Frequency‐dependent transmission of Batrachochytrium salamandrivorans in eastern newts

Abstract

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 R0 = 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.

Host density has limited effects on pathogen invasion, disease‐induced declines and within‐host infection dynamics across a landscape of disease

“…Thus, elucidating the effects of host density on various branch points is important to describe variability in response trajectories in host–parasite systems. However, many studies either (i) focus on the effects of density on transmission in small‐scale experiments and then extrapolate these effects up to natural populations (Dallas et al, 2018; Tompros et al, 2022) or (ii) focus on the effects of density at a single branch point in the field (e.g. pathogen invasion, Davis et al, 2007), even though the effects of host density can vary across the response trajectory (Begon et al, 2019).…”

“…We therefore predict that the magnitude of host population declines following Bd invasion should increase for higher host population density prior to decline (branch point 2–4). Finally, while it is often assumed that the accumulation of Bd infection intensity following initial infection is driven by within‐host processes and is largely decoupled from the density of hosts in the environment (DiRenzo et al, 2018; Wilber et al, 2017), there is some evidence that host density can influence chytrid infection intensity (Bosch et al, 2020; Tompros et al, 2022). We test this hypothesis and predict that host density in enzootic and epizootic scenarios will not have a clear effect on Bd infection intensity (branch point 2).…”

Batrachochytrium salamandrivorans Threat to the Iberian Urodele Hotspot

“…Anthropogenic movement of indigenous species (e.g., I. alpestris [ 40 ]; C. lusitanica [ 84 ]) equally carries the risk of pathogen spread. Since several of these species may develop chronic, high level infections, they may act as disease reservoirs that facilitate frequency-dependent Bsal transmission, which increases the probability of extirpation of highly susceptible species [ 15 , 85 ] and hence should be considered in risk assessments.…”

Temperature and duration of exposure drive infection intensity with the amphibian pathogen Batrachochytrium dendrobatidis

“…In a notable amphibian pathogen, Batrachochytrium dendrobatidis (hereafter Bd ), evidence suggests that infection intensity is one of a number of epidemiological parameters, including prevalence of infection, that is positively associated with pathogen transmission ( Vredenburg et al, 2010 ; Tompros et al, 2021 ). This generalist pathogen is known to infect multiple hosts around the world, and its effects are known to vary both inter- and intra-specifically ( Rosa et al, 2017 ), in part as a result of infection intensity ( Whitfield et al, 2017 ).…”