Data from: Is exposure to chytrid fungus and ranavirus higher in ponds invaded by American bullfrogs?

Aim – The spread of emerging infectious diseases (EIDs) and non-native invasive species (NIS) are interconnected processes driving biodiversity loss. A key example involves Batrachochytrium dendrobatidis (Bd) and ranavirus (Rv), two pathogens that have contributed to global amphibian declines. Their occurrence has been linked to invasive American bullfrogs (Lithobates catesbeianus) serving as asymptomatic vectors. To determine the relationship between Bd and Rv exposure and bullfrogs, we investigated whether pathogen occurrence and environmental loads are correlated with the presence and abundance of bullfrogs, whether the exposure window is extended by year-round tadpole presence, and whether pathogen spread is promoted through juvenile migration.

Location – The bullfrog-invaded area in Belgium (~400 km²).

Methods – We sampled 157 ponds, including four with bullfrog and Bd presence monitored monthly for two years, using quantitative environmental DNA (eDNA) barcoding. We validated a novel duplexed assay to simultaneously detect Bd and Rv, and quantified eDNA concentrations of bullfrogs, Bd, and Rv serving as proxies for abundance and pathogen loads. We then related the presence and (seasonal) abundance of bullfrogs to that of Bd and Rv.

Results – Bd was detected more frequently and at higher loads when bullfrogs were present and abundant. Rv detection likelihood, but not load, increased with bullfrog abundance, yet no relationship was found between bullfrog presence and Rv presence or load. Seasonal fluctuations in Bd loads varied between ponds with and without breeding activity. In the former, Bd was consistently detected, likely due to overwintering tadpoles. In the latter, Bd was detected inconsistently, with peak eDNA concentrations coinciding with juvenile immigration. 

Conclusions – Our findings indicate that amphibian exposure to Bd, and to a lesser extent Rv, is higher when bullfrogs are present and abundant. The duplexed eDNA assay provides a cost-efficient tool for monitoring pathogens threatening amphibians globally, aiding in invasive bullfrog management and EID mitigation.