Batrachochytrium: Biology and Management of Amphibian Chytridiomycosis

Abstract

Two pathogenic fungi in the genus Batrachochytrium are impacting amphibian populations globally. Batrachochytrium–amphibian systems have become models for understanding processes and testing theory in disease ecology. Reservoir hosts and nonamphibian hosts may be critical for both the spread of disease and host–pathogen dynamics. Environmental factors affecting pathogen growth, and host resistance and tolerance characteristics including behaviour, immune function and microbiome, are considered in the context of disease management. Recovery of wild populations informs current disease management strategies and provides hope for proactive policies and research directed towards disease control and amphibian conservation.

Key Concepts

  • Batrachochytrium is the only genus of Rhizophydiales to colonise and infect vertebrate hosts.
  • Extensive lineage and genotype variation has been uncovered for B. dendrobatidis, but still need to be identified for B. salamandrivorans, if present.
  • Unlike B. dendrobatidis, B. salamandrivorans produces nonmotile, cell‐walled zoospores that can persist in environmental substrates for longer durations.
  • Both pathogens are characterised by expansions of protease and peptidase gene families.
  • B. dendrobatidis‐induced chytridiomycosis more broadly affects amphibian hosts with affinity towards anurans, while B. salamandrivorans‐induced chytridiomycosis affects primarily urodelans (caudates).
  • Nonamphibian hosts for Batrachochytrium potentially include crayfish, fish and waterfowl.
  • Tolerance and resistance to Batrachochytrium‐induced chytridiomycosis varies greatly among host species, and it is yet to be determined how prior infection with one may alter infection dynamics of the other.
  • Rediscoveries of ‘lost’ species raise the possibility that adaptations in wild frog populations may lead to population recovery. The mechanisms behind such recovery likely differ among species, populations and life stages, and research in this area has just begun.
  • Batrachochytrium‐induced chytridiomycosis looms over native amphibians globally, and continued research efforts and development of novel policies at a national and international scale are urgently needed to minimise their impacts.

Keywords: chytrid fungus; disease ecology; virulence; disease susceptibility; reservoir host; disease mitigation; population recovery

Figure 1. Conceptual cost–benefit analysis of Batrachochytrium dendrobatidis (Bd) or B. salamandrivorans (Bsal) virulence and pathogen fitness. Pathogen strains may differ in virulence, as mediated by optimum pathogen fitness with maximal benefit at minimal cost. The most virulent strain may not be the most fit, and rapid reproduction may lead to morbidity or early host death and reduced opportunities for transmission to new hosts.
Figure 2. Skin mucosal defences of amphibians including innate and adaptive immune defences, the microbiome and interactions with Batrachochytrium. Inset zoosporangium artwork courtesy of Robert Brucker. Physiological host defences include skin defence compounds secreted from granular glands in amphibian skin, the microbiome including beneficial bacteria and fungi and the acquired or adaptive immune system. These defences can all be altered by exposure to Batrachochytrium, either through pathogen recognition and induction of host or microbiome responses or by pathogen immune evasion and suppression of host defences.
Figure 3. Batrachochytrium spp. infection success is influenced by aspects of landscape ecology, community and behavioural ecology and host immunological ecology. These factors may act on Bd and Bsal independently or in tandem and should all be considered when approaching management of chytrid pathogens (Table).
Figure 4. Growth of Batrachochytrium salamandrivorans in response to abiotic environmental parameters. (a) Growth of Bsal across a pH gradient ranging from 6 to 9. (b) Growth of Bsal across a salinity gradient ranging from 0 to 14 ppt. Growth is presented as the change in optical density through time (i.e. slope). The duration of the assay was 5 days and absorbance measurements were read at 492 nm. The low reading observed at pH 6.5 is suspected to be an abnormality in the conducted assay. In other testing, growth at 6.5 was comparable to that of pH 6 and 7.
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Woodhams, Douglas C, Barnhart, Kelly L, Bletz, Molly C, Campos, Alberto J, Ganem, Steven J, Hertz, Andreas, LaBumbard, Brandon C, Nanjappa, Priya, and Tokash‐Peters, Amanda G(Jul 2018) Batrachochytrium: Biology and Management of Amphibian Chytridiomycosis. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0027207]