Overview of Amphibian Diseases
Disease has been implicated as a factor in the decline of amphibian populations worldwide (Blaustein et al. 1994b, Laurance et al. 1996, Berger et al. 1998, Daszak 2000, Kiesecker et al. 2001) as have other factors including habitat loss and fragmentation, chemical pollution, climate change, introduction of exotic species, increased ultraviolet radiation, and natural population fluctuations (Pechmann et al. 1991, Blaustein et al. 1994a, Blaustein and Wake 1995, Knapp and Matthews 2000, Hero and Shoo in press). Several of these other factors have been suggested to work synergistically with disease (Fellers et al. 2001, Kiesecker at al. 2001).
Emerging diseases are those that have increased in incidence, virulence or geographic range, have shifted hosts or have recently evolved new strains. When outbreaks of an infectious disease become widespread, resulting in mass mortality, it is considered an epidemic. Epidemics play an important role in the dynamics of many animal populations, including humans. For example, the Black Plague killed an estimated one third of the European population. Epidemics can have severe consequences on wildlife populations if they sufficiently reduce population densities to allow future stochastic events to cause a local or global extinction (Scott 1988, Harwood and Hall 1990).
The human species now numbers 6.3 billion, and collectively we have altered between one-third and one-half of the Earth’s land surface (Kiesecker et al. 2004). At least 20 major human diseases have reemerged in more virulent forms in the past two decades and more than 30 new diseases, including Ebola, AIDS and SARS, have emerged. In addition, several wildlife species and domestic animals have been killed in large numbers in the past 10 years by new diseases such as the canine distemper virus and mad cow disease. How is the global decline of amphibians related to increased disease prevalence among humans and wildlife? An article by Kiesecker et al. (2004) reviews the relationship between the global decline in amphibians and the rise in new and resurgent human infectious diseases. This article also gives detailed examples of the role of infectious diseases in amphibian population declines. Here, we briefly review infectious diseases reported in amphibians with an emphasis on diseases associated with population decline.
Examples of pathogens causing infectious disease in amphibians include viral, bacterial, water mold, metazoan, trematode, and fungal agents and are associated with varying levels of mortality and population decline (Wright and Whitaker 2001).
Viruses belonging to the family Iridoviridae have been associated with mass mortality in the common frog (Rana temporaria) and the federally listed Sonora tiger salamanders (Ambystoma tigrinum stebbinsi) (Cunningham et al. 1996, Jancovich et al. 1997). Iridoviruses (Ranavirus spp.) have been recently isolated in several other amphibians (Zupanovic et al. 1998, Mao et al. 1999) and have been associated with disease outbreaks in both captive and wild populations (Chinchar 2002).
The bacterial pathogen characteristic of red-legged disease, Aeromonas hydrophila, was isolated in the mountain yellow-legged frog (Rana muscosa) in Kings Canyon National Park, California, and believed to be responsible for a massive die-off in 1979 (Bradford 1991), as well as implicated in die-offs of boreal toads (Bufo boreas boreas) (Carey 1993).
A metazoan agent in Class Myxozoa (currently identified as Myxozoan sp.) has been found recently in mountain yellow-legged frog (Rana muscosa) tadpoles (J. Parker, pers. comm.). Further investigation is required to identify this agent to species and to understand how widespread it may be in nature.
Trematode infestation has been implicated in limb deformities in the pacific treefrog (Pseudacris regilla) (Johnson et al. 1999) and several other species of amphibians (Johnson et al. 2002). For further information please see: amphibian malformations research.
Fungal pathogens affecting amphibians include the pathogenic fungus Batrachochytrium dendrobatidis (Bd), which has been has been implicated as the cause of significant population decline of amphibians in the Americas and Australia (Berger et al. 1998, Carey et al. 1999; see section below for further discussion). A pathogenic water mold, Saprolegnia ferax, an important worldwide pathogen of fish, has been identified in western toad eggs (Bufo boreas) and appears to be largely responsible for egg mortality in several amphibian species (Blaustein et al. 1994b, Kiesecker et al. 2001). Extinctions of Wyoming toads (Bufo baxteri) are due primarily to the parasitic fungus Basidiobolus ranarum (Taylor et al. 1999), and the fungus Mucor amphibiorum has been reported in wild native tree frogs in Australia (Litoria caerulea) with unknown incidence and population effects (Berger et al. 1997).
Chytridiomycosis, the disease caused by the chytrid fungus. While many diseases have been observed within amphibian populations, the pathogenic chytrid fungus, Batrachochytrium dendrobatidis, has been identified as having severe impacts upon amphibian populations at a number of locations around the world, specifically in Australia (Berger et al. 1998), Central America (Lips 1998), USA (Fellers et al. 2001), South America (Young et al. 2001), and Spain (Bosch et al. 2001). For more information, see below for spatialepidemiology.net's map of worldwide distribution of chytrid fungus in amphibians. In several cases chytrid fungus was found on dead and dying frogs suggesting that chytridiomycosis was likely to be the primary cause of death (Berger et al. 1998, Bosch et al. 2001). This has been supported by a number of experiments where inoculation with chytrid fungus spores caused death in amphibians (Nichols et al. 1999, Parker et al. in prep.). Chytrid fungus infects keratinized mouthparts of tadpoles and keratinized adult frog skin, and has caused significant dieoffs in the Sierra Nevada mountains of California, USA.
Worldwide Distribution of Batrachochytrium dendrobatidis (the "chytrid fungus")
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Taken from Fisher et al. (2009); the image is a screenshot from
It is uncertain whether these infectious diseases are new agents that have been recently spread to the amphibian's geographic range (Laurance et al. 1996, Lips 1999), or old agents that previously co-existed with the amphibian but with recently increased pathogenicity, or whether amphibian immune function has recently decreased (Carey 1993). Disease introduction and/or transmission may involve human mediation. It is most likely that multiple factors are involved and that no one factor can explain all amphibian declines (Blaustein and Wake 1995, Kiesecker et al. 2001).
Written by Lara Rachowicz, Department of Integrative Biology, University of California, Berkeley, larar AT socrates.berkeley.edu
Vance Vredenburg, August 6, 2008, Department of Biology, San Francisco State University, San Francisco, California, USA, vancev AT sfsu.edu
Berger, L., Speare, R., and Humphrey, J. 1997. Mucormycosis in a free-ranging green tree frog from Australia. Journal of Wildlife Diseases 33: 903-907.
Berger, L., Speare, R., Daszak, P., Green, D. E., Cunningham, A. A., Goggin, C. L., Slocombe, R., Ragan, M. A., Hyatt, A. D., McDonald, K. R., Hines, H. B., Lips, K. R., Marantelli, G., and Parkes, H. 1998. Chytridiomycosis causes amphibian mortality associated with population declines in the rain forests of Australia and Central America. Proceedings of the National Academy of Sciences of the United States of America 95: 9031-9036.
Blaustein, A. R., Hoffman, P. D., Hokit, D. G., Kiesecker, J. M., Walls, S. C., and Hays, J. B. 1994a. UV repair and resistance to solar UV-B in amphibian eggs: A link to population declines? Proceedings of the National Academy of Sciences of the United States of America 91: 1791-1795.
Blaustein, A. R., Hokit, D. G., O'Hara, R. K., and Holt, R. A. 1994b. Pathogenic fungus contributes to amphibian losses in the Pacific Northwest. Biological Conservation 67: 251-254.
Blaustein, A. R., and Wake, D. B. 1995. The puzzle of declining amphibian populations. Scientific American 272: 52-57.
Bosch, J., Martinez-Solano, I., and Garcia-Paris, M. 2001. Evidence of a chytrid fungus infection involved in the decline of the common midwife toad (Alytes obstetricans) in protected areas of central Spain. Biological Conservation 97: 331-337.
Bradford, D. F. 1991. Mass mortality and extinction in a high elevation population of Rana muscosa. Journal of Herpetology 25: 369-377.
Bradley G. A., Rosen, P. C., Sred, M. J., Jones, T. R., and Longcore, J. E. 2002. Chytridiomycosis in native Arizona Frogs. Journal of Wildlife Diseases, 38(1): 206–212
Carey, C. 1993. Hypothesis concerning the causes of the disappearance of boreal toads from the mountains of Colorado. Conservation Biology 7: 355-362.
Carey, C., Cohen, N., and Rollins-Smith, L. 1999. Amphibian declines: An immunological perspective. Developmental & Comparative Immunology 23: 459-472.
Chinchar, V.G. 2002. Ranaviruses (family Iridoviridae): emerging cold-blooded killers. Arch. Virol. 147: 447-470.
Cunningham, A. A., Langton, T. E. S., Bennett, P. M., Lewin, J. F., Drury, S. E. N., Gough, R. E., and Macgregor, S. K. 1996. Pathological and microbiological findings from incidents of unusual mortality of the common frog (Rana temporaria). Philosophical Transactions of the Royal Society of London B Biological Sciences 351: 1539-1557.
Daszak, P., Cunningham, A. A., and Hyatt, A. D. 2000. Emerging infectious diseases of wildlife: threats to biodiversity and human health. Science 287: 443-449.
Daszak, P., Scott, D. E., Kilpatrick, A. M., Faggoni, C., Gibbons, J. W., and Porter, D. 2005. Amphibian population declines at savannah river site are linked to climate, not chytridiomycosis. Ecology, 86(12): 3232–3237.
Fellers, G. M., Green, D. E., and Longcore, J. E. 2001. Oral chytridiomycosis in the mountain yellow-legged frog (Rana muscosa). Copeia 2001: 945-953.
Harris R. N., James, T. Y., Lauer, A., Simon, M. A., and Patel, A. 2006. Amphibian pathogen Batrachochytrium dendrobatidis is inhibited by the cutaneous bacteria of amphibian species. EcoHealth. 3(1): 53-56.
Harwood, J., and Hall, A. 1990. Mass mortality in marine mammals: its implications for population dynamics and genetics. Trends in Ecology and Evolution 5:254-257.
Jancovich, J. K., Davidson, E. W., Morado, J. F., Jacobs, B. L., and Collins, J. P. 1997. Isolation of a lethal virus from the endangered tiger salamander Ambystoma tigrinum stebbinsi. Diseases of Aquatic Organisms 31: 161-167.
Jennings, M. R., and Hayes, M. P. 1994. Amphibian and reptile species of special concern. California Department of Fish and Game, Rancho Cordova, CA, USA.
Johnson, P. T. J., Lunde, K. B., Ritchie, E. G. and Launer, A. E. 1999. The effect of trematode infection on amphibian limb development and survivorship. Science 284: 802-804.
Johnson, P. T. J., Lunde, K. B., Thurman, E. M., Ritchie, E. G., Wray, S. N., Sutherland, D. R., Kapfer, J. M., Frest, T. J., Bowerman, J., and Blaustein, A. R. 2002. Parasite (Ribeiroia ondatrae) infection linked to amphibian malformations in the western United States. Ecological Monographs 72: 151-168.
Kiesecker, J. M., Belden, L. K., Shea, K., and Rubbo, M. J. 2004. Amphibian decline and emerging disease. American Scientist 92: 138-147.
Kiesecker, J. M., Blaustein, A. R., and Belden, L. K. 2001. Complex causes of amphibian population declines. Nature 410: 681-684.
Knapp, R. A., and Matthews, K. R. 2000. Non-native fish introductions and the decline of the mountain yellow-legged frog from within protected areas. Conservation Biology 14: 428-438.
Laurance, W. F., McDonald, K. R., and Speare, R. 1996. Epidemic disease and the catastrophic decline of Australian rain forest frogs. Conservation Biology 10: 406-413.
Lips, K. R. 1998. Decline of a tropical montane amphibian fauna. Conservation Biology 12:106-117.
Lips, K. R. 1999. Mass mortality and population declines of anurans at an upland site in western Panama. Conservation Biology 13: 117-125.
Lips K. R., Green, D. E., and Papendick, R. 2003. Chytridiomycosis in wild frogs from southern Costa Rica. Journal of Herpetology 37(1): 215-218.
Longcore J. E., Pessier, A. P., and Nichols, D. K. 1999. Batrachochytrium dendrobatidis gen. et sp. nov., a chytrid pathogenic to amphibians. Mycologia 91(2): 219-227.
Mao, J., Green, D. E., Fellers, G., and Chinchar, V. G. 1999. Molecular characterization of iridoviruses isolated from sympatric amphibians and fish. Virus Research 63: 45-52.
McCallum, H. 2005. Inconclusiveness of chytridiomycosis as the agent in widespread frog declines. Conservation Biology. 19(5): 1421-1430.
Morgan J. A. T., Vredenburg, V. T., Rachowicz, L. J., Knapp, R. A., Stice, M. J., Tunstall, T., Bingham, R. E., Parker, J. M., Longcore, J. E., Moritz, C., Briggs, C. J., and Taylor, J. W. 2007. Population genetics of the frog-killing fungus Batrachochytrium dendrobatidis. Proceedings of the National Academy of Sciences 104(34): 13845-13850.
Nichols, D. K., Lamirande, E. W., Pessier, A. P., and Longcore, J. E. 2001. Experimental transmission of cutaneous chytridiomycosis in dendrobatid frogs. Journal of Wildlife Diseases 37: 1-11.
Pechmann, J. H. K., Scott, D. E., Semlitsch, R. D., Caldwell, J. P., Vitt, L. J., and Gibbons, J. W. 1991. Declining amphibian populations: The problem of separating human impacts from natural fluctuations. Science (Washington D C) 253: 892-895.
Pessier, A. P., Nichols, D. K., Longcore, J. E., and Fuller, M. S. 1999. Cutaneous chytridiomycosis in poison dart frogs (Dendrobates spp.) and White’s tree frogs (Litoria caerulea). Journal of Veterinary Diagnostic Investigation 11: 194–199.
Piotrowski J. S., Annis, S. L., and Longcore, J. E. 2004. Physiology of Batrachochytrium dendrobatidis, a chytrid pathogen of amphibians. Mycologia. 96(1): 9-15.
Rachowicz L. J., and Vredenburg, V. T. 2004. Transmission of Batrachochytrium dendrobatidis within and between amphibian life stages. Diseases of Aquatic Organisms. 61: 75-83.
Scott, M. E. 1988. The impact of infection and disease on animal populations: implications for conservation biology. Conservation Biology 2: 40-56.
Taylor, S. K., Williams, E. S., Thorne, E. T., Mills, K. W., Withers, D. I., and Pier, A. C. 1999. Causes of mortality of the Wyoming toad. Journal of Wildlife Diseases 35: 49-57.
Wright, K. M., and Whitaker, B. R. 2001. Amphibian medicine and captive husbandry. Krieger, Florida.
Young, B. E., Lips, K. R., Reaser, J. K., Ibanez, R., Salas, A. W., Cedeno, J. R., Coloma, L. A., Ron, S., La Marca, E., Meyer, J. R., Munoz, A., Bolanos, F., Chaves, G., and Romo, D. 2001. Population declines and priorities for amphibian conservation in Latin America. Conservation Biology 15: 1213-1223.
Zupanovic, Z., Musso, C., Lopez, G., Louriero, C. L., Hyatt, A. D., Hengstberger, S., and Robinson, A. J. 1998. Isolation and characterization of iridoviruses from the giant toad Bufo marinus in Venezuela. Diseases of Aquatic Organisms 33:1-9.