects and plant or animal host, and in developing new strategies for controlling insect pests.

Emerging infectious diseases and amphibian population declines

We review recent research on the pathology, ecology, and biogeography of two emerging infectious wildlife diseases, chytridiomycosis and ranaviral disease, in the context of host-parasite population biology. We examine the role of these diseases in the global decline of amphibian populations and propose hypotheses for the origins and impact of these panzootics. Finally, we discuss emerging infectious diseases as a global threat to wildlife populations.