Host–pathogen dynamics among the invasive American bullfrog ( Lithobates catesbeianus ) and chytrid fungus ( Batrachochytrium dendrobatidis )

The combination of introduced host species and emerging pathogens can result in unanticipated disease dynamics and novel host–pathogen interactions. The American bullfrog (Lithobates catesbeianus) is a successful invasive amphibian in the western U.S. that can act as a host to the emerging fungal pathogen, Batrachochytrium dendrobatidis (Bd) implicated in the decline of amphibian populations worldwide. We examined if wild-caught invasive bullfrogs were differentially susceptible to two regionally distinct isolates of Bd. Newly metamorphosed bullfrog individuals were exposed to either a Bd strain originally isolated from bullfrogs in their endemic range or a strain from the invaded range in the western U.S. We quantified initial infection load and compared mortality rates and changes in infection load after 30 days to determine strain-specific susceptibility. Wild-caught bullfrogs from the western U.S. were particularly susceptible to an eastern Bd strain. Although infection loads were not different between strains, individuals exposed to the western strain survived, suggesting the ability to reduce their infection. Our findings highlight differences between strains and response variation of an invasive species host.