The epidemiology of the amphibian pathogen Batrachochytrium dendrobatidis in the UK
Creators: name: Smith, Freya
Publisher: name: Imperial College London
Year: 2013

description: Batrachochytrium dendrobatidis (Bd) is a recently emerged multihost fungal pathogen, which has rapidly spread globally. It has been detected in 52 different countries, and in over 500 amphibian species. In susceptible hosts, it causes the disease chytridiomycosis. Although globally, Bd causes amphibian declines (and in some parts of the world, has resulted in a 40% loss of amphibian species), host responses are inconsistent and Bd appears able to coexist with some amphibian species or assemblages of species in a state of endemism. Batrachochytrium dendrobatidis was first detected in wild amphibians in the UK in 2004, in an introduced population of North American bullfrogs (Lithobates catesbeianus). Apparent spillover of infection into native species, including deaths in wild common toad (Bufo bufo) metamorphs at the index site, and in natterjack toads (Epidalea calamita) bred for reintroduction, has led to considerable concern for the conservation status of our native amphibian species. This thesis reports the results of cross-sectional, longitudinal and experimental investigations into the epidemiology of Bd in the UK. National cross-sectional surveys were conducted in 2008 and 2011, during which skin swabs were collected from almost 9000 amphibians and tested for the presence of Bd DNA using real-time PCR. Infection was detected at 30 sites (16% total), in all six native amphibian species tested, and in three of four non-native amphibian species tested. There was no evidence of change in either the distribution or prevalence of infected sites between the two survey years. There was no obvious spatial clustering and intra-site prevalence was almost uniformly low (median 10-12%). The results showed a strong positive association between Bd occurrence and the presence of non-native amphibian species, suggesting that co-introduction with non-native amphibians may have played a significant role in the current distribution of infection in the UK. Infection was also associated with sites occupied by natterjack toads, a species that has been the subject of long-term conservation management (including translocation events), providing further evidence that human-assisted movement of amphibians has contributed to the current distribution of Bd in the UK. A longitudinal study conducted in 2010 found a strong seasonal pattern of infection in newt species, with a higher prevalence of infection during the later, warmer months of the breeding season. Seasonal variation in Bd infection (and chytridiomycosis) has been commonly reported. However previous efforts have typically been conducted in tropical rather than temperate climates, where this pattern is reversed. This study also found, in terrestrially sampled common toads, an apparent increase in the prevalence of infection post-breeding suggesting that the breeding period is associated with an increased exposure or susceptibility to Bd (or both). Finally, two large-scale experiments were conducted to investigate infection in smooth newts (Lissotriton vulgaris) and palmate newts (Lissotriton helveticus). There was no evidence that experimental exposure to Bd was associated with increased mortality, or morbidity. In addition, even at high levels of exposure, infection was both rare and short lived, suggesting that these two native species are unlikely to provide a reservoir for infection in the UK. Overall, these results show a widespread but patchy distribution of infection, consistent with multiple point-introductions. At infected sites, the prevalence of Bd was low, and infected individuals had low infectious burdens, suggesting that the full expression of chytridiomycosis is not present in the UK at this time. A strong correlation with non-native amphibians and no evidence of broad-scale range expansion between 2008 and 2011 suggest that natural colonisation of ponds may not play a substantial role in the epidemiology of infection in this country. As a result, a combination of strict biosecurity protocols for fieldworkers, avoidance of long-distance translocations of native species and the prevention of future release of non-native species may be sufficient to control further spread of infection in the UK.