Disease ecology has traditionally focused on a single-pathogen-single-host system, but growing evidence suggests that addressing the more realistic situation of multiple pathogens within a host leads to more accurate knowledge of host pathology, pathogen transmission, and virulence evolution. Here, we address how coinfection in amphibians affects pathogen prevalence and intensity in multiple species of hosts. The pathogens investigated were Batrachochytrium dendrobatidis (Bd), a chytrid fungus that grows on the skin of post-metamorphic amphibians and is implicated in worldwide amphibian declines, Ranavirus, a virus that replicates in the internal organs of amphibians causing hemorrhaging and mass mortality, and Echinostoma trivolvis, a trematode that encysts in the kidneys of amphibians and can cause impaired growth or osmoregulation. Pacific tree frogs (Pseudacris regilla) and western toads (Anaxyrus boreas) were exposed to one, two, or all three pathogens in a laboratory setting.
Results/Conclusions:
Snout-vent length, Gosner developmental stage and survival all decreased as the intensity of infection or exposure to more than one pathogen increased. While infection intensity of Ranavirus and Bd did not change when an animal was exposed to multiple pathogens, but in H. regilla, Echinostoma trivolvis metacercariae loads were reduced by 52 percent when individuals were coinfectioned with Bd or Ranavirus. This result suggests either competition among pathogens, or that the amphibian immune response to cercariae is heightened by exposure to other pathogens. Growth rates and, in turn, reproduction is likely to be lower in co-infected amphibians than predicted when addressing single infections. These results suggest that coinfections could have important but underappreciated effects on amphibians, which are one of the most threatened vertebrate taxa. Consequently, further studies are needed to more fully understand the consequences of infections with multiple pathogens.