Fewer frogs with extra legs: parasites

Charting the relationships between parasites and amphibians is important since few studies have examined the influence of parasite diversity on disease, and the fact that amphibians are declining faster than any group of animals on the planet due to human activities like habitat loss, pollution, and emerging diseases, says Pieter Johnson, assistant professor of ecology and evolutionary biology at the University of Colorado. For a new study published in the Proceedings of the National Academy of Sciences, researchers sampled 134 California ponds for parasites, known as trematodes, comparing their abundance and distribution to the health of more than 2,000 Pacific chorus frogs. The team combined the field studies with extensive lab experiments that charted the health of the frogs in the presence of different combinations of the six most common amphibian parasites, including the Ribeiroia group whose larvae burrow into tadpole limb regions and form cysts that disrupt normal frog and toad leg development, causing extra or missing limbs. When the chorus frogs were exposed to all six trematode types simultaneously, the infection success rate was 42 percent lower than for frogs exposed to only a single species of parasite.  “Our results show increases in parasite diversity consistently cause a decrease in infection success by the most virulent parasite,” Johnson says. 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, Johnson says. In the new 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 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. One surprising finding is 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 says. 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 says. “Collectively, our findings illustrate the importance of considering the hidden role of parasite diversity in affecting disease risk,” he explains.  “While our study was on amphibian diseases, there is ample evidence to suggest similar processes can be occurring in humans and other groups of animals.” Recent studies also have shown similar relationships between host diversity and the risk of disease in some plants, mammals, birds, and coral. A decrease in vertebrate host species for ticks carrying Lyme disease, for example, can increase the risk of Lyme disease in humans. “It could be that the most dangerous parasites occur in greater numbers in disturbed environments,” says co-author Jason Hoverman, a postdoctoral researcher working with Johnson who recently accepted a position as assistant professor at Purdue University’s forestry and natural resources department. “If we are trying to minimize disease risk in humans or in threatened groups of animals like amphibians, studies like this will be able to tell us which scenarios are most likely to occur.” The project was funded by grants from the National Science Foundation and a David and Lucile Packard Foundation fellowship awarded to Johnson in 2008. Photo/copyright David Herasimtschuk, Freshwaters Illustrated.