In vitro modeling of Batrachochytrium dendrobatidis infection of the amphibian skin

The amphibian invitrome: Past, present, and future contributions to our understanding of amphibian immunity

Many amphibian populations are declining worldwide, and infectious diseases are a leading cause. Given the eminent threat infectious diseases pose to amphibian populations, there is a need to understand the host-pathogen-environment interactions that govern amphibian susceptibility to disease and mortality events. However, using animals in research raises an ethical dilemma, which is magnified by the alarming rates at which many amphibian populations are declining. Thus, in vitro study systems such as cell lines represent valuable tools for furthering our understanding of amphibian immune systems. In this review, we curate a list of the amphibian cell lines established to date (the amphibian invitrome), highlight how research using amphibian cell lines has advanced our understanding of the amphibian immune system, anti-ranaviral defence mechanisms, and Batrachochytrium dendrobatidis replication in host cells, and offer our perspective on how future use of amphibian cell lines can advance the field of amphibian immunology.

Virulence and Pathogenicity of Chytrid Fungi Causing Amphibian Extinctions

Ancient enzootic associations between wildlife and their infections allow evolution to innovate mechanisms of pathogenicity that are counterbalanced by host responses. However, erosion of barriers to pathogen dispersal by globalization leads to the infection of hosts that have not evolved effective resistance and the emergence of highly virulent infections. Global amphibian declines driven by the rise of chytrid fungi and chytridiomycosis are emblematic of emerging infections. Here, we review how modern biological methods have been used to understand the adaptations and counteradaptations that these fungi and their amphibian hosts have evolved. We explore the interplay of biotic and abiotic factors that modify the virulence of these infections and dissect the complexity of this disease system. We highlight progress that has led to insights into how we might in the future lessen the impact of these emerging infections. Expected final online publication date for the Annual Review of Microbiology, Volume 75 is October 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Batrachochytrium fungi: stealth invaders in amphibian skin

Amphibian populations around the world have been affected by two pathogenic fungi within the phylum Chytridiomycota. Batrachochytrium dendrobatidis (Bd) has infected hundreds of species and led to widespread declines and some species extinctions. Batrachochytrium salamandrivorans (Bsal) has devastated some native European salamanders, especially the iconic fire salamanders (Salamandra salamandra). Comparative genomic studies show that Bd is more diverse and widespread than previously thought, and global lineages occur together allowing for the development of hybrid lineages. New studies raise the concern of greater pathogenesis if both Bd and Bsal infect the same host. Although amphibians possess robust immune defenses, co-infected and many single-infected hosts seem unable to mount effective immune responses. A strong defense may actually be harmful. Analysis of Bd and Bsal secretions documents small metabolites that signal high density to limit their growth and to suppress adaptive immune defenses, thus enabling a stealth presence in the skin compartment.

Presence of low virulence chytrid fungi could protect European amphibians from more deadly strains

Wildlife diseases are contributing to the current Earth’s sixth mass extinction; one disease, chytridiomycosis, has caused mass amphibian die-offs. While global spread of a hypervirulent lineage of the fungus Batrachochytrium dendrobatidis (BdGPL) causes unprecedented loss of vertebrate diversity by decimating amphibian populations, its impact on amphibian communities is highly variable across regions. Here, we combine field data with in vitro and in vivo trials that demonstrate the presence of a markedly diverse variety of low virulence isolates of BdGPL in northern European amphibian communities. Pre-exposure to some of these low virulence isolates protects against disease following subsequent exposure to highly virulent BdGPL in midwife toads (Alytes obstetricans) and alters infection dynamics of its sister species B. salamandrivorans in newts (Triturus marmoratus), but not in salamanders (Salamandra salamandra). The key role of pathogen virulence in the complex host-pathogen-environment interaction supports efforts to limit pathogen pollution in a globalized world.

The pathogen Batrachochytrium dendrobatidis (BD) associated with widespread amphibian declines is present in Europe but has not consistently caused disease-induced declines in that region. Here, the authors suggest that an endemic strain of BD with low virulence may protect the hosts upon co-infection with more virulent strains.