Keeping threatened amphibian species afloat                        

  Deutsch |  English |  Español | Search   

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: 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 28-100 km/year.

Where Bd thrives, generally in moist cool habitats, 50% of amphibian species and 80% of individuals can be expected to disappear within one year (Lips et al. 2006; www.amphibianark.org/pdf/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 six 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:

                            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 (1 part bleach to 9 parts water) or other suitable disinfectant and left to stand for 24 hours before release into the sewerage system.

Alytes muletensis - Mallorcan midwife toad

Photo: 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 positive 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.

References
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, 12-15^th 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 US $10.00 each for zoos and aquariums.

Questions can be addressed to:
Dr. Allan Pessier
Email: apessier@sandiegozoo.org
619-231-1515, Ext 4510

Home | Who we are |  Donations  |  News  |  Links  |  Downloads
Send a message to webmaster(at)amphibianark.org with questions or comments about the web site.
Copyright © 2009 amphibianark.org
Last update: Saturday October 10, 2009