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1. Batrachochytrium salamandrivorans is necronotic: carcasses could play a role in Bsal transmission

   https://doi.org/10.3389/famrs.2024.1284608  

   Carter, E Davis; DeMarchi, Joseph A; Wilber, Mark Q; Miller, Debra L; Gray, Matthew J (February 2024, Frontiers in Amphibian and Reptile Science)

   IntroductionBatrachochytrium salamandrivorans(Bsal) poses a major threat to global amphibian biodiversity. It is essential we understandBsaltransmission to develop better-informed management strategies. Infected carcasses are an important source of transmission for several human and wildlife disease systems; however, they have not been examined as sources forBsalexposure. Here, we evaluated whether infected newt carcasses could contribute toBsaltransmission dynamics. MethodsWe cohoused infected carcasses with susceptible newts in two cohousing chamber types (partitioned or non-partitioned) at three timepoints post-mortem ([0,24[, [24,48, [48,72] hrs). The partitioned chamber prevented newt-to-newt contact hence only allowed indirect, waterborne transmission of zoospores. We measured shedding rates of infected carcasses at each post-mortem timepoint and monitored infection status and mortality of susceptible newts which were exposed during cohousing events. ResultsOur results indicate carcasses are capable of transmittingBsalto susceptible newts up to at least 72 hrs post-mortem, even without live newts directly contacting carcasses. All susceptible newts in each chamber type and post-mortem period became infected and >90% experienced disease-induced mortality.Bsalgenomic copies/uL in skin swabs taken from infected carcasses were high, averaging 7.4x10⁵, 8.6x10⁵, and 2.0x10⁶at 24, 48, and 72 hrs post-mortem, respectively. Water samples collected from cohousing chambers averaged 2743Bsalgenomic copies/uL (approximately 1357 zoospores) and did not decline over 72 hrs. DiscussionOur results indicateBsalinfection can occur rapidly between infected carcasses and susceptible aquatic salamanders via indirect and direct transmission pathways, and carcasses may prolong outbreaks by increasing the duration that infected individuals remain infectious. Carcass removal may be a strategy to reduceBsaltransmission and the impacts of outbreaks. 

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2. Environmental DNA ‐based detection of pathogens in trade and captive settings: Best practices and validation for Batrachochytrium salamandrivorans

   https://doi.org/10.1111/2041-210X.14217  

   Brunner, Jesse L.; Yarber, Christian M.; Pearhill, Robert A. I.; Goldberg, Caren S. (September 2023, Methods in Ecology and Evolution)

   Abstract Detecting pathogens in the live animal trade is critical for tracking and preventing their movement, introduction and spillover into susceptible fauna. However, the scale of the live animal trade makes individually testing animals infeasible for all but the most economically important taxa. For instance, while the fungal pathogen,Batrachochytrium salamandrivorans(Bsal), threatens amphibian, particularly caudate diversity, in Europe and the Americas, screening even a fraction of the millions of live amphibians imported into the United States, alone, is impractically laborious and expensive. A promising alternative to individual‐level sampling (e.g. swabbing the skin of salamanders) is to instead collect DNA from the animals' environment (e.g. housing container or water) which allows us to screen a whole group of animals at a time.We used a series of experiments withBsal‐spiked water and substrates and experimentally infected rough‐skinned newts (Taricha granulosa) to determine which methods yield the mostBsalenvironmental DNA (eDNA) and evaluate the capacity of these methods to detectBsal‐infected animals in conditions found in captive settings and trade.We found that filtering water housing infected animals for even an hour can consistently recover detectable levels ofBsaleDNA, that there is little evidence ofBsaleDNA being clumped in housing containers or swamped or inhibited by dirty housing containers, and that eDNA‐based methods achieves an equivalent or higher chance of detectingBsalinfections in a (virtual) population of co‐housed newts with fewer samples than individual swabs.By sampling the genetic materials accumulated from a whole group of animals, eDNA‐based methods are a powerful means of detecting pathogens, such asBsal, in shipments and captive populations. These methods bring routine pathogen surveillance into reach in many more contexts and can thus be an important tool in conservation and disease control. 

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3. Putative resistance and tolerance mechanisms have little impact on disease progression for an emerging salamander pathogen

   https://doi.org/10.1111/1365-2435.13754  

   Wilber, Mark Q.; Carter, Edward Davis; Gray, Matthew J.; Briggs, Cheryl J.; Pedersen, ed., Amy (January 2021, Functional Ecology)

   Abstract Resistance and tolerance are unique host defence strategies that can limit the impacts of a pathogen on a host. However, for most wildlife–pathogen systems, there are still fundamental uncertainties regarding (a) how changes in resistance and tolerance can affect disease outcomes and (b) the mechanisms underlying resistance and tolerance in host populations.Here, we first compared observed patterns of resistance and tolerance and their effects on disease outcomes among salamander species that are susceptible to infection and mortality from the emerging fungal pathogenBatrachochytrium salamandrivorans(Bsal). We then tested whether two putative mechanisms that contribute to host resistance and tolerance, skin sloughing and skin lesion reduction, predicted reducedBsalgrowth rate or increased host survival during infection, respectively.We performed multi‐doseBsalchallenge experiments on four species of Salamandridae found throughout North America. We combined the laboratory experiments with dynamic models and sensitivity analysis to examine how changes in load‐dependent resistance and tolerance functions affectedBsal‐induced mortality risk. Finally, we used our disease model to test whether skin sloughing and lesion reduction predicted variability in infection outcomes not described byBsalinfection intensity.We found that resistance and tolerance differed significantly among salamander species, with the most susceptible species being both less resistance and less tolerant ofBsalinfection. Our dynamic model showed that the relative influence of resistance versus tolerance on host survival was species‐dependent—increasing resistance was only more influential than increasing tolerance for the least tolerant species where changes in pathogen load had a threshold‐like effect on host survival. Testing two candidate mechanisms of resistance and tolerance, skin sloughing and lesion reduction, respectively, we found limited support that either of these processes were strong mechanisms of host defence.Our study contributes to a broader understanding of resistance and tolerance in host–pathogen systems by showing that differences in host tolerance can significantly affect whether changes in resistance or tolerance have larger effects on disease outcomes, highlighting the need for species and even population‐specific management approaches that target host defence strategies. A freePlain Language Summarycan be found within the Supporting Information of this article. 

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4. Inhibitory Bacterial Diversity and Mucosome Function Differentiate Susceptibility of Appalachian Salamanders to Chytrid Fungal Infection

   https://doi.org/10.1128/aem.01818-21  

   Jiménez, Randall R.; Carfagno, Amy; Linhoff, Luke; Gratwicke, Brian; Woodhams, Douglas C.; Chafran, Liana Soares; Bletz, Molly C.; Bishop, Barney; Muletz-Wolz, Carly R. (April 2022, Applied and Environmental Microbiology)

   Reguera, Gemma (Ed.)

   ABSTRACT Mucosal defenses are crucial in animals for protection against pathogens and predators. Host defense peptides (antimicrobial peptides, AMPs) as well as skin-associated microbes are key components of mucosal immunity, particularly in amphibians. We integrate microbiology, molecular biology, network-thinking, and proteomics to understand how host and microbially derived products on amphibian skin (referred to as the mucosome) serve as pathogen defenses. We studied defense mechanisms against chytrid pathogens, Batrachochytrium dendrobatidis (Bd) and B. salamandrivorans (Bsal), in four salamander species with different Batrachochytrium susceptibilities. Bd infection was quantified using qPCR, mucosome function (i.e., ability to kill Bd or Bsal zoospores in vitro ), skin bacterial communities using 16S rRNA gene amplicon sequencing, and the role of Bd-inhibitory bacteria in microbial networks across all species. We explored the presence of candidate-AMPs in eastern newts and red-backed salamanders. Eastern newts had the highest Bd prevalence and mucosome function, while red-back salamanders had the lowest Bd prevalence and mucosome function, and two-lined salamanders and seal salamanders were intermediates. Salamanders with highest Bd infection intensity showed greater mucosome function. Bd infection prevalence significantly decreased as putative Bd-inhibitory bacterial richness and relative abundance increased on hosts. In co-occurrence networks, some putative Bd-inhibitory bacteria were found as hub-taxa, with red-backs having the highest proportion of protective hubs and positive associations related to putative Bd-inhibitory hub bacteria. We found more AMP candidates on salamanders with lower Bd susceptibility. These findings suggest that salamanders possess distinct innate mechanisms that affect chytrid fungi. IMPORTANCE How host mucosal defenses interact, and influence disease outcome is critical in understanding host defenses against pathogens. A more detailed understanding is needed of the interactions between the host and the functioning of its mucosal defenses in pathogen defense. This study investigates the variability of chytrid susceptibility in salamanders and the innate defenses each species possesses to mediate pathogens, thus advancing the knowledge toward a deeper understanding of the microbial ecology of skin-associated bacteria and contributing to the development of bioaugmentation strategies to mediate pathogen infection and disease. This study improves the understanding of complex immune defense mechanisms in salamanders and highlights the potential role of the mucosome to reduce the probability of Bd disease development and that putative protective bacteria may reduce likelihood of Bd infecting skin. 

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5. Superinfection and the evolution of an initial asymptomatic stage

   https://doi.org/10.1098/rsos.202212  

   Saad-Roy, Chadi M.; Grenfell, Bryan T.; Levin, Simon A.; Pellis, Lorenzo; Stage, Helena B.; van den Driessche, P.; Wingreen, Ned S. (January 2021, Royal Society Open Science)

   null (Ed.)

   Pathogens have evolved a variety of life-history strategies. An important strategy consists of successful transmission by an infected host before the appearance of symptoms, that is, while the host is still partially or fully asymptomatic. During this initial stage of infection, it is possible for another pathogen to superinfect an already infected host and replace the previously infecting pathogen. Here, we study the effect of superinfection during the first stage of an infection on the evolutionary dynamics of the degree to which the host is asymptomatic (host latency) in that same stage. We find that superinfection can lead to major differences in evolutionary behaviour. Most strikingly, the duration of immunity following infection can significantly influence pathogen evolutionary dynamics, whereas without superinfection the outcomes are independent of host immunity. For example, changes in host immunity can drive evolutionary transitions from a fully symptomatic to a fully asymptomatic first infection stage. Additionally, if superinfection relative to susceptible infection is strong enough, evolution can lead to a unique strategy of latency that corresponds to a local fitness minimum, and is therefore invasible by nearby mutants. Thus, this strategy is a branching point, and can lead to coexistence of pathogens with different latencies. Furthermore, in this new framework with superinfection, we also find that there can exist two interior singular strategies. Overall, new evolutionary outcomes can cascade from superinfection. 

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