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      Pesticide Mixtures, Endocrine Disruption, and Amphibian Declines: Are We Underestimating the Impact?

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          Abstract

          Amphibian populations are declining globally at an alarming rate. Pesticides are among a number of proposed causes for these declines. Although a sizable database examining effects of pesticides on amphibians exists, the vast majority of these studies focus on toxicological effects (lethality, external malformations, etc.) at relatively high doses (parts per million). Very few studies focus on effects such as endocrine disruption at low concentrations. Further, most studies examine exposures to single chemicals only. The present study examined nine pesticides (four herbicides, two fungicides, and three insecticides) used on cornfields in the midwestern United States. Effects of each pesticide alone (0.1 ppb) or in combination were examined. In addition, we also examined atrazine and S-metolachlor combined (0.1 or 10 ppb each) and the commercial formulation Bicep II Magnum, which contains both of these herbicides. These two pesticides were examined in combination because they are persistent throughout the year in the wild. We examined larval growth and development, sex differentiation, and immune function in leopard frogs ( Rana pipiens). In a follow-up study, we also examined the effects of the nine-compound mixture on plasma corticosterone levels in male African clawed frogs ( Xenopus laevis). Although some of the pesticides individually inhibited larval growth and development, the pesticide mixtures had much greater effects. Larval growth and development were retarded, but most significantly, pesticide mixtures negated or reversed the typically positive correlation between time to metamorphosis and size at metamorphosis observed in controls: exposed larvae that took longer to metamorphose were smaller than their counterparts that metamorphosed earlier. The nine-pesticide mixture also induced damage to the thymus, resulting in immunosuppression and contraction of flavobacterial meningitis. The study in X. laevis revealed that these adverse effects may be due to an increase in plasma levels of the stress hormone corticosterone. Although it cannot be determined whether all the pesticides in the mixture contribute to these adverse effects or whether some pesticides are effectors, some are enhancers, and some are neutral, the present study revealed that estimating ecological risk and the impact of pesticides on amphibians using studies that examine only single pesticides at high concentrations may lead to gross underestimations of the role of pesticides in amphibian declines.

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          Chytridiomycosis causes amphibian mortality associated with population declines in the rain forests of Australia and Central America.

          Epidermal changes caused by a chytridiomycete fungus (Chytridiomycota; Chytridiales) were found in sick and dead adult anurans collected from montane rain forests in Queensland (Australia) and Panama during mass mortality events associated with significant population declines. We also have found this new disease associated with morbidity and mortality in wild and captive anurans from additional locations in Australia and Central America. This is the first report of parasitism of a vertebrate by a member of the phylum Chytridiomycota. Experimental data support the conclusion that cutaneous chytridiomycosis is a fatal disease of anurans, and we hypothesize that it is the proximate cause of these recent amphibian declines.
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            A simplified table for staging anuran embryos larvae with notes on identification

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              Global Amphibian Declines: A Problem in Applied Ecology

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                Author and article information

                Journal
                Environ Health Perspect
                Environmental Health Perspectives
                National Institute of Environmental Health Sciences
                0091-6765
                April 2006
                24 January 2006
                : 114
                : S-1
                : 40-50
                Affiliations
                Laboratory for Integrative Studies in Amphibian Biology, Department of Integrative Biology, Museum of Vertebrate Zoology, Group in Endocrinology, and Energy and Resources Group, University of California, Berkeley, California, USA
                Author notes
                Address correspondence to T.B. Hayes, Laboratory for Integrative Studies in Amphibian Biology, Department of Integrative Biology, University of California, Berkeley, CA 94720-3140 USA. Telephone: (510) 643-1054. Fax: (510) 643-6264. E-mail tyrone@ 123456berkeley.edu

                The authors declare they have no competing financial interests.

                Article
                ehp0114s1-000040
                10.1289/ehp.8051
                1874187
                16818245
                91612fac-0f46-474d-a626-e2c7ce3b7acc
                This is an Open Access article: verbatim copying and redistribution of this article are permitted in all media for any purpose, provided this notice is preserved along with the article's original DOI
                History
                : 31 January 2005
                : 23 January 2006
                Categories
                Monograph

                Public health
                amphibians,corticosterone,atrazine,development,immunosuppression,endocrine disruption,mixtures,amphibian declines,growth

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