Updated January 20, 2021, Michelle Koo; Original August 6, 2008, Lara Rachowicz
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 non-native species, 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 stressors 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, in humans, 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 7.9 billion (US Census, March 2021), and collectively we have altered about a third to a 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. The 2020 coronovirus (COVID-19) pandemic is a vivid reminder of this fact. 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 among many others. 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: 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 menu above for more information). 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 (Anaxyrus 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). 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.
Chytrid Research Resources
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 be exacerbated by 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).
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