An investigation into the effects of Batrachochytrium dendrobatidis (Bd) on natterjack toad (Bufo calamita) populations in the UK

The chytridiomycete Batrachochytrium dendrobatidis (Bd) is a parasite which has been blamed for amphibian declines across the world. This study was designed to investigate the effects of Bd on natterjack toads (Bufo calamita), following the discovery of Bd populations of this species in the UK. The effect of Bd on natterjack toads was assessed by fieldwork and experiments. Wild adult natterjacks were tagged and repeatedly tested for Bd during 2009-2011. Captive adults and juveniles from infected populations were also tested in response to changes in environmental conditions. Swabs were used to collect Bd DNA from the skin of study animals. Swabbing did not reliably diagnose infection but the quantity of Bd DNA in swabs (Bd score) provided an indication of infection activity. Immersion in water appeared to trigger Bd zoospore emergence from the skin, resulting in an increase in the likelihood of Bd detection and increases in Bd score. Bd dynamics in natterjack populations were also influenced by salinity. Natterjacks in the UK are found mainly in coastal habitat, where ponds are often inundated by high tides. Adults captured in brackish water were less likely to test positive than those caught in fresh water. Bd isolated from coastal natterjacks was killed in vitro by a salinity equivalent to 50% seawater. The isolate grew fastest at low salinities, suggesting that it may have become adapted to brackish conditions. Despite this adaptation, tidal inundation may be sufficient to disinfect ponds and limit Bd transmission. Capture-mark-recapture (CMR) data from adult natterjacks revealed a weak negative correlation between Bd score and survival in the wild. Males had higher Bd scores than females but survival did not differ between sexes and there was no correlation between Bd score and growth. An experiment showed BD could kill natterjacks if infection activity was boosted by wet conditions. However only 6% of wild adults recorded Bd scores in excess of a mortality threshold derived from this experiment. Many adults and juvenile natterjacks can tolerate Bd infection and act as reservoirs of this pathogen. Despite detection of Bd in at least 14 UK natterjack populations by 2011, no mass mortalities of adult natterjacks have been reported and spawning has continued at all sites. Bd does not appear to have a major effect on natterjacks but this situation may not persist and vigilance should be maintained. Bd isolated from natterjack toads in this study belomgs to a global panzootic lineage (GPL) of Bd which Farrer et al (2011) claim has achieved a global distribution as a result of human activities.