Significant reductions of host abundance weakly impact infection intensity of Batrachochytrium dendrobatidis

Abstract

Infectious diseases are considered major threats to biodiversity, however strategies to mitigate their impacts in the natural world are scarce and largely unsuccessful. Chytridiomycosis is responsible for the decline of hundreds of amphibian species worldwide, but an effective disease management strategy that could be applied across natural habitats is still lacking. In general amphibian larvae can be easily captured, offering opportunities to ascertain the impact of altering the abundance of hosts, considered to be a key parameter affecting the severity of the disease. Here, we report the results of two experiments to investigate how altering host abundance affects infection intensity in amphibian populations of a montane area of Central Spain suffering from lethal amphibian chytridiomycosis. Our laboratory-based experiment supported the conclusion that varying density had a significant effect on infection intensity when salamander larvae were housed at low densities. Our field experiment showed that reducing the abundance of salamander larvae in the field also had a significant, but weak, impact on infection the following year, but only when removals were extreme. While this suggests adjusting host abundance as a mitigation strategy to reduce infection intensity could be useful, our evidence suggests only heavy culling efforts will succeed, which may run contrary to objectives for conservation.

In situ treatment of juvenile frogs for disease can reverse population declines

“…Our results differ from previous field studies that did not find evidence of persistent effects of itraconazole treatment on apparent survival or Bd infection (Bosch et al 2015;Hudson et al 2016;Geiger et al 2017; Knapp et al 2022 [this study found persistent effects of treatment for adults, but not for earlier life stages]). Our treatment targeted a specific period (shortly after metamorphosis) when Cascades frogs are highly susceptible to Bd and experience dramatic increases in Bd prevalence (Hardy et al 2015).…”

“…In addition, we hypothesize that metamorphs emerging from surface-water pools may be more susceptible to infection than those emerging from fens because temperatures in pools may regularly exceed the frog's thermal optimum (i.e., the thermal mismatch hypothesis for cool-adapted species; Cohen et al 2017;Sauer et al 2020), and because rapid dry down of pools may decrease post-metamorphic immune function (Gervasi & Foufopoulos 2008;Rollins-Smith et al 2011;Kohli et al 2019). However, it is also possible that larval habitats differed in other factors that could be related to Bd infection, including density and contact rates (Rachowicz & Briggs 2007;Briggs et al 2010;Bosch et al 2020) and host genetic factors related to Bd resistance (e.g., Savage & Zamudio 2011Bataille et al 2015). Such effects of larval habitat may play an important role in determining the impact of climatic change on interactions between amphibians and Bd.…”

“…Once Bd becomes established in populations of at-risk amphibians, there is a limited set of management options available, including translocation, habitat manipulation, and treatment of hosts or environments with chemical or biological antifungal agents (Woodhams et al 2011;Scheele et al 2014Scheele et al , 2021Langwig et al 2015;Garner et al 2016;Brannelly et al 2021). While eradication of the pathogen is possible (Bosch et al 2015), it is unlikely to be feasible in most situations, leading to a renewed focus on facilitating coexistence with Bd Brannelly et al 2021). In situ treatment of amphibian hosts with antifungal chemicals, such as itraconazole, has emerged as one of the more reliable interventions for reducing Bd infections in individuals (e.g., Hudson et al 2016;Geiger et al 2017;Knapp et al 2022).…”

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

“…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).…”

Unraveling disparate roles of organisms, from plants to bacteria, and viruses on built cultural heritage

“…In Europe, there has been a dramatic decline in some species that are highly susceptible to Batrachochytrium , such as the midwife toad ( Alytes obstetricans , Fig. 2 ) (Bosch et al 2020 ). Microbiological studies of heritage monuments, particularly those using metagenomic tools, should focus on detecting and highlighting the occurrence of emergent pathogens in cultural sites, to prevent making these frequently visited site sources of the spread of hazards either to humans or to other organisms.…”