Browse Subject Areas

Click through the PLOS taxonomy to find articles in your field.

For more information about PLOS Subject Areas, click here.

  • Loading metrics

Pathogen invasion and non-epizootic dynamics in Pacific newts in California over the last century

  • Shruti Chaukulkar ,

    Contributed equally to this work with: Shruti Chaukulkar, Hasan Sulaeman, Andrew G. Zink, Vance T. Vredenburg

    Roles Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Validation, Visualization, Writing – original draft, Writing – review & editing

    Affiliation Department of Biology, San Francisco State University, San Francisco, California, United States of America

  • Hasan Sulaeman ,

    Contributed equally to this work with: Shruti Chaukulkar, Hasan Sulaeman, Andrew G. Zink, Vance T. Vredenburg

    Roles Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Validation, Visualization, Writing – original draft, Writing – review & editing

    Affiliation Department of Biology, San Francisco State University, San Francisco, California, United States of America

  • Andrew G. Zink ,

    Contributed equally to this work with: Shruti Chaukulkar, Hasan Sulaeman, Andrew G. Zink, Vance T. Vredenburg

    Roles Conceptualization, Formal analysis, Funding acquisition, Investigation, Methodology, Project administration, Resources, Supervision, Validation, Visualization, Writing – original draft, Writing – review & editing

    Affiliation Department of Biology, San Francisco State University, San Francisco, California, United States of America

  • Vance T. Vredenburg

    Contributed equally to this work with: Shruti Chaukulkar, Hasan Sulaeman, Andrew G. Zink, Vance T. Vredenburg

    Roles Conceptualization, Funding acquisition, Investigation, Methodology, Project administration, Resources, Supervision, Writing – original draft, Writing – review & editing

    Affiliation Department of Biology, San Francisco State University, San Francisco, California, United States of America

Pathogen invasion and non-epizootic dynamics in Pacific newts in California over the last century

  • Shruti Chaukulkar, 
  • Hasan Sulaeman, 
  • Andrew G. Zink, 
  • Vance T. Vredenburg


Emerging infectious disease is a growing threat to global biodiversity. The infectious disease chytridiomycosis, caused by the fungal pathogen Batrachochytrium dendrobatidis (Bd) has led to the decline and extinction of hundreds of amphibian species. Severe Bd-caused epizootics have been documented in North, Central and South America—with many of the research focused on anurans. California, where Bd-related epizootics and amphibian declines have been reported, has some of the highest diversity of salamanders. After more than a decade since the first known epizootic in California, little is known about Bd disease dynamics in salamanders. Pacific newts (Genus: Taricha) are ideal study species because of their abundance, wide geographic range, occurrence in both aquatic and terrestrial habitats, and how little is known about Bd infection dynamics for this group. We conducted a retrospective study to determine the relationship between Pacific newts and the fungal pathogen. We tested 1895 specimens collected between 1889–2009 and found no evidence of Bd-infected Pacific newts until the late 1940’s. Although we estimate that Bd emerged in this genus and rapidly spread geographically throughout California, we did not find evidence for epizootic dynamics. Bd infection prevalence and intensity, two measures commonly used to estimate dynamics, remained consistently low over time; suggesting Pacific newts may not be highly susceptible. Also, we found the timing of first Bd emergence in Pacific newts predate Bd emergence in other California salamander species. In addition, we found several environmental and anthropogenic factors correlated with Bd prevalence which may help explain Bd disease dynamics in the genus Taricha. Pacific newts may be a reservoir species that signal pathogen invasion into California salamanders, though further studies are needed.


Amphibian population declines have occurred globally beginning in the late 1970’s [1,2]. While there are many causes for the declines, emerging infectious disease is one of the main factors [3]. This phenomenon is considered the worst case of disease caused die-offs recorded and is attributable to a single pathogen [4]. The fungal pathogen, Batrachochytrium dendrobatidis, was first discovered in in dead and dying frogs in Central America [5, 6], and later in North America, Europe and Australia. [7, 8, 4, 9], and causes the disease chytridiomycosis. Bd infects the skin of the amphibians and causes hyperkeratosis. Hyperkeratosis is the thickening of the amphibian skin that disrupts the osmotic balance as the infection moves across the skin, leading to death by cardiac arrest [6, 1012]. In the palmate newt (Lissotriton helveticus), Bd was shown to decrease ventral water absorption rates after repeated exposure to Bd [13].

Emergence and dynamics of Bd, since its discovery, are still not fully understood. For example, it has been debated whether Bd is an invasive pathogen [14, 15, 8, 9], or whether it was already present globally and only recently became pathogenic [16]. Other factors that influence disease dynamics, such as transmission, host vulnerability, and pathogen strain variation, are also not fully understood. Several studies have shown the rapid invasion of Bd through Mexico and Central America [17,8] resulting in epizootics that caused many extinctions [18]. Unfortunately, many declines occurred before Bd was described [17]. Retrospective studies analyzing the presence of the pathogen on museum-collected specimens can help create a timeline for disease emergence and transmission. Previous historical studies in California found Bd infected amphibians long before the pathogen was first described. Huss et al., 2014 found Bd-positive Rana catesbeiana in 1928, but found no evidence that epizootics immediately followed. Other studies found Bd first appearing in specimens collected in California in the late 1950s, 1960s, and 1970s [19, 20, 21]. The emergence patterns of Bd in both frogs and salamander specimens led to the conclusion that Bd emerged as a novel pathogen in California [19, 20, 21] and help explain epizootics documented in other species [9]. These retrospective data improve our understanding of Bd dynamics and with further study may help identify the origin of Bd.

We propose that Pacific newts may be reservoir species for Bd, similar to Pacific chorus frogs (Hyliola regilla) [22]. A reservoir species maintains the pathogen in hosts and can spread the pathogen to other host species [22]. Pacific newts are abundant and widespread in California, migrate and have a large home range. There are currently four known species of pacific newts; Taricha torosa (California newt), Taricha granulosa (Rough skin newt), Taricha rivularis (Red-bellied newt) and Taricha sierrae (Sierra newt) [23]. Pacific newts also utilize both aquatic and terrestrial habitats and have a short larval stage of 4–6 months in water followed by metamorphosis and a move to terrestrial habitats until they reach sexual maturity. [24,25]. Pacific newts are completely aquatic during breeding season and move to a terrestrial habitat until the next breeding cycle [26, 27, 28]. Additionally, there are over 9,000 specimens collected from 1880–2015 and are currently housed in permanent museum collections. This allows for random sampling of museum specimens to provide a better insight on Bd emergence. We conducted a randomly sampled retrospective survey using the museum specimens to describe the spread of Bd across California on the genus Taricha. We also evaluated historical Bd prevalence and intensity in association with several biotic and abiotic factors that would affect the ecology of Bd. Lastly, we used a Bayesian analysis to estimate the time of invasion [21, 29]. From this study, we discovered various insights into the temporal and spatial dynamics of Bd on Pacific newts in California, along with various ecological drivers of Bd infections.

Materials and methods

Museum sampling

In order to create the timeline for Bd prevalence, we created a sampling regime from the specimen database for Taricha (n = 9,774) from The samples were selected from the permanent collections housed at the Museum of Vertebrate Zoology, University of California, Berkeley and the California Academy of Sciences, San Francisco. Samples were randomly selected in a blocked design, where 20 replicate samples per species, per decade (1889–2009) were selected for skin swab collection. The selection process led to total of 1895 museum specimens. These specimens were then sampled for Bd presence. The swabbing and qPCR results from our experiment can be accessed on the amphibian disease portal [30].

Swab processing

To reduce cross-contamination between specimens kept in the same jar at the natural history museums, every specimen was rinsed with 70% EtOH prior to swabbing and then swabbed 30 times using a sterile medical swab (MW113, Medical Wire and Equipment, Corsham, UK) across its dorsal and ventral surfaces along with the toes and the mouthparts; changing gloves between each specimen. Swabs collected were kept dry in 1.5 mL microcentrifuge tubes at 4°C until processed. Before extraction process, swabs were dried in a Spin Vac (Savant Instruments, Farmingdale, NY, USA) to remove EtOH. DNA Extraction was performed using 40μL of Prepman Ultra (Applied Biosystems, Carlsbad, CA, USA) [17, 31, 32] and diluted 1:10 with 0.25 × TE buffer. We analyzed each sample in singlicate, using 5 μL of the diluted DNA extract. When run in singlicate, on specimens identified as Bd-infected from histological examination, qPCR correctly detected Bd 60% of the time [17]. Universal DNA standards from the global pandemic lineage strain (provided by A.S. Hyatt) were used to calibrate the qPCR (0.1, 1.0, 10, and 100 zoospore equivalents per reaction). Negative controls were also included during extraction and qPCR to detect contamination. Samples were run on an Applied Biosystems 7300 Real-Time PCR thermocycler. We calculated the number of zoospores in terms of Zswab (i.e., estimated Bd zoospore genomic equivalents on each swab) by multiplying qPCR results by 80 to account for sample dilution (40 μL Prepman × 10 dilution/ 5 μL for reaction = 80). A Bd-positive sample was described as having a Zswab score greater than zero.


To characterize the temporal and spatial distribution of Bd on Pacific newts in California, we calculated 95% confidence intervals for Bd prevalence for the genus Taricha we sampled from each decade based on a binomial probability distribution. To estimate the arrival date of Bd in the California, we used a Bayesian modeling approach. In this model, the process of Bd arrival is described using a threshold model where Bd switches from absent to present with some mean prevalence. The number of infected newts in each year was treated as a draw from a binomial distribution with a sample size equal to the number of newts sampled in that year [29, 21]. We also calculated the probability of detecting zero positives for each decade prior to the decade of first detection to check if Bd was present prior to first detected positive in California. These calculations are based on the binomial distribution and utilizes the total number of samples evaluated in a particular decade as the number of trials. A previous study that used a qPCR technique to depict Bd endemism in North American museum specimens found that 11% of specimens tested positive for Bd [33] Therefore, we used 0.11 as the “true” probability of finding a Bd-positive individual for our binomial confidence interval.

All statistical analyses were performed using the statistical software R (version 3.4.2). We did a linear regression for Bd infection status as a response variable, assuming a binomial distribution as animals can only either be infected or not infected. We used the following explanatory variable groups for our response variable: human footprint, precipitation, temperature, distance to water body, elevation, amphibian species richness, and soil-water balance. Elevation and topographic information were extracted from USGS (,, soil-water balance data was used from the consortium for spatial information (, temperature and precipitation data was used from WorldClim ( We include human footprint data from a recent study that creates a human footprint index based on anthropogenic factors (e.g. human population size, light pollution, number of roads and railways, etc.) [34]. To reduce the number of factors, we first did a Pearson correlation test to eliminate highly correlated factors (r > 0.9 or < -0.9). We then performed a stepwise regression to choose the best-fit model based on the AIC [35, 36].


Museum sampling

Of the 1895 archived Pacific newt specimens in the retrospective study, 58 tested positive for Bd, with an overall prevalence of 3.06% (Figs 1 and 2). The earliest positive was a 1948 specimen in San Diego County and since then, Bd has spread throughout California over time (Fig 1). The Bayesian analysis gave 95% credible interval for the date of Bd arrival in Californian Pacific newts between 1945 to 1948 with a post-arrival Bd prevalence between 4% to 5% (mean: ~4.5%). Death follows in adult frogs when the individual reaches an infection intensity of 10,000 zoospores. Though there are currently no data showing a similar threshold for salamanders, we listed 10,000 zoospores as a measure of comparison for a potentially deadly infection load on an individual [9, 37, 17]. None of the infected individuals from the museum study had infection intensities greater than the 10,000 zoospore genomic equivalents value associated with mortality in anurans. Based on the binomial distribution calculation, the probability of finding no Bd-positive samples in each decade was less than 0.001 (Table 1).

Fig 1.

(a) Spatial and temporal distribution of Pacific newts in California that tested positive for Bd from 1889–2009. (b) Spatial and Temporal distribution of Pacific newts in California that tested negative for Bd from 1889–2009.

Fig 2. Emergence of Bd in Pacific newts from 1889–2009.

Infection prevalence (solid line) and infection intensity (broken line) patterns over time. Gray bars represent number of samples analyzed per decade. Infection intensity is represented with a broken line, while the dashed line at Log10 Zscore = 4 represents the 10,000 zoospore genomic equivalents shown to be associated with mortality in anurans as a basis for comparison.

Table 1. Batrachochytrium dendrobatidis (Bd) prevalence in all four species of Pacific newts.

Linear regression

In the model with the best AIC (AIC = -6680.91), we found that infection status has a positive relationship with the following factors: elevation and annual precipitation (p<0.01, p = 0.03; respectively). We found that infection status has a negative relationship with the following factors: precipitation of the driest quarter and mean actual evapotranspiration (p = 0.01, p = 0.01; respectively). Infection status was not shown to have a significant relationship with the following factors: snout to vent length, maximum temperature of the warmest month (Table 2).

Table 2. Linear regression output.

Environmental factors and their relationship to Bd presence.


Bd has been associated with various amphibian declines in multiple regions throughout the world [8, 9]. Museum samples offer evidence of the timing as well as location of the pathogen arrival leading to amphibian decline [17]. However, museum specimens were collected for reasons unrelated to our study, and thus the specimen collections contain sampling biases that are not related to our study. The emergence of Bd in the 1960's in our samples and its following rise in prevalence concurs with other known die offs in California beginning in the 1970's [38, 39, 9, 20, 21]. Our earliest positive was from 1948, though it is possible that there are earlier positives we didn’t detect from a different Bd strain [40]. In addition, it is also possible that Bd was already present but at such low prevalence that no die offs were recorded and most species were not found to be infected. However, with our robust sample size up to our first Bd infected animal (n = 641 before 1950), we have sufficient power to detect even a very low prevalence (Table 1). Our Bayesian analysis predict the arrival of the Bd strain we tested for between 1944–1948 in California Pacific newts with a mean prevalence of around 4%. This supports our hypothesis that Pacific newts may be a reservoir species for chytridiomycosis (at least in terrestrial salamanders) [22]. As a Bd reservoir, newts would help maintain and spread Bd to other hosts [41]. Specimens collected prior to 1940 (432 samples) tested negative, consistent with the hypothesis that Bd emerged as an epizootic in California. Our study suggests that Bd possibly spread throughout California with multiple points of entry considering the distance between the earliest positive to present day (Fig 1A and 1B). This coincides with the wide spread geographic range and use of multiple habitats of Pacific newts. Recently, a new chytrid fungus specific to salamanders, Batrachochytrium salamandrivorans (Bsal) was described in the Netherlands during a mass die-off in the European fire salamanders. [42]. Bsal poses a major threat to the salamander diversity in North America [42, 43] and has not been shown to be present in North America. Therefore, there is a need for further studies regarding Pacific newts’ possible role as a reservoir species for the pathogen.

Consistent with other studies, our linear regression output found that infection status had a positive relationship with elevation [4446]. It is interesting given that we had a relatively small range of elevations (mean = 443 meters; range = 2-1800m). The upper elevation limit in our sampling is due to Pacific newts’ elevation limit of 2000m [47]. Our results also suggest a negative relationship between Bd infection and precipitation of the driest quarter and a positive relationship between Bd infection and annual precipitation, consistent with past studies [16, 44, 48, 49]. Lastly, we found that mean actual evapotranspiration, the rate of which the water in the soil evaporates, had a negative relationship with Bd infection. Bd has been known to reside and even to survive in moist soils [50, 51], thus soil run-offs and soil transport (e.g. during construction or landscaping being described as possible means to spread Bd [52, 53]. Consequently, higher soil evapotranspiration translates to a drier soil and a less suitable environment for Bd. Our linear regression analysis is limited by data availability for some of the variables (e.g. human footprint).

Our study focuses on four newt species that occur along western North America, where Bd epizootics have been documented [9]. However, there are several field surveys of Bd infections in newts from other regions where Bd epizootics are not known. For example, a study in the Eastern newt complex (N. viridescens) showed Bd-infections in wild populations had low zoospore counts and low prevalence [54, 55, 56, 57]. Another study suggested that Eastern newts may act as a Bd reservoir [56] and can develop acquired Bd-immunity as they mature [57]. These studies in other systems provide a framework for understanding how Bd may interact with other salamanders in the family Salamandridae, but comparisons must be done with the understanding that those species may have different evolutionary relationships with this pathogen. For example, there are no known epizootics of Bd where Eastern newts occur, and some have suggested that Bd may have a longer evolutionary history with amphibian species on Eastern US [33].

In our study, we provide new evidence that Bd is a novel pathogen in California, suggesting that Bd emerged in the last 4–5 decades. This is important because the pathogen was described almost 20 years after the first mass die offs were reported in California [21, 38]. In this study we found evidence that Bd invaded and became established in populations of Pacific newts earlier than other salamander species in the California region. Pacific newts have a widespread geographic range, use multiple habitats (aquatic/terrestrial) for extended periods of time (i.e. months), have large home range size, and have large populations. We found the pattern of emergence, where Bd was absent and then spread geographically (Fig 1) and increased in prevalence over time, to be similar to that found in other salamanders in the region. However, we also found that Bd dynamics in Pacific newts seem to represent a non-epizootic dynamic, where Bd infection intensities remain low. This may indicate that Pacific newts may not experience epizootic conditions in nature. We suggest that additional studies including laboratory and field-based Bd susceptibility studies are necessary to fully describe the relationship between Bd and Pacific newts and whether or not Pacific newts would make for a reservoir species that maintains the pathogen in amphibian communities.


We are grateful to all the undergraduate students at San Francisco State University who assisted with salamander swabbing in the museum and swab processing: Stephenie Huynh, Kurt Lutz, Adrienne Le, Michael Gibson, Kelly Hyde. Thanks to Dr. Ed Connor for invaluable advice on the statistical methods used. We are grateful to the California Academy of Sciences and Museum of Vertebrate Zoology at Berkeley for providing access to their collections and to Tiffany Yap, Michelle Koo, Jens Vindum, Lauren Scheinberg and Carol Spencer, for their help and patience.


  1. 1. Stuart SN, Chanson JS, Cox NA, Young BE, Rodrigues AS, Fischman DL, Waller RW. Status and trends of amphibian declines and extinctions worldwide. Science. 2004 Dec 3;306(5702):1783–6. pmid:15486254
  2. 2. Wake DB, Vredenburg VT. Are we in the midst of the sixth mass extinction? A view from the world of amphibians. Proceedings of the National Academy of Sciences. 2008 Aug 12;105(Supplement 1):11466–73.
  3. 3. Daszak P, Scott DE, Kilpatrick AM, Faggioni C, Gibbons JW, Porter D. Amphibian population declines at Savannah River site are linked to climate, not chytridiomycosis. Ecology. 2005 Dec 1;86(12):3232–7.
  4. 4. Skerratt LF, Berger L, Speare R, Cashins S, McDonald KR, Phillott AD et al. Spread of chytridiomycosis has caused the rapid global decline and extinction of frogs. EcoHealth. 2007 Jun 1;4(2):125.
  5. 5. Berger L, Speare R, Daszak P, Green DE, Cunningham AA, Goggin CL et al. Chytridiomycosis causes amphibian mortality associated with population declines in the rain forests of Australia and Central America. Proceedings of the National Academy of Sciences. 1998 Jul 21;95(15):9031–6.
  6. 6. Longcore JE, Pessier AP, Nichols DK. Batrachochytrium dendrobatidis gen. et sp. nov., a chytrid pathogenic to amphibians. Mycologia. 1999 Mar 1:219–27.
  7. 7. Laurance WF, McDonald KR, Speare R. Epidemic disease and the catastrophic decline of Australian rain forest frogs. Conservation Biology. 1996 Apr 1;10(2):406–13.
  8. 8. Lips KR, Brem F, Brenes R, Reeve JD, Alford RA, Voyles J et al. Emerging infectious disease and the loss of biodiversity in a Neotropical amphibian community. Proceedings of the national academy of sciences of the United States of America. 2006 Feb 28;103(9):3165–70. pmid:16481617
  9. 9. Vredenburg VT, Knapp RA, Tunstall TS, Briggs CJ. Dynamics of an emerging disease drive large-scale amphibian population extinctions. Proceedings of the National Academy of Sciences. 2010 May 25;107(21):9689–94.
  10. 10. Berger L, Hyatt AD, Speare R, Longcore JE. Life cycle stages of the amphibian chytrid Batrachochytrium dendrobatidis. Diseases of aquatic organisms. 2005 Dec 30;68(1):51–63. pmid:16465834
  11. 11. Voyles J, Young S, Berger L, Campbell C, Voyles WF, Dinudom A et al. Pathogenesis of chytridiomycosis, a cause of catastrophic amphibian declines. Science. 2009 Oct 23;326(5952):582–5. pmid:19900897
  12. 12. Voyles J, Vredenburg VT, Tunstall TS, Parker JM, Briggs CJ, Rosenblum EB. Pathophysiology in mountain yellow-legged frogs (Rana muscosa) during a chytridiomycosis outbreak. PLoS One. 2012 Apr 25;7(4):e35374. pmid:22558145
  13. 13. Wardziak T, Luquet E, Plenet S, Léna JP, Oxarango L, Joly P. Impact of both desiccation and exposure to an emergent skin pathogen on transepidermal water exchange in the palmate newt Lissotriton helveticus. Diseases of aquatic organisms. 2013 Jun 13;104(3):215–24. pmid:23759559
  14. 14. Rachowicz LJ, HERO J, Alford RA, Taylor JW, Morgan JA, Vredenburg VT, et al. The novel and endemic pathogen hypotheses: competing explanations for the origin of emerging infectious diseases of wildlife. Conservation Biology. 2005 Oct 1;19(5):1441–8.
  15. 15. Rachowicz LJ, Knapp RA, Morgan JA, Stice MJ, Vredenburg VT, Parker JM, et al. Emerging infectious disease as a proximate cause of amphibian mass mortality. Ecology. 2006 Jul 1;87(7):1671–83. pmid:16922318
  16. 16. Pounds JA, Bustamante MR, Coloma LA, Consuegra JA, Fogden MP, Foster PN, et al. Widespread amphibian extinctions from epidemic disease driven by global warming. Nature. 2006 Jan 12;439(7073):161. pmid:16407945
  17. 17. Cheng TL, Rovito SM, Wake DB, Vredenburg VT. Coincident mass extirpation of neotropical amphibians with the emergence of the infectious fungal pathogen Batrachochytrium dendrobatidis. Proceedings of the National Academy of Sciences. 2011 Jun 7;108(23):9502–7.
  18. 18. Crawford AJ, Lips KR, Bermingham E. Epidemic disease decimates amphibian abundance, species diversity, and evolutionary history in the highlands of central Panama. Proceedings of the National Academy of Sciences. 2010 Aug 3;107(31):13777–82.
  19. 19. Padgett-Flohr GE, Hopkins II RL. Batrachochytrium dendrobatidis, a novel pathogen approaching endemism in central California. Diseases of Aquatic Organisms. 2009 Jan 28;83(1):1–9. pmid:19301630
  20. 20. Sette CM, Vredenburg VT, Zink AG. Reconstructing historical and contemporary disease dynamics: A case study using the California slender salamander. Biological Conservation. 2015 Dec 31;192:20–9.
  21. 21. De León ME, Vredenburg VT, Piovia-Scott J. Recent emergence of a chytrid fungal pathogen in California Cascades frogs (Rana Cascadae). EcoHealth. 2017 Mar 1;14(1):155–61. pmid:27957606
  22. 22. Reeder NM, Pessier AP, Vredenburg VT. A reservoir species for the emerging amphibian pathogen Batrachochytrium dendrobatidis thrives in a landscape decimated by disease. PLoS One. 2012 Mar 12;7(3):e33567. pmid:22428071
  23. 23. Twitty VC. Migration and speciation in newts. Science. 1959 Dec 25;130(3391):1735–43. pmid:17812829
  24. 24. Twitty V, Grant D, Anderson O. Long distance homing in the newt Taricha rivularis. Proceedings of the National Academy of Sciences. 1964 Jan 1;51(1):51–8.
  25. 25. Licht P, Brown AG. Behavioral Thermoregulation and Its Role in the Ecology of the Red‐Bellied Newt, Taricha Rivularis. Ecology. 1967 Jul 1;48(4):598–611.
  26. 26. Gordon K. The amphibia and reptilia of Oregon. Oregon State College; 1939.
  27. 27. Bishop DW. Polydactyly in the tiger salamander. Journal of Heredity. 1947 Oct;38(10):291–3.
  28. 28. Stebbins RC. A field guide to western amphibians and reptiles. Peterson Field Guide Series. Houghton Mifflin Company, Boston, MA. 1985.
  29. 29. Phillips BL, Puschendorf R. Do pathogens become more virulent as they spread? Evidence from the amphibian declines in Central America. Proceedings of the Royal Society of London B: Biological Sciences. 2013 Sep 7;280(1766):20131290.
  30. 30. Sulaeman H. 2018 "Pathogen invasion and non-epizootic dynamics in Pacific newts in California over the last century" AmphibiaWeb: Amphibian Disease Portal. <> Accessed 07 Mar 2018.
  31. 31. Boyle DG, Boyle DB, Olsen V, Morgan JA, Hyatt AD. Rapid quantitative detection of chytridiomycosis (Batrachochytrium dendrobatidis) in amphibian samples using real-time Taqman PCR assay. Diseases of aquatic organisms. 2004 Aug 9;60(2):141–8. pmid:15460858
  32. 32. Boyle AH, Olsen V, Boyle DB, Berger L, Obendorf D, Dalton A, et al. Diagnostic assays and sampling protocols for the detection of Batrachochytrium dendrobatidis. Diseases of aquatic organisms. 2007 Jan 18;73(3):175–92. pmid:17330737
  33. 33. Talley BL, Muletz CR, Vredenburg VT, Fleischer RC, Lips KR. A century of Batrachochytrium dendrobatidis in Illinois amphibians (1888–1989). Biological Conservation. 2015 Feb 28;182:254–61.
  34. 34. Venter O, Sanderson EW, Magrach A, Allan JR, Beher J, Jones KR, et al. Global terrestrial Human Footprint maps for 1993 and 2009. Scientific data. 2016 Aug 23;3:sdata201667.
  35. 35. Gabor CR, Fisher MC, Bosch J. A non-invasive stress assay shows that tadpole populations infected with Batrachochytrium dendrobatidis have elevated corticosterone levels. PLoS One. 2013 Feb 13;8(2):e56054. pmid:23418508
  36. 36. Schloegel LM, Picco AM, Kilpatrick AM, Davies AJ, Hyatt AD, Daszak P. Magnitude of the US trade in amphibians and presence of Batrachochytrium dendrobatidis and ranavirus infection in imported North American bullfrogs (Rana catesbeiana). Biological Conservation. 2009 Jul 31;142(7):1420–6.
  37. 37. Kinney VC, Heemeyer JL, Pessier AP, Lannoo MJ. Seasonal pattern of Batrachochytrium dendrobatidis infection and mortality in Lithobates areolatus: Affirmation of Vredenburg's “10,000 Zoospore Rule”. PLoS One. 2011 Mar 10;6(3):e16708. pmid:21423745
  38. 38. Bradford DF. Mass mortality and extinction in a high-elevation population of Rana muscosa. Journal of Herpetology. 1991 Jun 1:174–7.
  39. 39. Weinstein SB. An aquatic disease on a terrestrial salamander: individual and population level effects of the amphibian chytrid fungus, Batrachochytrium dendrobatidis, on Batrachoseps attenuatus (Plethodontidae). Copeia. 2009 Dec 21;2009(4):653–60.
  40. 40. Jenkinson TS, Román B, Lambertini C, Valencia‐Aguilar A, Rodriguez D, Nunes‐de‐Almeida CH, et al. Amphibian‐killing chytrid in Brazil comprises both locally endemic and globally expanding populations. Molecular ecology. 2016 Jul 1;25(13):2978–96 pmid:26939017
  41. 41. Schloegel LM, Ferreira CM, James TY, Hipolito M, Longcore JE, Hyatt AD, et al. The North American bullfrog as a reservoir for the spread of Batrachochytrium dendrobatidis in Brazil. Animal Conservation. 2010 Dec 1;13(s1):53–61.
  42. 42. Martel A, Spitzen-van der Sluijs A, Blooi M, Bert W, Ducatelle R, Fisher MC, et al. Batrachochytrium salamandrivorans sp. nov. causes lethal chytridiomycosis in amphibians. Proceedings of the National Academy of Sciences. 2013 Sep 17;110(38):15325–9.
  43. 43. Yap TA, Koo MS, Ambrose RF, Wake DB, Vredenburg VT. Averting a North American biodiversity crisis. Science. 2015 Jul 31;349(6247):481–2. pmid:26228132
  44. 44. Ron SR. Predicting the distribution of the amphibian pathogen Batrachochytrium dendrobatidis in the New World. Biotropica. 2005 Jun 1;37(2):209–21.
  45. 45. Brem FM, Lips KR. Batrachochytrium dendrobatidis infection patterns among Panamanian amphibian species, habitats and elevations during epizootic and enzootic stages. Diseases of aquatic organisms. 2008 Sep 24;81(3):189–202. pmid:18998584
  46. 46. Blaustein AR, Romansic JM, Scheessele EA, Han BA, Pessier AP, Longcore JE. Interspecific variation in susceptibility of frog tadpoles to the pathogenic fungus Batrachochytrium dendrobatidis. Conservation Biology. 2005 Oct 1;19(5):1460–8.
  47. 47. Stebbins RC, McGinnis SM. Field guide to amphibians and reptiles of California: revised edition. Univ of California Press; 2012 Sep 4.
  48. 48. Olson DH, Aanensen DM, Ronnenberg KL, Powell CI, Walker SF, Bielby J, et al. Mapping the global emergence of Batrachochytrium dendrobatidis, the amphibian chytrid fungus. PloS one. 2013 Feb 27;8(2):e56802. pmid:23463502
  49. 49. Fisher MC, Garner TW, Walker SF. Global emergence of Batrachochytrium dendrobatidis and amphibian chytridiomycosis in space, time, and host. Annual review of microbiology. 2009 Oct 13;63:291–310. pmid:19575560
  50. 50. Johnson ML, Speare R. Survival of Batrachochytrium dendrobatidis in water: quarantine and disease control implications. Emerging infectious diseases. 2003 Aug;9(8):922. pmid:12967488
  51. 51. Johnson ML, Speare R. Possible modes of dissemination of the amphibian chytrid Batrachochytrium dendrobatidis in the environment. Diseases of aquatic organisms. 2005;65:181–6. pmid:16119886
  52. 52. Pauza MD, Driessen MM, Skerratt LF. Distribution and risk factors for spread of amphibian chytrid fungus Batrachochytrium dendrobatidis in the Tasmanian Wilderness World Heritage Area, Australia. Diseases of Aquatic Organisms. 2010 Nov 25;92(2–3):193–9. pmid:21268981
  53. 53. Roedder D, Schulte U, Toledo LF. High environmental niche overlap between the fungus Batrachochytrium dendrobatidis and invasive bullfrogs (Lithobates catesbeianus) enhance the potential of disease transmission in the Americas. North-Western Journal of Zoology. 2013 Jun 1;9(1).
  54. 54. Richards-Hrdlicka KL, Richardson JL, Mohabir L. First survey for the amphibian chytrid fungus Batrachochytrium dendrobatidis in Connecticut (USA) finds widespread prevalence. Diseases of Aquatic Organisms. 2013 Feb 28;102(3):169–80 pmid:23446966
  55. 55. Rothermel BB, Walls SC, Mitchell JC, Dodd CK Jr, Irwin LK, Green DE, et al. Widespread occurrence of the amphibian chytrid fungus Batrachochytrium dendrobatidis in the southeastern USA. Diseases of Aquatic Organisms. 2008 Oct 16;82(1):3–18. pmid:19062748
  56. 56. Strauss A, Smith KG. Why does amphibian chytrid (Batrachochytrium dendrobatidis) not occur everywhere? An exploratory study in Missouri ponds. PloS one. 2013 Sep 25;8(9):e76035. pmid:24086681
  57. 57. Raffel TR, Michel PJ, Sites EW, Rohr JR. What drives chytrid infections in newt populations? Associations with substrate, temperature, and shade. EcoHealth. 2010 Dec 1;7(4):526–36. pmid:21125308