Shelly Lyser Treatment of a Frog Fungal Pathogen May 8 2006 Water Temperature and Chytridiomycosis in Rana muscosa Larvae Shelly Lyser Abstract The fungal pathogen Batrachochytrium dendrobatidis also known as chytrid fungus has been implicated in amphibian declines worldwide Chytridiomycosis is a water borne disease that interferes with osmoregulation in amphibian skin and has the ability to rapidly wipe out populations Laboratory studies have shown that exposing adult frogs to elevated temperatures can reduce or even eliminate their infection My field study tests whether elevated temperatures can alter infection rates in frog larvae which can be a vital reservoir for the pathogen Seventy two tadpoles were removed from alpine lakes and placed in small pools with a density of twelve tadpoles per pool Half of the pools were left exposed to sunlight to allow the water to receive solar heating during the day and the other half shaded to attempt to simulate temperature fluctuations similar to a lake An eight degree difference in peak temperatures was achieved between the two groups and a 3 5 degree difference in the averages however no significant change in infection level was found after two weeks of treatment These results suggest that higher or more sustained temperature increases are required to reduce infection in tadpoles p 1 Shelly Lyser Treatment of a Frog Fungal Pathogen May 8 2006 Introduction Many studies have demonstrated that populations of amphibians are experiencing decline on a 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 on populations of other species such as reduced community stability and species diversity For example studies have shown that the dramatic decline in amphibian populations significantly alters the trophic structure of food webs in alpine lakes Finlay and Vredenburg manuscript 2005 Factors implicated in amphibian decline include habitat destruction Gray and Smith 2005 introduced species Vredenburg 2004 pesticide contamination Davidson 2004 Fellers et 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 the fungal pathogen Batrachochytrium dendrobatidis hereafter referred to as the chytrid fungus which causes the emerging infectious disease chytridiomycosis in frogs This disease has been implicated 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 fungus infects the keratinized tissue in the skin of amphibians leading to secondary skin infections internal desiccation and interference with osmoregulation which normally occurs through the skin Berger et al 1998 Nichols et al 2001 However the exact mechanisms that cause mortality have not yet been determined Amphibian larvae which carry the infection are not mortally affected until they metamorphose although one study has shown that it may create a competitive disadvantage Parris and Cornelius 2004 Some amphibian populations previously decimated by chytrid infection have rebounded in recent 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 to develop 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 some species Pounds et al 2005 These findings highlight the importance of developing management 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 chytrid infection Some of these treatment methods could have management implications A study by p 2 Shelly Lyser Treatment of a Frog Fungal Pathogen May 8 2006 Parker in 2002 shows that placing infected adult frogs in tanks containing the fungicide malachite green for twenty four hours will rid the frogs of their infection Unfortunately this fungicide is highly carcinogenic and should not be used on a large scale or in delicate ecosystems Another study conducted by Woodhams 2003 demonstrates that when infected frogs of the species Litoris chloris are subjected to periodic elevations in water temperature up to 37 C they have a greater chance of survival than those kept at room temperature Parker also specifically tested the effects of temperature treatment of Rana muscosa adults unpublished data Chytrid is known to grow equally well at 18 or 23 degrees in isolated cultures but the frog 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 the fungus as a result of these higher temperatures However no studies have tested whether elevated temperatures can effectively cure amphibians in the field For my study I have chosen to conduct a field experiment with the mountain yellow legged frog Rana muscosa an aquatic species that lives at high elevation in the Sierra Nevada This experiment addresses the questions How will elevated water temperature affect chytrid load in R muscosa larvae and Can temperature be used to treat infected populations in the field as an aid to population recovery Using larvae as the developmental stage for intervention is relevant because of how chytrid affects Rana muscosa populations In adult frogs the infection is most prevalent on the feet and ventral regions of the animal near the inner thighs 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 be fatal at this stage Individuals of this species usually remain in larval form for three to four years before they metamorphose and thus the larvae can persist and carry the infection for several years after all the adults in the population have died from infection If the larvae that remain can be cleared of the infection there is a greater chance the site will be recolonized by individuals from the metapopulation Methods My study site is Sixty Lake Basin
View Full Document
Unlocking...