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According to CU-Boulder Assistant Professor Pieter Johnson,scientists are concerned about how changes in biodiversity affectthe risk of infectious diseases in humans and wildlife. Chartingthe relationships between parasites and amphibians is importantsince few studies have examined the influence of parasite diversityon disease, and the fact that amphibians are declining faster thanany group of animals on the planet due to human activities likehabitat loss, pollution and emerging diseases, Johnson said. In the new study, the team sampled 134 California ponds for theparasites, known as trematodes, comparing their abundance anddistribution with the health of more than 2,000 Pacific chorusfrogs. The CU team combined the field studies with extensive labexperiments that charted the health of the frogs in the presence ofdifferent combinations of the six most common amphibian parasites,including the Ribeiroia group whose larvae burrow into tadpole limbregions and form cysts that disrupt normal frog and toad legdevelopment, causing extra or missing limbs. The new study showed when the chorus frogs were exposed to all sixtrematode types simultaneously, the infection success rate was 42percent lower than for frogs exposed to only a single species ofparasite. "Our results show increases in parasite diversityconsistently cause a decrease in infection success by the mostvirulent parasite," said Johnson of the ecology andevolutionary biology department. A paper by Johnson and co-author Jason Hoverman, a CU-Boulderpostdoctoral researcher, appears in this week's issue of the Proceedings of the National Academy of Sciences . The project was funded by grants from the National ScienceFoundation and a David and Lucile Packard Foundation fellowshipawarded to Johnson in 2008. While the six parasites used in the study are responsible for about95 percent of trematode infections in the wild, most of the world'sparasites cause limited damage to host individuals, said Johnson.In the PNAS study, only two parasites, Ribeiroia and a parasitegroup 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 biodiversitycan help protect against certain diseases, but few previous studieshad considered the diversity of the parasites themselves. Becausemany parasites compete with each other, ecological systems richerin parasites can act as a buffer against virulent pathogens.Johnson said the combination of extensive field and lab work helpedstrengthen the study results. One surprising study finding was that under certain conditions,increases in parasite diversity could increase or decrease hostdisease. In that aspect of the study, the infection rates weredependent on the order in which the six parasite species were addedto the habitats of the frogs, and whether newly added parasitespecies replaced other parasites or were added alongside them, hesaid. If a dangerous parasite is first on the scene, it tends to bereplaced when less dangerous species are added, decreasing the oddsof host disease. But if a dangerous parasite species is added to anenvironment already harboring parasites, the study showed either aneutral effect or an increase in disease, Johnson said. "Collectively, our findings illustrate the importance ofconsidering the hidden role of parasite diversity in affectingdisease risk," said Johnson. "While our study was onamphibian diseases, there is ample evidence to suggest similarprocesses can be occurring in humans and other groups ofanimals." Recent studies also have shown similar relationships between hostdiversity and the risk of disease in some plants, mammals, birdsand coral. A decrease in vertebrate host species for ticks carryingLyme disease, for example, can increase the risk of Lyme disease inhumans, said Johnson. "It could be that the most dangerous parasites occur ingreater numbers in disturbed environments," said Hoverman, whorecently accepted a position as assistant professor at PurdueUniversity's forestry and natural resources department. "If weare trying to minimize disease risk in humans or in threatenedgroups of animals like amphibians, studies like this will be ableto tell us which scenarios are most likely to occur." The new study has implications for declining biodiversity beingseen across the planet as a result of human activities, includingamphibians, said Johnson. Roughly 40 percent of amphibian speciesaround the world are in decline, and more than 200 have goneextinct since the 1970s, some as a result of the often-fatalchytrid fungus that infects amphibian skin. Some scientists arguethat rapid global amphibian decline seen today is driving the nextgreat mass extinction event, he said. Trematodes have a complex life cycle that involve snails,amphibians and predators. Host snails release parasite larvae inthe water, infecting amphibians and causing deformities thatinclude extra or missing legs. Deformed frogs and toads rarelysurvive long because of their susceptibility to predators likewading birds, which ingest them and later release trematodes thatinfect other snails, completing the life cycle. Deformed frogs first gained attention in the mid-1990s when a groupof Minnesota schoolchildren discovered a pond where more than halfof the leopard frogs had missing or extra limbs, said Johnson.Since then reports of deformed amphibians have been widespread inthe United States, leading to speculation they were being caused byfactors like pollution, increased ultraviolet radiation orparasitic infection. A 2008 study by Johnson showed American toads who pal around withgray tree frogs reduce their chances of parasitic infections knownto cause limb malformations because trematode larva that infecttree frog tadpoles are killed by the tadpoles' immune systems. In2007, Johnson led a study showing high levels of nutrients likenitrogen and phosphorus used in North American farming and ranchingactivities fuel trematode infections by elevating the abundance andreproduction of snail species that host the parasites. 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