Download Chytrid fungus (Batrachochytrium dendrobatidis) infection

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.
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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.
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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:
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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.
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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
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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.
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