Batrachochytrium dendrobatidis

Chytrid Frog Fungus

Synonym(s): Chytridiomycosis
Class: Chytridiomycetes
Order: Rhizophydiales
Family: Chytridiaceae

Photographer: Forrest Brem, Riders of a Modern-Day Ark
Source: http://www.plosbiology.org/article/info:doi/

Description

The fungus attacks the parts of a frog's skin that have keratin in them. Since frogs use their skin in respiration, this makes it difficult for the frog to breathe. The fungus also damages the nervous system, affecting the frog's behavior. A sick frog may have discolored skin be sloughing, or peeling on the outside layers of its skin. The skin can vary from obvious peeling of wet skin, to a roughness of the frog's skin.  Also, a telling symptom is the frog having its legs spread slightly away from itself, rather than keeping them tucked close to its body. In more extreme cases, the frog's body will be rigid, and its back legs will trail behind it.

Tadpoles can be infected around their mouths, but this is not enough to kill them. It's only when they start turning into frogs, and grow keratin in other areas, that the Chytrid fungus can spread throughout their bodies.

Ecological Threat: Scientists think the decline and disappearance of some frog species in Australia and New Zealand may be partly due to a disease caused by a Chytrid fungus. Once a pond has become infected with Chytrid fungus, the fungus may stay in the water forever. This means it is very important not to move frogs from one area to another; which is almost impossible to control. Also, now Chytrid is now present on every frog-inhabited continent on Earth. Once an area becomes marked as "pathogen positive", it soon enough sees a swift deterioration in frog numbers and frog diversity, with as many as 90%  of the frogs in the area disappearing forever.

Frogs are important creatures.  If frog populations were to undergo a bust, insect populations would inevitably follow suit with a huge boom. This could mean increased crop devastation and incidence of insect-borne disease. Malaria kills over one million people every year; without frogs to control mosquito population growth, one can only imagine the devastation that would ensue, furthermore emphasizing the importance of maintaining lethal fungi like Batrachochytrium dendrobatidis.

Biology: Batrachochytrium dendrobatidis has two primary life stages: a sessile, reproductive zoosporangium and a motile, uniflagellated zoospore released from the zoosporangium. The zoospores are known be active only for a short period of time, and can travel short distances of one to two centimeters. However, the zoospores are capable of chemotaxis, and can move towards a variety of molecules that are present on the amphibian surface, such as sugars, proteins and amino acids. Once the zoospore reaches its host, it forms a cyst underneath the surface of the skin, and initiates the reproductive portion of its life cycle. While this fungus is not a new pathogen, has only now entered the global stage. The most compelling theory for this is the theory that rising temperatures increase the virility of Chytrid, especially in the tropical rainforests, where warmer temperatures have led to an increase in cloud cover. More cloud cover means cooler days and warmer nights in the rainforest, making it a very hospitable environment for Chytrid, which grows and thrives best between 63 and 77oF.

History: The native range of this fungus is unknown but Africa mat be the source, because the earliest known case of infection is on a museum specimen of the African clawed frog (Xenopus laevis) collected in 1938. However, it is unknown on how it got into the United States, but there are two theories. The first is in regards to importation of the African clawed frog; while the second is that the fungus has been around for decades but went unnoticed. A leopard frog museum specimen from 1974 in Colorado was infected with the fungus, reaffirming that the timeline of introduction of Batrachochytrium dendrobatidis is unknown.

U.S. Habitat: Being an aquatic fungus Batrachochytrium dendrobatidis requires water as well as a. It will survive in any body of water from streams and lakes to artificial containers but dies above temperatures of 82oF.

Distribution

Native Origin: Unknown; supposedly Africa

U.S. Present:  Throughout

 

Management

Since the Chytrid frog fungus has been eliminating native frogs off of Australia and New Zealand their Departments of Conservation and frog researchers have been collaborating on strict frog hygiene protocols. These protocols include including not taking packs, bags or gaiters into frog areas, and treating clothing, footwear and research equipment with a biocide known to kill the fungus.

Hopefully amphibians will soon develop immunity to the fungus, as African Clawed Frogs have, but the rate of decline (and extinction) is already so high that by the time they develop immunity, it might already be too late. Several zoos across the United States have established captive breeding programs for critically endangered frog species, with curators from the Atlanta Zoo going so far as to airlift frogs out of Panama in Tupperware containers.

Studies conducted in Virginia have shown that the addition of the anti-chytrid (antifungal) bacteria Janthinobacterium lividum to the skin of B. dendrobatidis-susceptible amphibians (i.e. Rana muscosa juveniles) increases the concentration of the antifungal metabolite violacein, which in turn decreases the mortality rate due to infection by B. dendrobatidis and also increases survivability.

 

Text References

Becker M., Brucker R., Schwantes C., Harris R., Minbiole K. 2009. The bacterially-produced metabolite violacein is associated with survival in amphibians infected with a lethal disease. AEM 21:1294–1302.

Covacevich, J.A. and McDonald, K.R. 1993. Distribution and conservation of frogs and reptiles of Queensland rainforests. Memoirs of the Queensland Museum 34:189-199.

Garner, T.W., Perkins, M.W., Govindarajulu, P., Seglie, D., Walker, S., Cunningham, A.A., Fisher, M.C. September 2006. The emerging amphibian pathogen Batrachochytrium dendrobatidis globally infects introduced populations of the North American bullfrog, Rana catesbeiana. Biol. Lett. 2(3):455–9

Harris R., Brucker R., Minbiole K., Walke J., Becker M., Schwantes C. et al. 2009. Skin microbes on frogs prevent morbidity and mortality caused by a lethal skin fungus. ISME Journal 3(7):818–824.

Lam B., Walke J., Vredenburg V., Harris R. 2009. Proportion of individuals with anti-Batrachochytrium dendrobatidis skin bacteria is associated with population persistence in the frog Rana muscosa. Biological Conservation 143:529–531

Laurance, W.F., McDonald, K.R. and Speare, R. 1996. Epidemic disease and the catastrophic decline of Australian rainforest frogs. Conservation Biology 10:406-413.

Moss, A.S., Reddy, N.S., Dortaj, I.M., San Francisco M.J. 2008. Chemotaxis of the amphibian pathogen Batrachochytrium dendrobatidis and its response to a variety of attractants. Mycologia 100(1):1–5.

Symonds EP, Trott DJ, Bird, P.S., Mills, P. 2008. Growth characteristics and enzyme activity in Batrachochytrium dendrobatidis isolates. Mycopathologia 166(3):143–147.

Woodward, Susan L., and Joyce Ann. Quinn. 2011. Chytrid Frog Fungus. Encyclopedia of Invasive Species: From Africanized Honey Bees to Zebra Mussels. Santa Barbara, CA: Greenwood. 18-20. Print.   

Internet Sources

http://www.newislander.com/ports/2009/11/say_goodbye_to_kermit/

http://www.nzfrogs.org/NZ+Frogs/Conservation.html

http://www.environment.nsw.gov.au/animals/FrogChytridFungus.htm

 

 

 

 

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