Shelly Lyser Treatment of a Frog Fungal Pathogen May 8 2006p. 1Water Temperature and Chytridiomycosis in Rana muscosa LarvaeShelly LyserAbstract The fungal pathogen Batrachochytrium dendrobatidis, also known as chytrid fungus,has been implicated in amphibian declines worldwide. Chytridiomycosis is a water-bornedisease that interferes with osmoregulation in amphibian skin, and has the ability to rapidly wipeout populations. Laboratory studies have shown that exposing adult frogs to elevatedtemperatures can reduce or even eliminate their infection. My field study tests whether elevatedtemperatures can alter infection rates in frog larvae, which can be a vital reservoir for thepathogen. Seventy-two tadpoles were removed from alpine lakes and placed in small pools witha density of twelve tadpoles per pool. Half of the pools were left exposed to sunlight to allow thewater to receive solar heating during the day, and the other half shaded to attempt to simulatetemperature fluctuations similar to a lake. An eight-degree difference in peak temperatures wasachieved between the two groups and a 3.5-degree difference in the averages, however nosignificant change in infection level was found after two weeks of treatment. These resultssuggest that higher or more sustained temperature increases are required to reduce infection intadpoles.Shelly Lyser Treatment of a Frog Fungal Pathogen May 8 2006p. 2IntroductionMany studies have demonstrated that populations of amphibians are experiencing decline ona global scale (Blaustein et al. 1993, Hecnar 1996, Alford and Richards 1999, Stuart et al. 2004).The loss of amphibians from communities is likely to create both direct and indirect effects onpopulations of other species, such as reduced community stability and species diversity. Forexample, studies have shown that the dramatic decline in amphibian populations significantlyalters the trophic structure of food webs in alpine lakes (Finlay and Vredenburg manuscript2005). Factors implicated in amphibian decline include habitat destruction (Gray and Smith2005), introduced species (Vredenburg 2004), pesticide contamination (Davidson 2004, Fellerset al. 2004), acid rain (Watkins-Colwell and Watkins-Colwell 1998), increased UV radiation(Diamond et al. 2002), and disease (Greer et al. 2005, Hero and Morrison 2004). I focus on thefungal pathogen Batrachochytrium dendrobatidis, hereafter referred to as the ‘chytrid fungus’,which causes the emerging infectious disease chytridiomycosis in frogs. This disease has beenimplicated in the disappearance or decline of many frog populations in Europe, Australia,Central America, North America, and parts of Asia (Berger et al 1998). The chytrid fungusinfects the keratinized tissue in the skin of amphibians, leading to secondary skin infections,internal desiccation, and interference with osmoregulation which normally occurs through theskin (Berger et al. 1998, Nichols et al. 2001). However, the exact mechanisms that causemortality have not yet been determined. Amphibian larvae which carry the infection are notmortally affected until they metamorphose, although one study has shown that it may create acompetitive disadvantage (Parris and Cornelius 2004). Some amphibian populations previously decimated by chytrid infection have rebounded inrecent years, displaying new resistance to the fungus (McDonald et al. 2005, Rollins-Smith et al.2005). Their persistence could be the result of natural selection which allows for populations todevelop immunological defenses to the fungus or a change in behavior to inhibit fungal growth.Not all populations recover, however, and the initial outbreak resulted in the extinction of somespecies (Pounds et al. 2005). These findings highlight the importance of developingmanagement strategies to help populations survive the initial epidemic of chytrid infection,thereby providing the opportunity for more gradual adaptation and recovery (McDonald et al.2005). Laboratory studies have been conducted to test methods for clearing frogs of chytridinfection. Some of these treatment methods could have management implications. A study byShelly Lyser Treatment of a Frog Fungal Pathogen May 8 2006p. 3Parker in 2002 shows that placing infected adult frogs in tanks containing the fungicidemalachite green for twenty-four hours will rid the frogs of their infection. Unfortunately, thisfungicide is highly carcinogenic and should not be used on a large scale or in delicateecosystems. Another study conducted by Woodhams (2003) demonstrates that when infectedfrogs of the species Litoris chloris are subjected to periodic elevations in water temperature (upto 37°C), they have a greater chance of survival than those kept at room temperature. Parker alsospecifically tested the effects of temperature treatment of Rana muscosa adults (unpublisheddata). Chytrid is known to grow equally well at 18 or 23 degrees in isolated cultures, but thefrog’s infection level declined in this experiment at the higher end of this temperature spectrum.This result implies that the frogs may exhibit an immunological or metabolic response to thefungus as a result of these higher temperatures. However, no studies have tested whetherelevated temperatures can effectively cure amphibians in the field. For my study, I have chosento conduct a field experiment with the mountain yellow-legged frog, Rana muscosa, an aquaticspecies that lives at high elevation in the Sierra Nevada.This experiment addresses the questions: “How will elevated water temperature affectchytrid load in R. muscosa larvae?” and “Can temperature be used to treat infected populations inthe field as an aid to population recovery?” Using larvae as the developmental stage forintervention is relevant because of how chytrid affects Rana muscosa populations. In adultfrogs, the infection is most prevalent on the feet and ventral regions of the animal near the innerthighs, which are important areas for respiration, water uptake and gas exchange. In the larvae,however, chytrid infection is found only in the mouthparts and its effects do not appear to befatal at this stage. Individuals of this species usually remain in larval form for three to four yearsbefore they metamorphose, and thus the larvae can persist and carry the infection for severalyears after all the adults in the population have died from infection. If the larvae that remain canbe cleared of the infection, there
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