|
 Chytrid Fungus
See the sampling video on
our
Videos
page
Field-Sampling Protocol
for Batrachochytrium dendrobatidis from living amphibians, using alcohol
preserved swabs.
Article by Forrest Brem, Joseph R. Mendelson III & Karen R. Lips. Download pdf file.
Chytrid fungus and chytridiomycosis
| |
 |
| |
Litoria spenceri
photo by ©Michael MacFadden |
Chytrid fungi were long thought to be predominantly free-living saprophytes,
with a few species capable of infecting only invertebrates and vascular plants.
However, in 1999 a new species - Batrachochytrium dendrobatidis
(hereafter Bd) - was described infecting amphibians and causing the often fatal
disease, chytridiomycosis. Since that discovery, Bd has been identified in
association with amphibian population declines on every amphibian-inhabited
continent.
Bd is thought to have originated in South Africa, where
the earliest record occurs in a museum specimen from the 1930s, and initially
spread by the commercial trade in clawed frogs (Xenopus). For more
information on the origins and spread of Bd, see the article by Weldon et al
at
http://www.cdc.gov/ncidod/EID/vol10no12/03-0804.htm
From the site of its introduction, Bd spreads through water courses and
amphibian-to-amphibian contact, and possibly by other mechanisms not yet fully
understood. In Central America, where the spread of Bd has been extensively
studied, its rate of progression has been calculated at 28-100km/yr.
Where Bd thrives, generally moist cool habitats, 50% of
amphibian species and 80% of individuals can be expected to disappear within 1
year (Lips et al. 2006;
www.amphibianark.org/Lips%20et%20al%202006.pdf).
Currently it cannot be stopped in the wild and a minority of species seem able
to survive with a Bd infection as larvae or as adults and these animals likely
serve as a reservoir and vectors for future outbreaks. Notable among resistant
species are worldwide invasive pest species including marine toads, American
bullfrogs and African clawed frogs.
Whilst the problem is undoubtedly severe, and may seem
overwhelming, there is some reason for hope. Though many susceptible species
decline rapidly and disappear, at least one appears to be coming back from the
brink of extinction after 10 years. For a detailed accounting of Bd and a model
regional response, see the Australian's ‘Threat Abatement Plan at
http://www.deh.gov.au/biodiversity/threatened/publications/tap/amphibians/pubs/amphibians.pdf
and ‘Action Plan for Australian Frogs’ at
http://www.deh.gov.au/biodiversity/threatened/action/frogs/
Chytrid in captivity
Captive amphibians are NOT SAFE from chytrid fungus and
mortalities in private and zoo collections have been reported during 2006/7 in
USA, Japan, Australia and Europe (including UK). Several different treatment
regimes have been used with varying degrees of success including various
anti-fungal drugs and exposure to high temperatures. Probably the most readily
available and cost-effective drug treatment is itraconazole, administered as a
bath (see
http://www.open.ac.uk/daptf/froglog/FROGLOG-46-1.html).
Owing to continuing lack of clarity on chytrid infection and prevalence cycles,
species resistance, and the inability to confirm with 100% certainty that a
negative result is truly negative, at least two treatment cycles are recommended
in conjunction with suitable barrier management, enclosure hygiene and repeat
PCR testing.
The
only sure way to avoid getting chytrid fungus in your collection is to never
bring in another amphibian - from the wild (global or local) or from any other
facility (zoo, commercial, lab, etc). Obviously this would make it impossible
for any conservation institution to function. The realistic way to proceed is to
do what any responsible institution should already be doing - quarantine all
in-coming amphibians.
Quarantine should consist of at least 2-3 months in a room isolated from all
other amphibians and serviced by staff not in contact with the established
amphibian collection. During this time, animals should be tested for chytrid
(see below) and kept at temperatures between 18 and 25°C in order for chytrid to
be at its most active and therefore detectable. Many tropical amphibians will be
dead by the time a problem is visibly detected but incubation period for the
disease can be 9-76 days, with most succumbing in 18-48 days. If a chytrid
infection is suspected a variety of treatments are plausible as discussed above.
Since some animals can carry the infection without signs of disease (NB - in
many species it is the freshly metamorphosed individuals which succumb) routine
testing for chytrid (and other infectious agents) is essential with both
in-coming amphibians and your established collection.
By
far the most sensitive and reliable technique is the (real-time) PCR test. A
simple dry, sterile swab is wiped along the ground-contacting under parts of the
test specimen (thighs, drink-patch, rear toes and webbing – see diagram below)
and sent to a suitable laboratory. At least two tests should be carried out 6
weeks apart even for amphibians testing negative, whilst post treatment testing
should probably be repeated several times over a period of a year.
The
following laboratories provide the service:
Diagnostic
Laboratory,
Wildlife Epidemiology
Zoological Society of
London (ZSL)
Wellcome Building
London NW1 4RY
UK
email :
matthew.perkins@ioz.ac.uk
Pisces Molecular
2200 Central Avenue, Suite F
Boulder, CO 80301
Voice: 303-546-9300
Fax: 303-546-9400
Email:
jwood@pisces-molecular.com
School of Biological Sciences
Center for Integrated Biotechnology
Washington State University
Pullman, WA 99164-4236
Andrew Storfer
Associate Professor
Phone: (509) 335-7922
Fax: (509) 335-3184
Email: astorfer@wsu.edu
Wildlife
Disease Laboratories
Institute for Conservation Research
San Diego Zoo *
Dr. Allan Pessier
Email: apessier@sandiegozoo.org
619-231-1515, Ext 4510
The Swiss
company Ecogenics (www.ecogenics.ch;
info@ecogenics.ch) will be offerering
commercially a PCR-based test for the detection of chytrid fungus from amphibian
tissue samples and non-invasive swabs. The test is the real-time PCR test
developed by Boyle et al. (2005, Diseases of Aquatic Organisms 60: 141-148).
Please contact Ecogenics directly for pricing and further details. Establishment
of the test by Ecogenics was financed by the Swiss federal office for the
environment through a contract with KARCH.
Swabbing an amphibian for a Chytrid PCR test:
 |
Swab the underside
of the frog over the shaded region, particularly focusing on the
'drink patch' (underside of the pelvic region involved in water
uptake through the skin), the inner thighs of the hind legs
and the underside of the digits on the hind feet - including
the webbing between the toes. ©ZSL |
Chytrid fungus spores can sometimes be confirmed in
clinically ill amphibians with a skin scraping viewed under a microscope (see
article
http://www.jcu.edu.au/school/phtm/PHTM/frogs/papers/briggs-2003.pdf),
and histological examination of deceased individuals can
confirm infection (see online instruction
http://www.jcu.edu.au/school/phtm/PHTM/frogs/histo/chhisto.htm).
Chytrid fungus travels easily in water and on damp materials, as well as on
amphibians themselves. In order to minimise the risk of transferring chytrid
from captive amphibians to native wild amphibians in your region, all waste
materials (soil, leaf litter, branches, moss, plants, faeces, spawn, dead
animals and WATER) from all enclosures housing amphibians should be
treated as CLINICAL WASTE. Solids should be bagged and incinerated. Water should
be disinfected with bleach (1part bleach to 9 parts water) or other suitable
disinfectant and left to stand for 24hr before release into the sewerage system.
Alytes muletensis - Mallorcan midwife toad
photo by ©Trent Garner, ZSL
REMEMBER - Most zoos get chytrid sooner or
later. While it does require a swift, thorough response, it is not the end of
the world. You have probably already had it in your collection and not even
known about it. The keys to managing chytrid are:
-
Test your
established collection comprehensively and regularly (e.g. annually),
-
Carry out strict
and thorough quarantine of new arrivals,
-
Treat any
suspected/proven +ve animals swiftly with an approved method but DO NOT consider them
chytrid-free until they have been repeatedly tested over an extended period, at
least a year. Continue to test them regularly and keep them isolated from your
collection,
-
Practice good
hygiene/barrier management between animal rooms/displays and between enclosures – use
separate equipment and disposable gloves for all enclosures and dispose of all
waste and waste water responsibly.
The following is a good
reference summarizing the evidence for spread of chytridiomycosis as the cause of
many amphibian declines:
Spread of Chytridiomycosis Has Caused the Rapid Global
Decline and Extinction of Frogs
Articles on
Batrachochytrium dendrobatidis: the amphibian chytrid
A guide to husbandry and biosecurity standards required for
the safe and responsible management of ex situ populations of amphibians
These standards are based upon those reported in the proceedings of the
CBSG/WAZA Amphibian Ex situ Conservation Planning Workshop, El Valle, Panama, 12-15th February 2006.
Survey protocol for detecting chytridiomycosis in all Australian
frog populations
Lee F. Skerratt, Lee Berger, Harry
B. Hines, Keith R. McDonald, Diana Mendez, Richard Speare
* Testing for
amphibian infectious diseases (Chytrid Fungus and Ranavirus) available from the
San Diego Zoo
The Wildlife Disease Laboratories of the Institute for Conservation Research at
the San Diego Zoo are pleased to be able to offer low-cost testing for the
amphibian chytrid fungus (Batrachochytrium dendrobatidis) and Ranaviruses.
The testing is subsidized by an Institute of Museum and Library Services (IMLS)
National Leadership grant “Infectious Disease Control and Bioresource Banking
for the Amphibian Extinction Crisis” awarded to the Zoological Society of San
Diego and Zoo Atlanta.
The goal of the subsidized testing is to encourage widespread surveillance of
zoo collections for these potentially population-limiting infectious diseases.
Hopefully, these efforts will facilitate eradication of chytrid fungal
infections from established zoo collections and enable the collection of data of
the occurrence and prevalence of these diseases that is needed to make use of
disease risk assessment tools for reintroduction programs.
The laboratory is also able to provide assistance in working up outbreaks of
infectious diseases in captive collections (especially molecular diagnostic
testing) working together with your facility veterinarian and pathologist.
Available Tests:
Real-Time (Taqman) PCR for Amphibian Chytrid Fungus
Conventional PCR for Ranavirus
Tests are U.S. $10.00 each for zoos and aquariums.
Questions can be addressed to:
Dr. Allan Pessier
Email:
apessier@sandiegozoo.org
619-231-1515, Ext 4510
|
