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Transcript
Chytrid fungus (Batrachochytrium dendrobatidis) infection If you wish to report finding dead wildlife, or signs of wildlife disease, please call Garden Wildlife Health on 0207 449 6685. Agent Batrachochytrium dendrobatidis (Bd). Bd is a fungus (phylum chytriomycota) that can parasitize amphibian skin. Depending on the species affected it can have no apparent adverse effects, sporadic deaths or 100% mortality. It has been implicated in mass die-offs and species extinctions since its discovering in 1999. However, some species can tolerate infection with no apparent effect on their population number suggesting it can behave as an endemic pathogen or commensal organism. Hence, its origin and true impact are currently under intense scrutiny. Species affected The infection can be carried in all native amphibian species. Adult skin and the mouthparts of tadpoles are normally affected. Clinical signs Clinical signs can vary from numerous dead amphibians visible in, and surrounding, water bodies to no dead amphibians visible (especially in areas where they are swiftly scavenged). Affected adult amphibians may have reddening of the skin, excessive shedding of skin, droopy eyelids, abnormal postures, seizures or unusual behaviours such as nocturnal species being active during the day. Chronically infected, inapparent carriers have been described. Therefore, laboratory testing is required to confirm the presence of infection. Disease transmission The most common form of transmission is by direct contact between uninfected and infected amphibians. Environmental persistence of Bd in moist organic environments has been demonstrated for several months. The zoospore’s flagellum enables the organism to travel in water sources and survive in moist or wet materials. Hence, movement of organic materials from wetlands can spread Bd. It has been postulated that Bd may be spread by people’s boots and equipment and by birds, livestock and invertebrates moving between water sources. The pet and scientific trade in amphibians is frequently implicated in the international spread of the disease. Outwardly healthy Bd carriers will serve as a source of infection where quarantine procedures are inadequate. 1 Larvae tend not to be affected but can carry organism in their keratinised mouth parts. Upon metamorphosis, these individuals can succumb to disease. Vectors Vectors comprise infected animals (especially those exhibiting carrier statuses) and mechanical transport of moist organic materials by water sources, human beings, livestock and other wildlife. Distribution and origin Since its discovery, Bd has been found in wild and captive amphibian populations on every amphibian-inhabited continent. It is actively spreading in South, Central, and western North America, as well as the Caribbean, Australia and Europe. Bd is also found in Africa, Asia, and eastern North America, but does not seem to be spreading in these locations. Bd is conspicuously absent from Madagascar, Borneo and New Guinea. The origin of Bd is currently under investigation. There are two main competing theories. The first, is that Bd was a commensal organism for certain species in certain parts of the world. The movement of amphibians and organic material into areas where Bd was novel resulted in an epidemic among naïve species. The second, is that Bd was a commensal in amphibian populations globally. Recent environmental or anthropogenic factors have created a novel environment in certain parts of the world where Bd is capable of parasitizing its amphibian host. Risk to human health No risk to human health. Risk to domestic animal health No risk to domestic animal health other than farmed and pet amphibians. Recently metamorphosed North American bullfrogs (Rana catesbeiana) farmed for the international restaurant trade have suffered losses of up to 95%. Diagnosis Ideally a full post mortem examination should be carried out to determine contributing causes of death. At a minimum, skin swab and toe clip samples be sent to an appropriate laboratory for diagnostic testing. Tests carried out on samples include: PCR, real-time PCR, electron microscopy and histology (followed by electron microscopy). Before collecting or sending any samples from animals with a suspected disease, the proper authorities should be contacted. Samples should only be sent under secure conditions and to authorised laboratories to prevent the spread of the disease. Although Bd is not known to be zoonotic, routine hygiene precautions are recommended when handling animals. Also, suitable precautions must be taken to avoid cross contamination of samples or cross-infection of animals. 2 If you wish to report finding dead wildlife, or signs of wildlife disease, please call Garden Wildlife Health on 0207 449 6685. Control There is no effective control for Bd infection. Mitigation is aimed at reducing the local number of zoospores by removing and burying dead amphibians as soon as possible (assuming a diagnosis has already been made) and by preventing movement of infective material (spawn, tadpoles, amphibians or water plants) to other sites. Prevention Environment Ensure that the site is regularly scanned for dead amphibians, fish and reptiles. Ideally any site containing a reasonable population of amphibians should be monitored for sick and dead animals as a matter of course. If sick or dead animals are found, they should be tested for Bd infection so that the site’s Bd status can be determined. People coming into contact with water or amphibians should ensure where possible that their equipment and footwear/clothing has been cleaned and fully dried before use if it has previously been used at another site. To properly clean footwear and equipment: first use a brush to clean off organic material e.g. mud and grass rinse with clean water soak in disinfectant rinse with clean water and allow to dry. If any clothing is particularly soiled during activities, then washing at 40oC with detergent should be sufficient to remove any contamination with Bd. Ideally, different sets of footwear should be used at the site than are used by staff at home. Biosecurity measures should be increased to reduce the chance of spread if disease is confirmed. Livestock It is important to reduce the chance that livestock moving between sites (especially those travelling from known infected sites) will carry infected material on their feet. This can be accomplished by ensuring that feet are clean before transport. Foot baths can be used and animals should be left in a dry area after the bath for their feet to fully dry before transport. Wildlife Do not allow the introduction of amphibians without thorough screening and quarantine for Bd. This screening may still not pick up all carrier individuals (such as natterjack toads which tend to carry the fungus with apparent disease) but will reduce the risk of actively infected animals being introduced to the site. Also, remember that the fungus can be introduced with water or aquatic plants. 3 Humans Humans must ensure that all biosecurity measures described above are followed to prevent introduction of the infectious agent into previously uninfected areas. Further information World Organisation for Animal Health (OIE). Disease card: infection with Batrachochytirum dendrobatidis. http://www.oie.int/fileadmin/Home/eng/Health_standards/aahc/2010/en_chapitre_1.8.1.h tm [Accessed April 2013]. Speare, R. (2003). Summary of formidable infectious diseases of amphibians. www.jcu.edu.au/school/phtm/PHTM/frogs/formidable.htm [Accessed April 2013]. World Organisation for Animal Health (OIE). Diagnostic manual for aquatic animal diseases. http://www.oie.int/doc/ged/D9568.PDF [Accessed April 2013]. OIE reference laboratories and collaborating centres for diseases of amphibians, crustaceans, fish and molluscs: http://www.oie.int/fileadmin/Home/eng/Health_standards/aahm/2010/3_LIST_OF_LABS.pdf [Accessed April 2013]. IOZ publications on Bd: St-Amour, V., Wong, W.M., Garner, T.W.J., Lesbarrères, D. (2008) Anthropogenic influence on the prevalence of two amphibian pathogens. Emerging Infectious Diseases, 14, 1175-1176. Walker, S.F., Salas, M.B., Jenkins, D., Garner, T.W.J., Cunningham, A.A., Hyatt, A.D., Bosch, J. and Fisher, M.C. (2007) Environmental detection of Batrachochytrium dendrobatidis in a temperate climate. Diseases of Aquatic Organisms 77: 105-112. DOI: 10.3354/dao01850. Garner, T.W.J., Perkins, M., Govindarajulu, P., Seglie, D., Walker, S.J., Cunningham, A.A. and Fisher, M.C. (2006) The emerging amphibian pathogen Batrachochytrium dendrobatidis globally infects introduced populations of the North American bullfrog, Rana catesbeiana. Biology Letters 2: 455-459. Cunningham, A.A., Garner, T.W.J., Aguilar-Sanchez, V., Banks, B., Foster, J., Sainsbury, A.W., Perkins, M., Walker, S.F., Hyatt, A.D. and Fisher, M.C. (2005) Emergence of amphibian chytridiomycosis in Britain. Veterinary Record 157: 386-387. Daszak, P., Berger, L., Cunningham, A.A., Hyatt, A.D., Green, D.E. & Speare, R. (1999) Emerging Infectious Diseases and Amphibian Population Declines. Emerging Infectious Diseases 5, 735-748. Berger, L., Speare, R., Daszak, P., Greene, 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. & Parkes, H. (1998) Chytridiomycosis causes amphibian mortality associated with population declines in the rain forests 4 of Australia and Central America. Proceedings of the National Academy of Sciences of the United States of America 95, 9031-9036. Garner, T.W.J., Rowcliffe, J.M., Fisher, M.C. (in press) Climate, chytridiomycosis or condition: an experimental test of amphibian survival. Global Change Biology. Fisher, M.C., Bosch, J., Yin, Z., Stead, D.A., Walker, J., Selway, L., Brown, A.J.P., Walker, L.A., Gow, N.A.R., Stajich, J.E., Garner, T.W. (2009) Proteomic and phenotypic profiling of the amphibian pathogen Batrachochytrium dendrobatidis shows that genotype is linked to virulence. Molecular Ecology, 18, 415-429. Garner, T.W.J., Walker, S., Bosch, J., Leech, S., Rowcliffe, J.M., Cunningham, A.A., Fisher, M.C. (2009) Life history trade-offs influence mortality associated with the amphibian pathogen Batrachochytrium dendrobatidis. Oikos, 118, 783-791. Ribas, L., Li, M-S, Doddington, B., Robert, J., Seidel, J.A., Kroll, J.S., Zimmerman, L., Grassly, N.C., Garner, T.W.J., Fisher, M.C. (2009) Expression profiling the temperature-dependent amphibian response to infection by Batrachochytrium dendrobatidis. PLoS ONE, 4, e8408. 5