Ontogeny of immunity and potential implications for co-infection

Immunity changes through ontogeny and can mediate facilitative and inhibitory interactions among co-infecting parasite species. In amphibians, most immune memory is not carried through metamorphosis, leading to variation in the complexity of immune responses across life stages. To test if the ontogeny of host immunity might drive interactions among co-infecting parasites, we simultaneously exposed Cuban treefrogs (Osteopilus septentrionalis) to a fungus (Bactrachochytrium dendrobaditis) and a nematode (Aplectana hamatospicula) at tadpole, metamorphic, and post-metamorphic life stages. We measured metrics of host immunity, host health, and parasite abundance. We predicted facilitative interactions between co-infecting parasites as the different immune responses hosts mount to combat these infectious are energetically challenging to mount simultaneously. We found ontogenetic differences in IgY levels and cellular immunity but no evidence that metamorphic frogs were more immunosuppressed than tadpoles. There was also little evidence that these parasites facilitated one another and no evidence that A. hamatospicula infection altered host immunity or health. However, Bd, which is known to be immunosuppressive, decreased immunity in metamorphic frogs. This made metamorphic frogs both less resistant and less tolerant of Bd infection than the other life stages. These findings indicate that changes in immunity altered host responses to parasite exposures throughout ontogeny.