title: Transmission of amphibian parasites: exploring the influences of host identity and exposure scenario on key transitions in the transmission pathway.
Creators: name: Allen, Bryony
Publisher: name: University of Liverpool Repository
Year: 2022

description: Batrachochytrium dendrobatidis (Bd) is an emerging fungal pathogen that threatens amphibian hosts globally. The observed mass mortality events in amphibian populations, multiple species declines and extinctions caused by this pathogen, have been attributed to its broad host range, widespread geographic distribution, and ability to infect all amphibian life-history stages. While the severity of the disease is worse in post-metamorphic animals, it is the pre-metamorphic (tadpole) stage that is seen as an important disease reservoir. The potential for tadpoles to act as a reservoir for Bd is dependent on their contribution to the environmental pool of zoospores, and therefore, their role in Bd transmission. However, few studies have explored transmission dynamics in multi-host tadpole communities. To explore Bd transmission in the tadpole communities, I break the transmission process into three discrete stages: (i) susceptibility (the probability of a naïve host becoming infected upon exposure to Bd zoospores in the environment), (ii) infectiousness (the rate at which infected hosts release Bd zoospores into the environment) and (iii) contact rate (the rate at which tadpoles encounter zoospores in the environment). I quantified the first two of these processes experimentally for a range of host species. I then used these data to parameterise a range of mathematical models to predict the consequences for Bd transmission in multi-host communities and in doing so, explored the sensitivity of my predictions to variations in contact rate (the third of the above processes). Standardised experiments exposing three tadpole host species individually to Bd showed that host susceptibility conformed to previous held species responses, with species lying along a continuum from largely resistant (rarely infected, or only infected to a very low level) to largely tolerant (highly likely to be infected, potentially to a high level), and there was little evidence of Bd-induced mortality for all species. I also co-exposed hosts to another amphibian pathogen of conservation concern, ranavirus, but this did not have a significant effect on Bd infections, although there were some additional mortalities of co-exposed hosts. Next, I explored zoospore shedding rates for the different host species, and found a clear power relationship between species-specific susceptibility and infectiousness. However, for a given pathogen load, hosts across all species did not differ in pathogen shedding rates. Hence these experiments revealed a strong host identity component to contribution to environmental zoospores, but this arises purely through variation in their own infection levels, which then directly determine their zoospore shedding rate. From these experimental chapters, I conclude that tadpoles can be classified into two broad host identities (tolerant and resistant), and I used this to develop and parameterise a series of mathematical models to assess Bd transmission dynamics in multi-host tadpole communities, considering variable contact and reinfection rates. The models revealed a clear effect of host community composition. Tolerant hosts contributed disproportionately to the environmental pool of zoospores, with the subsequent increase in the force of infection from the environment driving infection dynamics in a more resistant host species. These findings, however, suggest that the process by which reinfection occurs, which can lead to increases in on-host infection loads of tolerant host, can be highly influential in determining Bd-dynamics in the wider system. These predictions therefore highlight the need to understand the fate of zoospores shed from an infected host: whether they enter the aquatic environment and contribute towards the force of infection for other hosts, or whether they engage in immediate reinfection, contributing to increases in infection load of that host. My model predictions suggest that determining the relative occurrence of these two processes may be crucial for determining community-wide Bd transmission dynamics. Overall, my work shows that tadpole Bd-load dynamics and their resulting host identity associations, hold the potential to influence community-wide infection dynamics, an understanding of which could inform more targeted mitigation strategies than otherwise possible.