Initiation ofBatrachochytrium dendrobatidisinfection in the absence of physical contact with infected hosts – a field study in a high altitude lake

Abstract

Understanding transmission is a critical prerequisite for predicting disease dynamics and impacts on host populations. It is well established that Batrachochytrium dendrobatidis (Bd), the amphibian fungal pathogen responsible for chytridiomycosis, can be transmitted directly, through physical contact with an infected host. However, indirect pathways of transmission remain poorly investigated. We conducted a five-week long field infection experiment at a high altitude mountain lake in the French Pyrenees to investigate Bd transmission pathways in larval midwife toads Alytes obstetricans. Uninfected naïve tadpoles were co-housed either with infected tadpoles (direct and indirect transmission) or with uninfected ones (indirect transmission only). We found that physical contact with an infected host is not necessary for initial infection with Bd and that all tadpoles became infected after only four weeks. However, physical contact with infected tadpoles led to a faster spread within a tadpole group and resulted in higher Bd loads and subsequently higher mortality. Our findings clearly demonstrate that in A. obstetricans, Bd can quickly spread in a population even without physical contact. Our experiment therefore stresses the importance of indirect transmission of Bd zoospores in infected lakes for disease dynamics, especially when a reservoir species such as A. obstetricans is present.

Eco‐evolutionary rescue promotes host–pathogen coexistence

“…Previous studies have demonstrated that Bd occurs in the environment; given that amphibians can become infected in areas that do not contain infected amphibians (i.e., Courtois et al. , Fernández‐Beaskoetxea et al. , Voyles et al.…”

“…Negative host-density-dependent Bd transmission risk cannot be explained without assuming that Bd exists in environment reservoirs for long periods of time, given that Bd transmission risk is nearly 100% at sites where host abundance is one individual. Previous studies have demonstrated that Bd occurs in the environment; given that amphibians can become infected in areas that do not contain infected amphibians (i.e., Courtois et al 2016, Fern andez-Beaskoetxea et al 2016, Voyles et al 2018). However, without knowing if Bd can reproduce or how long it persists in the environment, we cannot support the implied assumptions of a Bd environmental reservoir.…”

Resistance, tolerance and environmental transmission dynamics determine host extinction risk in a load‐dependent amphibian disease

“…; Courtois et al . ), but the nature of this transmission has rarely been quantified. We experimentally quantified the transmission function in the R. muscosa ‐Bd system and used these results, in combination with a dynamic model, to predict how the environmental zoospore reservoir affected the ability of Bd to invade an amphibian population.…”

“…Uninfected frogs can acquire Bd infection through contact with other infected frogs and through contact with zoospores in an environmental Bd pool (Courtois et al . ). To account for these different pathways, we fit two sets of transmission models to the data.…”

“…If the load at time t + Dt was positive we assigned this data point a value of 1 (infected) and if the load was still zero we assigned it a value of 0 (uninfected). Uninfected frogs can acquire Bd infection through contact with other infected frogs and through contact with zoospores in an environmental Bd pool (Courtois et al 2017). To account for these different pathways, we fit two sets of transmission models to the data.…”