Amphibian Disease

Chytridiomycosis and Ranaviral disease are two amphibian diseases which may cause mass die-offs of amphibians and have the ability to reduce populations to dangerously low levels. The section which follows focuses on Chytridiomycosis, the disease which has been particularly implicated in the global decline of amphibians. Although outbreaks of Ranaviral disease may result in sharp population declines there is no evidence as yet for population extinctions due to this disease (Carey et al. 2003).

Ranaviral disease is caused by one of several closely related viruses in the genus Ranavirus. The Ranaviruses which infects amphibians may also infect fish and reptiles. Since the discovery of Ranavirus in the 1960s Ranaviral epizootics have been recorded across the globe. Ranaviral infections may result in external haemorrhaging (reddened skin or red spots), skin lesions and/or edema (swelling). However, in some cases there are no visible symptoms. When infected amphibians become more vulnerable to secondary bacterial infections (by Aeromonas hydrophila) “red-leg” may result.

 

For more information about Ranaviral disease see:

http://www.jcu.edu.au/school/phtm/PHTM/frogs/formidable.htm

http://www.zoo.cam.ac.uk/ioz/people/Garner.htm

http://www.zoo.cam.ac.uk/ioz/people/cunningham.htm

 

Chytridiomycosis

 

Infections of chytrid fungus…are today spreading rapidly and threatening entire species. There is thus the real possibility that much of an entire category of animals may become extinct worldwide – unless we prepare to act quickly

 

Sir David Attenborough

Patron, 2008 Year of the Frog

The Dominican mountain chicken is a victim of the chytrid

fungus in its native islands of Dominica and Montserrat.  Once the national dish of Dominica, its wild population has now declined by over 80% in the last decade as a result of chytridiomycosis.

Just over a decade ago analyses of dead and moribund amphibians from Australia and Panama led to the discovery of amphibian chytridiomycosis, a fungal disease caused by Batrachochytrium dendrobatidis (Berger et al. 1998). Since this time, chytrid (pronounced kit-rid) – related amphibian mortalities have been recorded worldwide. Today over 200 species are known to be infected with B.  dendrobatidis (Hyatt et al. 2006) . Chytridiomycosis has been the cause of severe population declines in Amphibia, localized population extinctions and in one case, the extinction of an entire species (link to Sharp-snouted day frog – section below). However, patterns of infection and the manifestation of the disease are context-dependent, depending in part upon the host species, the life history stage concerned and in part on extrinsic, particularly climate-related factors.

 

Several of the species classified by the Global Amphibian Assessment as ‘threatened’ and listed as EDGE-priority species are known to be under severe threat from chytridiomycosis. For other species, their susceptibility to chytridiomycosis is unknown.

 

Working in concert with amphibian EDGE is www.spatialepidemiology.net/bd (screenshot below), a mapping tool which serves as a platform for the collation of the results from screens for B. dendrobatidis across the globe. In addition to identifying those species and localities which are infected, the site enables the DNA fingerprints of B. dendrobatidis to be analyzed. This is important for furthering our understanding about potential routes of spread of the pathogen.

Emergence

The recent surge in amphibian extinctions and the discovery of chytridiomycosis follows a recent and unprecedented increase in the human-derived movement of amphibians across the globe. Species are transported beyond their natural range to meet the needs of research laboratories, food supplies, captive collections and pest control. The three most commonly introduced species are the North American Bullfrog, Rana catesbiana, the African-clawed frog, Xenopus laevis and the Cane toad, Bufo marinus. All three species are known to be asymptomatic carriers of B. dendrobatidis-infection (Fisher and Garner 2007).

The Out of Africa hypothesis for disease emergence suggests that the trade in X. laevis (a species used for pregnancy assays) from Africa to the USA, the UK and Australasia was key to the global spread of B. dendrobatidis) (Weldon et al. 2004). One of the earliest records for B. dendrobatidis corresponds to archived X. laevis from the 1930s, the period during which the trade in this species was taking off. Regardless of the contribution of amphibians from Africa the limited genetic diversity of B. dendrobatidis worldwide is consistent with the hypothesis of a recent spread. Today national imports of live amphibians may run into the millions (Schlaepfer et al. 2005). To mitigate against disease spread, measures are being advocated to avoid the co-housing of different amphibian species and to enforce a quarantine period of several months prior to the release of any individual into a collection.

The sudden emergence of chytridiomycosis has also been attributed to climatic changes.

Regardless of the historical presence of B. dendrobatidis in a locality, climatic influences and local weather patterns have been correlated with the prevalence and intensity of infection and the manifestation of disease e.g. (Berger et al. 2004; Pounds et al. 2006; Bosch et al. 2007).

Both species of gastic brooding frog (from the

family Rheobatrachidae) were driven to extinction

by 1985, possibly by chytridomycosis.

 

Biology of the amphibian chytrid

The microscopic amphibian chytrid, Batrachochytrium dendrobatidis is unique within the fungus family Chytridiomycota in being the only species known to parasitise a vertebrate host (Longcore et al. 1999). It is aquatic and reproduces by means of a zoospore, a uni-flagellate aquatic phase whose overall morphology resembles that of a tadpole (Fig - video coming soon). The zoospores attach to the keratinized cells of amphibians wherein they embed to form the reproductive zoosporangium which subsequently releases further zoospores (Fig - video coming soon). The survival, growth and reproduction of B. dendrobatidis is temperature-dependent. In-vitro growth occurs between 4 and 28°C (Piotrowski et al. 2004). However, optimum conditions are experienced between 17 and 25°C. 50 % mortality maybe incurred by cultures that are held at 30°C for eight days

 

Research has suggested that B. dendrobatidis may even be able to survive in the natural environment for a prolonged period of time in the absence of amphibians. In addition to growth in or on culture medium, B. dendrobatidis may survive and remain infectious in sterile natural media (river sand/water) for up to 12 weeks post-inoculation. It may also grow on dead amphibian and snake skin, dead algae, insect exoskeletons and feathers (Johnson and Speare 2005). Although sensitive to desiccation, B. dendrobatidis may also survive on feathers that have been dried for one to three hours (Johnson and Speare 2005).

 

 

Diagnostics and surveillance

The process for detecting B. dendrobatidis is determined by the distribution of keratin. The least invasive technique suitable for the molecular detection of B. dendrobatidis may be achieved by rolling a swab over the key areas of infection (Hyatt et al. 2007). The focal region for post-metamorphic amphibians is the rear half of the ventral surface particularly, the drink patch, the hind legs, the feet and the digits. For tadpoles, the swabbing is typically confined to the mouthparts, the only body-part which, in the early stages of development is keratinized.

A full description of diagnostic measures (using both molecular and histological techniques) may be found at:

Protocols and Techniques for Working with the Amphibian Chytrid Fungus.

 

The optimum timing to survey for chytrid-related mortalities is location specific. The heightened susceptibility of the recently metamorphosed amphibians of several temperate species often directs surveillance efforts to the period during which recently metamorphosed animals are leaving the water.

 

Case-studies

Featured below are several EDGE species which are known to be infected with B. dendrobatidis.

 

Archey's New Zealand frog (Leiopelma archeyi), EDGE rank 1

A New Zealand endemic listed as critically endangered which underwent a decline exceeding 80% within a decade. In one locality a population decline of 88% was recorded between 1996 and 2002 (Bell et al. 2004). Infections with B. dendrobatidis were reported at the time of the decline and chytridomycosis has been implicated. However, the finding of a B. dendrobatidis–infected population without evidence of declines suggests that other factors may be important in the actual emergence of the disease.

 

Sharp-snouted day frog (Taudactylus acutirostris) and Eungella day frog (T. eungellensis), EDGE rank 19=

Taudactylus acutirostris is an Australian endemic, formerly known from 9000 km sq. It is believed to have become extinct in the wild due to chytridiomycosis. The last captive individuals also succumbed to chytridiomycosis (Schoegel et al. 2005; GAA ref).

Taudactylus eungellensis, another Australian endemic which is critically endangered has undergone drastic declines. However, compared to T.  acutirostris the role of chytridomycosis in these declines is less clear.

Taudactylus acutirostris is one of the eight species from Australia which have become extinct (EX) or confined to captive populations (cp) over the last three decades (Schoegel et al. 2005): the southern gastric brooding frog Rheobatrachus silus (EX), the northern gastric brooding frog Rheobatrachus vitellinus (EX), the southern day frog, Taudactylus diurnus (EX), the armoured mist frog Litoria lorica (cp), the mountain mist frog Litoria nyakalensis (cp), the peppered tree frog Litoria piperata (cp), the yellow-spotted tree-frog Litoria castanea (cp). Although evidence to assess the reason for these latter declines is not available, chytridomycosis has been suspected (ref).

 

Dominican mountain chicken (Leptodactylus fallax), EDGE rank 158

The Dominican mountain chicken confined to the islands of Dominica and Montserrat is listed as Critically Endangered according to IUCN criteria, having undergone declines of over 80% within a decade. On Montserrat populations have been reduced severely by volcanic activity. On Dominica, it remained abundant despite heavy exploitation due to hunting, until 2002. Over the last six years the population has been decimated by chytridiomycosis.  Within 1.5 years the population of Dominican Mountain chickens declined by approximately 70% (Malhotra et al. 2007). Historically, an annual harvest of 8,000 -36,000 animals was taken. Last year conservationists from the Zoological Society of London were only able to find seven individuals on Dominica. These animals were captured for the purpose of captive breeding at the Zoological Society of London. To clear their infections they have been treated with Itraconazole.

 

Link to: Article in The Times

and blog http://spatialepidemiology.blogspot.com/

Mallorcan midwife toad (Alytes muletensis), EDGE rank = 55

On Mallorca, Alytes muletensis, the endemic Mallorcan midwife toad (local name = Ferreret), was until 1978, known only from fossils and thought to be extinct (Alcover and Mayol 1980; Tonge 1986; Román and Mayol 1997; Mayol in press). Today, subsequent to its rediscovery it is listed as vulnerable (Serra et al. 2004). It is endemic to the Tramuntana mountains of Mallorca and has a range of only 10 km2. In recent years two of the populations on the island are known to be heavily infected with B. dendrobatis (and a further two populations have a low prevalence of infections). Although the consequences of infection await formal assessment, if the population crashes associated with B. dendrobatidis-infected populations of the closely related species, the Common Midwife Toad (Alytes obstetricans), on mainland Spain (Bosch et al. 2001) (link to http://www.sosanfibios.org/spain.htm) are to be a precedent for the future of A. muletensis, increases in the level of conservation effort are required. Research is underway to assess whether A. muletensis incurs any survival costs due to infection and whether the strain of B. dendrobatis found on Mallorca may actually be less virulent than that found on the mainland. Eitherway, given the interplay between climate and the manifestation of disease, in this period of rapid climate change, high vigilance is required.

 

Conservation responses

(1.)  Vigilance against spread e.g. disinfecting equipment and footwear using recommended procedures, and always wearing disposable gloves when handling amphibians

 

(2.)  Stringent procedures for captive breeding, translocations and imports/exports of amphibians to avoid spread of disease

 

(3.)  The development of treatments for both the infected amphibians and their habitats

 

(4.) Field and laboratory-based research

 

 

LINKS

 

Control strategies for diseases in wild amphibians

Hygiene protocols for the handling of amphibians in field studies

The Threat Abatement Plan (written by the Australian government)

To view up to date information about currently debated issues and new research findings see the amphibian chytrid blog:

http://spatialepidemiology.blogspot.com/

http://www.jcu.edu.au/school/phtm/PHTM/frogs/ampdis.htm

http://www.sosanfibios.org/ 

Images

www.spatialepidemiology.net/bd 

 

ZSL Researchers

Dr. Andrew Cunningham

Dr. Matthew Fisher

Dr. Trent Garner

 

Reading List

Alcover JA, Mayol J (1980) Noticia del hallazgo de Baleaphyrne (Amphibia: Anura: Discoglossidae) viviente en Mallorca. Doñana, Acta Vertebrata 7(2): 266-269.

 

Bell BD, Carver S, Mitchell NJ, Pledger S (2004) The recent decline of a New Zealand endemic: how and why did populations of Archey's frog Leiopelma archeyi crash over 1996-2001? Biological Conservation 120: 189-199.

 

Berger L, Speare R, Daszak P, Green DE, Cunningham AA et al. (1998) Chytridiomycosis causes amphibian mortality associated with population declines in the rain forests of Australia and Central America. Proceedings of the National Academy of Science, USA 95: 9031-9036.

 

Berger L, Speare R, Hines H, Marantelli G, Hyatt AD et al. (2004) Effect of season and temperature on mortality in amphibians due to chytridiomycosis. Australian Veterinary Journal 82: 31-36.

 

Bosch J, Martínez-Solano I, García-París 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.

 

Bosch J, Carrascal LM, Duran L, Walker S, Fisher MC (2007) Climate change and outbreaks of amphibian chytridiomycosis in a montane area of Central Spain; is there a link? Proceedings of the Royal Society B-Biological Sciences 274(1607): 253-260.

 

Carey C, Pessier AP, Peace A (2003) Pathogens, Infectious Disease and Immune Defenses. In: Semlitsch RD, editor. Amphibian conservation. Washington and London: Smithsonian.

 

Fisher MC, Garner TWJ (2007) The relationship between the introduction of Batrachochytrium dendrobatidis, the international trade in amphibians and introduced amphibian species. Fungal Biology Reviews 21: 2-9.

 

Hyatt AD, Boyle DB, Olsen V. Detection of Batrachochytrium dendrobatidis (Bd): methods and recommendations

2006 5-7 November 2007; Tempe, Arizona, USA.

 

Hyatt AD, Boyle DG, Olsen V, Boyle DB, Berger L et al. (2007) Diagnostic assays and sampling protocols for the detection of Batrachochytrium dendrobatidis. Diseases of Aquatic Organisms 73: 175-192.

 

Johnson M, Speare R (2005) Possible modes of dissemination of the amphibian chytrid Batrachochytrium dendrobatidis in the environment. Diseases of Aquatic Organisms 65(3): 181-186.

 

Longcore JE, Pessier AP, Nichols DK (1999) Batrachochytrium dendrobatidis gen. et sp. nov., a chytrid pathogenic to amphibians. Mycologia 91: 219-227.

 

Malhotra A, Thorpe RS, Hypolite E, James A (2007) A report on the status of the herpetofauna of the Commonwealth of Dominica, West Indies Journal Applied Herpetology

 

Mayol, editor (in press) El Sapito Resicitado por la ciencia y salvado por la conservación. In (en prensa) Monografia de conservación de especies en España. . Generalitat Valenciana.

 

Piotrowski JS, Annis SL, Longcore JF (2004) Physiology of Batrachochytrium dendrobatidis, a chytrid pathogen of amphibians. Mycologia 96(1): 9-15.

 

Pounds AJ, Bustamante MR, Coloma LA, Consuegra JA, Fogden MPL et al. (2006) Widespread amphibian extinctions from epidemic disease driven by global warming. Nature 439: 161-167.

 

Román A, Mayol J (1997) Documents tècnics de conservació. La recuperación del ferreret, Alytes muletensis. Projecto life ferreret, Palma de Mallorca.

 

Schlaepfer MA, Hoover C, Dodd JCK (2005) Challenges in evaluating the impact of the trade in amphibians and reptiles on wild populations. Bioscience 55: 256–264.

 

Schoegel LM, Hero JM, Berger L, Speare R, McDonald K et al. (2005) The decline of the sharp-snouted day frog (Taudactylus acutirostris): the first documented case of extinction by infection in a free-ranging wildlife species? EcoHealth 3: 35-40.

 

Serra JM, Griffiths R, Bosch J, Beebee T, Schmidt B et al. (2004) Alytes muletensis. In: IUCN 2006. 2006 IUCN Red List of Threatened Species. Available: www.iucnredlist.org. Accessed 2007 Downloaded on 23 March 2007.

Tonge S (1986) Collecting the Mallorcan midwife toad. Oryx 20(2): 74-78.

 

Weldon C, du Preez LH, Hyatt AD, Muller R, Speare R (2004) Origin of the amphibian chytrid fungus. Emerging Infectious Diseases 10(12): 2100-2105.

 


The silent killer

Batrachochytrium dendrobatidis (bd or chytrid) is a disease-causing fungus responsible for numerous catastrophic declines of amphibians globally.

An international network of scientists is working to understand this disease and mitigate its impact on amphibians:

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