Does early life exposure to Batrachochytrium dendrobatidis promotes tolerance to chytrid fungal infection in post-metamorphic amphibians?

Aura Y. Muñiz Torres, Julia McCartney, Brandon LaBumbard and Douglas C. Woodhams, Department of Biology, University of Massachusetts-Boston, Boston, MA

Background/Question/Methods
Many amphibian populations worldwide have been experiencing declines due to the emerging fungal pathogen Batrachochytrium dendrobatidis (Bd), which causes the disease chytridiomycosis. This disease has affected more than 500 species, with 90 species reported extinct, but recent finding suggest that some species are resilient to the disease and co-exist with the pathogen. A possible driver of this host-pathogen coexistence is the modulation of immunity during development such that frogs exposed to Bd as tadpoles gain tolerance that persists through metamorphosis even upon re-exposure to Bd. For this experiment, we wanted to know if northern leopard frog (Lithobathes pipiens) exposed to Bd as tadpoles will become resistant to the pathogen and when re-exposed as metamorphs fight the infection. In addition, we wanted to know if a particular strain of Bd is more virulent than the local Vermont isolate when infecting on a laboratory setting. We exposed L.pipiens for approximately eight weeks. Froglets were exposed to either Bd (Vermont newt strain) or (California section line strain). Frogs were swabbed, measured and weighed every other week during the experiment. Changes in mass were tested for differences among treatments, and DNA from skin swabs was extracted for qPCR to detect Bd load through time.
Results/Conclusions
Our results suggest that over 35% of the individuals survived the infection. Frogs previously exposed to Bd are more resistant to the infection when re-exposed. Even though Section line Bd is more virulent than the Vermont newt Bd, at least when infecting L.pipiens it did not appear to be more virulent. There was no significant difference between the treatments in terms of mortality/survival, but we notice that frogs exposed to the Vermont strain who were not exposed as tadpoles had more mortality, especially in the first days of infection. Also, there was no difference in changes in body conditions (weight and SVL) between the treatments. In general, the laboratory exposures showed some mortality but not as high as we expected at the beginning of the experiment, maybe they still maintain a possible infection from tadpole exposure. Our results suggest that early pathogen exposure may be beneficial to wild population since the tadpole’s immune system could adapt overtime to the infection and survival rates could be higher after reinfection.