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1. Frequency‐dependent transmission of Batrachochytrium salamandrivorans in eastern newts

   https://doi.org/10.1111/tbed.14043  

   Tompros, Adrianna; Dean, Andrew D.; Fenton, Andy; Wilber, Mark Q.; Carter, Edward Davis; Gray, Matthew J. (April 2021, Transboundary and Emerging Diseases)

   null (Ed.)

   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 R_0 = 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. 

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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. Electrolyte imbalances and dehydration play a key role in Batrachochytrium salamandrivorans chytridiomycosis

   https://doi.org/10.3389/fvets.2022.1055153  

   Sheley, Wesley C.; Gray, Matthew J.; Wilber, Mark Q.; Cray, Carolyn; Carter, E. Davis; Miller, Debra L. (January 2023, Frontiers in Veterinary Science)

   Introduction One of the most important emerging infectious diseases of amphibians is caused by the fungal pathogen Batrachochytrium salamandrivorans (Bsal) . Bsal was recently discovered and is of global concern due to its potential to cause high mortality in amphibians, especially salamander species. To date, little has been reported on the pathophysiological effects of Bsal ; however, studies of a similar fungus, B. dendrobatidis (Bd) , have shown that electrolyte losses and immunosuppression likely play a key role in morbidity and mortality associated with this disease. The goal of this study was to investigate pathophysiological effects and immune responses associated with Bsal chytridiomycosis using 49 rough-skinned newts ( Taricha granulosa ) as the model species. Methods Taricha granulosa were exposed to a 1 × 10 7 per 10 mL dose of Bsal zoospores and allowed to reach various stages of disease progression before being humanely euthanized. At the time of euthanasia, blood was collected for biochemical and hematological analyses as well as protein electrophoresis. Ten standardized body sections were histologically examined, and Bsal -induced skin lesions were counted and graded on a scale of 1–5 based on severity. Results Results indicated that electrolyte imbalances and dehydration induced by damage to the epidermis likely play a major role in the pathogenesis of Bsal chytridiomycosis in this species. Additionally, Bsal -infected, clinically diseased T. granulosa exhibited a systemic inflammatory response identified through alterations in complete blood counts and protein electrophoretograms. Discussion Overall, these results provide foundational information on the pathogenesis of this disease and highlight the differences and similarities between Bsal and Bd chytridiomycosis. 

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4. Overlapping outbreaks of multiple bark beetle species are rarely more severe than single‐species outbreaks

   https://doi.org/10.1002/ecs2.4478  

   Tutland, Niko_J; Rodman, Kyle_C; Andrus, Robert_A; Hart, Sarah_J (March 2023, Ecosphere)

   Abstract Biotic disturbances that overlap in space and time may result in important shifts in forest structure and composition, with potential effects on many ecosystem services. Starting in the late 1990s, outbreaks of multiple bark beetle species caused widespread mortality of three co‐occurring conifer species in the ca. 40,000‐km²subalpine zone of the southern Rocky Mountains (SRM), USA. To better understand the implications of such outbreaks, our goal was to determine if overlapping outbreaks of multiple bark beetle species caused greater tree mortality than single‐species outbreaks in stands with multiple susceptible host tree species. We mapped stand susceptibility to outbreaks of spruce beetle (SB,Dendroctonus rufipennis), mountain pine beetle (MPB,Dendroctonus ponderosae), and western balsam bark beetle (WBBB,Dryocoetes confusus) by combining aerial survey data and forest composition variables in a random forest modeling framework. Then, we used existing maps of cumulative forest mortality from bark beetles to investigate the extent and severity of overlapping outbreaks from 1999 to 2019. We found that 46% of stands with two or more of the three studied hosts species—Engelmann spruce (Picea engelmannii), lodgepole pine (Pinus contortavar.latifolia), or subalpine fir (Abies lasiocarpa)—were susceptible to overlapping outbreaks (25% of all sampled stands). Of those stands, 31% experienced outbreaks of two or more beetle species. Stands affected by outbreaks of both MPB and SB had higher tree mortality than stands affected by one species alone, though stands susceptible to both MPB and SB were uncommon (<4% of all sampled stands). No other combinations of beetle outbreaks increased tree mortality above levels caused by single‐species outbreaks. Thus, contrary to expectations, overlapping outbreaks were rarely more severe than single‐species outbreaks in the SRM. This suggests that diverse forest communities may buffer against the most severe effects of bark beetle outbreaks, even during warm, dry conditions. 

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5. The geographic mosaic in parallel: Matching patterns of newt tetrodotoxin levels and snake resistance in multiple predator–prey pairs

   https://doi.org/10.1111/1365-2656.13212  

   Reimche, Jessica S.; Brodie, Jr., Edmund D.; Stokes, Amber N.; Ely, Erica J.; Moniz, Haley A.; Thill, Vicki L.; Hallas, Joshua M.; Pfrender, Michael E.; Brodie, III, Edmund D.; Feldman, Chris R.; et al (April 2020, Journal of Animal Ecology)

   Abstract The Geographic Mosaic Theory of Coevolution predicts that coevolutionary arms races will vary over time and space because of the diverse ecological settings and population histories of interacting species across the landscape. Thus, understanding coevolution may require investigating broad sets of populations sampled across the range of the interaction. In addition, comparing coevolutionary dynamics between similar systems may reveal the importance of specific factors that structure coevolution.Here, we examine geographic patterns of prey traits and predator traits in the relatively unstudied interaction between the Sierra garter snake (Thamnophis couchii) and sympatric prey, the rough‐skinned newt (Taricha granulosa), Sierra newt (Ta. sierrae) and California newt (Ta. torosa). This system parallels, in space and phenotypes, a classic example of coevolution between predatory common garter snakes (Th. sirtalis) and their toxic newt prey exhibiting hotspots of newt tetrodotoxin (TTX) levels and matching snake TTX resistance.We quantified prey and predator traits from hundreds of individuals across their distributions, and functional trait matching at sympatric sites.We show strong regional patterns of trait covariation across the shared ranges ofTh. couchiiand newt prey. Traits differ significantly among localities, with lower newt TTX levels and snake TTX resistance at the northern latitudes, and higher TTX levels and snake resistance at southern latitudes. Newts and snakes in northern populations show the highest degree of functional trait matching despite possessing the least extreme traits. Conversely, newts and snakes in southern populations show the greatest mismatch despite possessing exaggerated traits, with some snakes so resistant to TTX they would be unaffected by any sympatric newt. Nevertheless, individual variation was substantial, and appears to offer the opportunity for continued reciprocal selection in most populations.Overall, the three species of newts appear to be engaged in a TTX‐mediated arms race withTh. couchii. These patterns are congruent with those seen between newts andTh. sirtalis, including the same latitudinal gradient in trait covariation, and the potential ‘escape’ from the arms race by snake predators. Such concordance in broad scale patterns across two distinct systems suggests common phenomena might structure geographic mosaics in similar ways. 

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