Host-consumed resources increase pathogen load of endoparasites more than integument-infecting parasites

Pathogens use host resources for replication. Consequently, pathogen traits, such as where they infect a host, might allow pathogens to capitalize on host-consumed nutrients before the host. For instance, parasites with short generation times in the host gut or blood might benefit more from host-consumed resources than the host, whereas parasites in or on the host integument (e.g. skin, cuticle, exoskeleton), such as ectoparasites, might benefit less from host-consumed resources than the host. Thus, it is unclear whether increased food consumption by hosts should reduce or amplify pathogen levels. We conducted experiments on Cuban treefrogs Osteopilus septentrionalis to test how food availability affects infection levels of ranavirus and the fungal pathogen Batrachochytrium dendrobatidis (Bd), which are both associated with mass die-offs of amphibians. Given that ranavirus is an endoparasite with a much shorter generation time than the skin-infecting Bd, we postulated that ranavirus might be able to capitalize on host-consumed resources more quickly than Bd. Hence, we hypothesized that increased food availability to hosts might reduce Bd infections more than ranavirus infections. As predicted, augmenting food access decreased Bd infection intensity but increased ranavirus infection intensity. Future work should assess whether pathogen traits, such as generation time and where they infect hosts, generally affect whether food resources more positively benefit hosts or pathogens.