Effects of Emerging Infectious Diseases on Amphibians: A Review of Experimental Studies

Abstract

Numerous factors are contributing to the loss of biodiversity. These include complex effects of multiple abiotic and biotic stressors that may drive population losses. These losses are especially illustrated by amphibians, whose populations are declining worldwide. The causes of amphibian population declines are multifaceted and context-dependent. One major factor affecting amphibian populations is emerging infectious disease. Several pathogens and their associated diseases are especially significant contributors to amphibian population declines. These include the fungi Batrachochytrium dendrobatidis and B. salamandrivorans, and ranaviruses. In this review, we assess the effects of these three pathogens on amphibian hosts as found through experimental studies. Such studies offer valuable insights to the causal factors underpinning broad patterns reported through observational studies. We summarize key findings from experimental studies in the laboratory, in mesocosms, and from the field. We also summarize experiments that explore the interactive effects of these pathogens with other contributors of amphibian population declines. Though well-designed experimental studies are critical for understanding the impacts of disease, inconsistencies in experimental methodologies limit our ability to form comparisons and conclusions. Studies of the three pathogens we focus on show that host susceptibility varies with such factors as species, host age, life history stage, population and biotic (e.g., presence of competitors, predators) and abiotic conditions (e.g., temperature, presence of contaminants), as well as the strain and dose of the pathogen, to which hosts are exposed. Our findings suggest the importance of implementing standard protocols and reporting for experimental studies of amphibian disease.

The amphibian‐killing fungus in a biodiversity hotspot: identifying and validating high‐risk areas and refugia

“…This is consistent with other studies that have also found a positive relationship between Bd occurrence and human influence at both local (Adams et al 2010) and regional (James et al 2015) scales, but not at the global scale (Liu et al 2013a,b). Second, this pattern might occur if the human footprint translates into reduced habitat quality for amphibians or increases other stressors, which may increase their susceptibility to Bd infections (Daszak et al 2003, Gray et al 2007, Ramsey et al 2010, Becker et al 2016, Blaustein et al 2018, potentially increasing pathogen occurrence.…”

“…, Blaustein et al. ). Overall, our understanding of the factors underlying the distribution and prevalence of Bd has greatly increased over recent years (Berger et al.…”

“…At the local scale, Bd prevalence (i.e. number of positive animals/number sampled) and the host response to Bd exposure are likely the result of the complex interaction between the host, the pathogen, and the environmental conditions they encounter (Murray and Skerratt 2012, Spitzen-Van Der Sluijs et al 2014, Blaustein et al 2018. Overall, our understanding of the factors underlying the distribution and prevalence of Bd has greatly increased over recent years (Berger et al 2016).…”

Effects of invasive larval bullfrogs (Rana catesbeiana) on disease transmission, growth and survival in the larvae of native amphibians

“…Bd may be saprophytic (Daszak et al 1999;Longcore et al 1999;Briggs et al 2005) and is most pathogenic in moist, cool climates (Berger et al 2004;Piotrowski et al 2004;La Marca et al 2005;Whitfield et al 2016). Anuran susceptibility to Bd can vary with host life stage, species, population, age, Bd strain to which hosts are exposed, and environmental conditions (Blaustein et al 2005(Blaustein et al , 2018Garcia et al 2006;Di Rosa et al 2007;Searle et al 2011;Gervasi et al 2013aGervasi et al , 2017Sapsford et al 2013;Bradley et al 2015;Dang et al 2017;Mesquita et al 2017). For instance, Rumschlag and Boone (2018) observed increased winter mortality of leopard frogs (Rana pipiens) exposed to Bd before overwintering, which could reduce population growth rates due to reduced recruitment.…”

Emerging infectious disease threats to European herpetofauna

“…Although for now the toads seem to be safe, this may change in the future due to the effects of climate change and other such environmental effects. For more information on the environmental impact of hostpathogen interactions, which can be hugely deterministic in some cases, see Blaustein et al (2018) for an in-depth discussion. Experiments have also shown that some salamanders such as those in the genus Speleomantes also have some immunity to Bd (Pasmans et al, 2013).…”