While the six parasites used in the study are responsible for about 95 percent of trematode infections in the wild, most of the world's parasites cause limited damage to host individuals, said Johnson. In the PNAS study, only two parasites, Ribeiroia and a parasite group called Echinostoma -- which can trigger amphibian mortality -- were known to be particularly dangerous to their host species.
The primary study results support the idea that higher biodiversity can help protect against certain diseases, but few previous studies had considered the diversity of the parasites themselves. Because many parasites compete with each other, ecological systems richer in parasites can act as a buffer against virulent pathogens. Johnson said the combination of extensive field and lab work helped strengthen the study results.
One surprising study finding was that under certain conditions, increases in parasite diversity could increase or decrease host disease. In that aspect of the study, the infection rates were dependent on the order in which the six parasite species were added to the habitats of the frogs, and whether newly added parasite species replaced other parasites or were added alongside them, he said.
If a dangerous parasite is first on the scene, it tends to be replaced when less dangerous species are added, decreasing the odds of host disease. But if a dangerous parasite species is added to an environment already harboring parasites, the study showed either a neutral effect or an increase in disease, Johnson said.
"Collectively, our findings illustrate the importance of considering the hidden role of parasite diversity in affecting disease risk," said Johnson. "While our study was on amphibian diseases, there is ample evidence to suggest similar processes can be occurring in humans and other
|Contact: Pieter Johnson|
University of Colorado at Boulder