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The More Parasites, the Better?

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Science  29 Nov 2013:
Vol. 342, Issue 6162, pp. 1041
DOI: 10.1126/science.342.6162.1041-a

S. Altizer et al.'s Review “Climate change and infectious diseases: From evidence to a predictive framework” (2 August, p. 514) suggests that increased temperatures will favor several attributes of virulent pathogens that will adversely affect host health. Altizer et al. predict a decline in parasite biodiversity but overlook an important problem: This loss could have dire consequences to ecosystems.

Parasite diversity benefits ecosystems by regulating host population dynamics, increasing connectivity and stability in food webs (1) and decreasing community-level disease risk. For example, increased parasite richness reduced transmission of the virulent trematode Ribeiroia to amphibian hosts by more than 50% (2). This decrease in disease risk may be due to either increased parasite competition within intermediate hosts (2) or antiparasite immune responses increasing immune genetic diversity in hosts (3). Pathogens can also have a mediating effect on interspecific competition between shared hosts, as in the case of Anolis gingivinus lizards in the Caribbean, which exclude sister species A. wattsi except when their competitive ability is diminished by the presence of Plasmodium azurophilum (4). Parasites likely mediate such interactions largely through immune costs, with hosts trading off resource use between immune responses and reproduction and growth (5). Thus, although some virulent parasite populations may increase with climate change, we anticipate that the loss of parasite biodiversity will result in more widespread and unpredictable threats to ecosystem health. We therefore call for further research into parasite ecology and host-parasite coextinctions as tools for quantifying ecosystem vulnerability to climate change.

Frog with parasite-induced (Ribeiroia ondatrae) limb malformation.

In creased parasite diversity reduced the transmission of Ribeiroia to amphibian hosts.

CREDIT: BRETT A. GOODMAN, PIETER T. J. JOHNSON/WIKIMEDIA COMMONS

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