patterns of amphibian infection prevalence across wetlands on the
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DISEASES OF AQUATIC ORGANISMSDis Aquat Org
Vol. 121: 114, 2016doi: 10.3354/dao03039
Published August 31
At least 40% of all amphibian populations areexperiencing declines or extinctions caused by bothanthropogenic and natural stressors (Alford &Richards 1999, Voyles et al. 2009, Blaustein et al.2011 and references therein). How these stressorsinteract and influence population declines is not wellunderstood, although infectious disease is frequently
associated with many of the documented amphibiandeclines. Our understanding of amphibian diseasedynamics and amphibian population declines can beimproved by investigating populations with knowndisease prevalence and varying degrees of diseasesensitivity. Here, we investigated infection preva-lence and co-infection rate of 2 amphibian patho-gens, Batrachochytrium dendrobatidis (Bd; a fungusthat can cause chytridiomycosis) and ranavirus, in
Inter-Research 2016 www.int-res.com*These authors contributed equally to this work**Corresponding author: email@example.com
Patterns of amphibian infection prevalence across wetlands on the Savannah River Site,
South Carolina, USA
Cara N. Love1,2,*, Megan E. Winzeler1,2,*, Rochelle Beasley1,*, David E. Scott1,*, Schyler O. Nunziata3, Stacey L. Lance1,*,**
1Savannah River Ecology Laboratory, University of Georgia, Aiken, South Carolina 29802, USA2Odum School of Ecology, University of Georgia, Athens, Georgia 30602, USA
3Department of Biological Sciences, University of Kentucky, Lexington, Kentucky 40506, USA
ABSTRACT: Amphibian diseases, such as chytridiomycosis caused by Batrachochytrium dendro-batidis (Bd) and ranaviral disease caused by ranaviruses, are often linked to global amphibianpopulation declines, yet the ecological dynamics of both pathogens are poorly understood. Thegoal of our study was to determine the baseline prevalence, pathogen loads, and co-infection rateof Bd and ranavirus across the Savannah River Site (SRS) in South Carolina, USA, a region withrich amphibian diversity and a history of amphibian-based research. We tested over 1000 individ-uals, encompassing 21 amphibian species from 11 wetlands for both Bd and ranavirus. The preva-lence of Bd across individuals was 9.7%. Using wetland means, the mean (SE) Bd prevalencewas 7.9 2.9%. Among toad species, Anaxyrus terrestris had 95 and 380% greater odds of beinginfected with Bd than Scaphiopus holbrookii and Gastrophryne carolinensis, respectively. Odds ofBd infection in adult A. terrestris and Lithobates sphenocephalus were 75 to 77% greater in metal-contaminated sites. The prevalence of ranavirus infections across all individuals was 37.4%. Meanwetland ranavirus prevalence was 29.8 8.8% and was higher in post-metamorphic individualsthan in aquatic larvae. Ambystoma tigrinum had 83 to 85% higher odds of ranavirus infection thanA. opacum and A. talpoideum. We detected a 4.8% co-infection rate, with individuals positive forranavirus having a 5% higher occurrence of Bd. In adult Anaxyrus terrestris, odds of Bd infectionwere 13% higher in ranavirus-positive animals and odds of co-infection were 23% higher in con-taminated wetlands. Overall, we found the pathogen prevalence varied by wetland, species, andlife stage.
KEY WORDS: Batrachochytrium Chytrid Metals Ranavirus Wetland
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Dis Aquat Org 121: 114, 2016
the southeastern USA, where no mass mortality eventshave been documented (Daszak et al. 2005) butwhere both pathogens have been observed (Petersonet al. 2007, Hoverman et al. 2012a). Investigatingcold spots or regions where either the pathogen isabsent or where it is present but not having negativeimpacts can provide important insights into diseasedynamics (James et al. 2015).
The Bd fungus has been detected in at least 56countries (Olson et al. 2013) and has been blamed fordramatic population declines worldwide (Berger etal. 1998, Daszak et al. 1999, Lips 1999). Additionally,ranavirus infections have been implicated in massmortality events of amphibians across North America(Green et al. 2002), South America (Fox et al. 2006),Europe (Cunningham et al. 2007, Balseiro et al.2009), Australia (Speare & Smith 1992, Laurance etal. 1996), and Asia (Une et al. 2009, Xu et al. 2010).These pathogens differ in modes of transmission andthe disease symptoms they cause. The Bd funguscauses an amphibian-specific disease that is trans-mitted through physical contact or aquatic transmis-sion. Ranavirus is found in amphibians, reptiles, andfish (Gray et al. 2009), and can be transmittedthrough ingestion or physical contact, as well as bymechanical, vertical, or aquatic transmission (Milleret al. 2011). For both Bd and ranaviruses, symptomsand transmissibility are correlated with higher loadsof zoospores (Voyles et al. 2009, 2011) or virus (Brun-ner et al. 2005), respectively. Co-infection of these 2pathogens is rarely investigated, although studies ofother pathogens demonstrate that infection with onepathogen can alter susceptibility to another (e.g.Jolles et al. 2008, Munson et al. 2008).
The US Department of Energys Savannah RiverSite (SRS; South Carolina, USA) offers an ideal oppor-tunity to investigate amphibian disease dy namicsacross a variety of wetland types with diverse amphib-ian communities (Gibbons & Semlitsch 1991). TheSRS has over 300 natural ephemeral wetlands, sev-eral constructed wetlands, and encompasses a broadrange of habitat types typical across the southeasternUSA. Bd has been present on the SRS at low fre-quency since at least the late 1970s, although it hasnot been linked to population declines (Daszak et al.2005) and is known to be widespread in the south-eastern USA. (Rothermel et al. 2008). Until now,ranaviruses have not been studied on the SRS, butthey do occur in the southeastern USA, and have ledto short-term population declines in North Carolina(Petranka et al. 2003). Furthermore, they are re -ported to be widespread in Tennessee (Hoverman etal. 2012a). The objectives of our study were to (1)
quantify a baseline prevalence of Bd and ranavirusacross multiple species and wetlands of the SRS, (2)examine patterns of disease prevalence across lifestages, (3) determine if disease prevalence differsbetween contaminated and reference wetlands, and(4) determine if Bd zoospore or ranavirus loads ofinfected individuals differ between contaminatedand reference wetlands.
MATERIALS AND METHODS
From 25 February 2012 to 12 July 2013, we oppor-tunistically sampled larval and adult amphibiansfrom 11 wetlands in Aiken and Barnwell Counties onthe US Department of Energys SRS in South Caro -lina (Fig. 1). We typically defined a population ofamphibians by the wetland boundary; however, mul-tiple ponds at one site (wetland complex A-01, seebelow) were considered a single population due toclose proximity (
Love et al.: Amphibian disease on the Savannah River Site
wetland complex was constructed in 2006/2007 totreat storm water discharge from several industrialfacilities, including a tritium treatment facility, and ithas elevated levels of metals including Cu and Zn(Flynn et al. 2015). From previous research in the H-02 wetlands conducted in 2010/2011, we had tissuesamples from 21 amphibians available to include inthis study. In the H-02 wetlands, Cu levels averaged28.51 12.75 (SD) g l1 in the influent ends and12.00 4.56 g l1 in the effluent ends, while Zn lev-els averaged 29.26 21.58 and 9.16 2.85 g l1 inthe influent and effluent ends, respectively (Flynn etal. 2015). The A-01 wetland complex, constructed in2000, is a treatment system consisting of a retentionbasin and 8 cells used to treat industrial runoff, and iselevated in Cu, Zn, and Pb concentrations (Knox etal. 2006). From April 2004 to February 2005, mean(SD) levels of Cu, Pb and Zn in the influent waterwere 25.8 11.8, 1.2 1.0, and 18.4 7.5 g l1,respectively. Despite these contaminated sites hav-ing different cocktails of chemical pollutants, weconsidered them to gether as contaminated for thepurpose of our study; we note, however, that there isno replication of contamination conditions.
To sample amphibians for pathogentesting, we used a combination ofminnow traps, active dip netting, andpitfall traps along drift fences. Whenmore than 1 individual was caught ina single trap we only sampled 1 indi-vidual from each trap to reducepotential pathogen cross-contamina-tion. We did not sanitize the dip netbetween dips or after an individualwas caught due to the limited time anindividual remained in the net andbecause each individual was prima-rily in contact with only the leaf litterfound in the net. For tadpoles andsalamander larvae, we collected asmall (~4 mm long) tail clip and foradults we collected toe clips alongwith a swab. We used tail clips forrana viral testing and swabs and tailclips for Bd testing. We did notdirectly sample the mouthparts of tad-poles. We stored all tissue samples in100% ethanol and froze swabs at20C. All handling and collectionsfollowed the guidelines outlined inPessier & Mendelson (2009). Ourstudy was conducted in accordancewith the Guide for the Care and Use
of Laboratory Animals of the National Institutes ofHealth, and the protocol was approved by the Uni-versity of Georgia Institutional Animal Care and UseCommittee (AUP A2009 10-175-Y2-A0).
We extracted DNA from tissue samples usingDNeasy Blood and Tissue kit (Qiagen) and followedthe standard extraction protocol, with the exceptionof eluting the DNA in 100 l of the included elutionbuffer. All real-time PCR (qPCR) assays were per-formed on an iCycler IQ PCR detection System (Bio-Rad).
To detect Bd, we performed qPCR reactions in 13 lcontaining 1 Taqman Universal Master Mix (LifeTechnolog