Investigating Differences across Host Species and Scales to Explain the Distribution of the Amphibian Pathogen Batrachochytrium dendrobatidis

Abstract

Many pathogens infect more than one host species, and clarifying how these different hosts contribute to pathogen dynamics can facilitate the management of pathogens and can lend insight into the functioning of pathogens in ecosystems. In this study, we investigated a suite of native and non-native amphibian hosts of the pathogen Batrachochytrium dendrobatidis (Bd) across multiple scales to identify potential mechanisms that may drive infection patterns in the Colorado study system. Specifically, we aimed to determine if: 1) amphibian populations vary in Bd infection across the landscape, 2) amphibian community composition predicts infection (e.g., does the presence or abundance of any particular species influence infection in others?), 3) amphibian species vary in their ability to produce infectious zoospores in a laboratory infection, 4) heterogeneity in host ability observed in the laboratory scales to predict patterns of Bd prevalence in the landscape. We found that non-native North American bullfrogs (Lithobates catesbeianus) are widespread and have the highest prevalence of Bd infection relative to the other native species in the landscape. Additionally, infection in some native species appears to be related to the density of sympatric L. catesbeianus populations. At the smaller host scale, we found that L. catesbeianus produces more of the infective zoospore stage relative to some native species, but that this zoospore output does not scale to predict infection in sympatric wild populations of native species. Rather, landscape level infection relates most strongly to density of hosts at a wetland as well as abiotic factors. While non-native L. catesbeianus have high levels of Bd infection in the Colorado Front Range system, we also identified Bd infection in a number of native amphibian populations allopatric with L. catesbeianus, suggesting that multiple host species are important contributors to the dynamics of the Bd pathogen in this landscape.

Extreme drought, host density, sex, and bullfrogs influence fungal pathogen infection in a declining lotic amphibian

“…Our findings that both the probability of Bd infection and the probability of Bd load are higher in R. boylii when bullfrogs are present are supported by a prior field study that showed a positive relationship between Bd prevalence and load and bullfrog density in native amphibian populations sympatric with non‐native bullfrogs (Peterson and McKenzie ). Because they prefer pools with little or no flow, bullfrog densities in rivers can increase during drought years in California's Mediterranean climate (i.e., cool, wet winters and warm, dry summers), particularly after years with low winter peak discharges (Kupferberg , Doubledee et al.…”

“…), suggesting that R. boylii males may experience direct Bd transmission from contact with bullfrog reservoir hosts. In addition, Bd‐infected bullfrogs have been observed shedding more infective zoospores than other native western species (Peterson and McKenzie ).…”

“…Based on the literature, we expected Bd prevalence and load to vary according to different biotic and abiotic factors (Table ). We hypothesized that drought conditions would positively affect Bd prevalence and load if amphibians become highly concentrated in some reaches as the stream flow becomes intermittent—we expected Bd transmission rates and Bd susceptibility (due to stress) to increase as a function of host density (Rachowicz and Briggs , Peterson and McKenzie , Brannelly et al. ).…”

Disentangling host, pathogen, and environmental determinants of a recently emerged wildlife disease: lessons from the first 15 years of amphibian chytridiomycosis research

“…Importantly, these two invasive species have been implicated in the global spread of the disease (Daszak et al 2001;Vredenburg et al 2013). In Colorado, where L. catesbeianus is invasive, the density of L. catesbeianus was positively correlated with Bd infection prevalence and load in co-occurring native fauna (Peterson and McKenzie 2014). On the other hand, a recent study failed to find evidence of increased Bd infection on native UK fauna due to the presence of invasive and Bd-infected Xenopus (Tinsley et al 2015).…”

Probiotic treatment restores protection against lethal fungal infection lost during amphibian captivity

“…Probiotic-treated individuals were given bacterial cells (1 Â 10 6 ) suspended in sterile water [4]. We used a Bd strain recently isolated from bullfrogs in Colorado that was confirmed as the global pandemic lineage (Bd-GPL) [14], and we used qPCR for measuring Bd loads [13]. In manipulation 1, Bd exposures (1 Â 10 5 Bd zoospores) occurred 2 days after the probiotic treatment.…”