Exploring Differing Host Cutaneous Microbiome and Immune Responses Contributing to Chytridiomycosis Susceptibility in Amphibians

Chytridiomycosis is an emerging infectious disease caused by Batrachochytrium dendrobatidis, a fungal pathogen affecting the skin of amphibians. Chytridiomycosis is differentially affecting amphibian species and populations across the world, causing severe declines and extinctions. It is spread by contact or zoospores travelling in water sources. It is not yet fully understood why susceptibility is so variable. Understanding differing susceptibility is crucial for realising any successful conservation efforts. Multiple factors appear responsible for the differing susceptibility. The two factors being examined in this literature review are ineffective immune responses and the limitations of the cutaneous microbiome. Relevant papers’ significance and limitations are discussed with their provenance and objectivity taken into consideration. The immune system of amphibians comprises innate and acquired defences. The innate immune system has been found to be counterproductive in some cases, much like how an allergic reaction in humans can be detrimental to health. An adaptive immune response has yet to be confirmed in previously exposed individuals. The cutaneous microbiome plays an initial role in the defence against harmful zoospores by making the environment unsuitable or by producing deadly secretions; this consequently prevents colonization by the zoospores. Differing levels of secretions have been measured in response to disease. Most amphibian immune systems and microbiomes are not adapted to deal with chytridiomycosis, and the fungi are adapted to exploit this weakness. There are many difficulties in studying this disease, such as recreating a natural habitat in laboratory conditions, which is vital to get accurate microbiome data. The variety of species and global spread of this disease is incredibly wide ranging with many factors to consider. Many studies are only focused on one aspect of the disease, so a holistic and global approach would be more beneficial.