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Keeping threatened amphibian species afloat |
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Chytrid Fungus
Chytrid fungus and chytridiomycosis
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, email : matthew.perkins@ioz.ac.uk
Pisces Molecular
Voice: 303-546-9300 Swabbing an amphibian for a Chytrid PCR test:
Chytrid fungus spores can sometimes be confirmed in clinically ill amphibians with a skin scraping viewed under a microscope (see online 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:
The following is a good reference summarising the evidence for spread of chytrdiomycosis as the cause of many amphibian declines: Spread of Chytridiomycosis Has Caused the Rapid Global Decline and Extinction of Frogs |
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