Wednesday, April 16, 2014

Chytrid in Amphibians: A Worldwide Problem

A victim of chytrid: Southern Corroboree frog.
 One of Australia's most endangered and colorful frogs.
Only about 50 remain in the wild.

             Amphibian populations are decreasing all over the world.  Chytrid is a deadly disease that only    affects amphibians.  Chytrid is a skin fungus, called Batrachochytrium dendrobatidis, or B.d. and was discovered in 1999 as the causative agent for rapid amphibian die-offs.  This fungus causes amphibians’ skin to thicken and leads to cardiac arrest.  At least 350 species of amphibians have been infected with chytrid, and 200 of those species have suffered massive population reductions or extinctions.  Chytrid fungus is the leading cause of amphibian extinction worldwide and has been documented in North America, South America, Africa, Australia, Asia, and Europe.  This disease is devastating amphibian populations all over the world.

The global distribution of chytrid., Batrachochytrium dendrobatidis (Bd) assessed in 2013.
Image credit: Olson et al. (2013)

North America
            Despite the fact that chytrid spreads easily and is deadly, frogs are still being imported and exported from country to country.  One example of importation is the American Bullfrog.  American Bullfrogs are native to the U.S., but they are raised all over the world in factory farms, and every year millions are imported as food and pets.  A study testing for the presence of chytrid was done on 493 fresh bought frogs from markets in San Francisco, Los Angeles, and New York.  The results showed that 62 percent of the frogs were infected with chytrid.  Although it’s not known if bullfrogs introduced chytrid to the U.S., they are still a constant source of infection.
            Amphibian populations in the United States are declining so quickly that they could actually disappear from half of their habitats within the next 20 years. Endangered amphibians in the U.S. could disappear from half of their habitats in 6 years.  A study discovered that on average, 3.7% of the amphibian population is lost every year in the U.S, and species that are threatened or endangered are declining at an average rate of 11.6% every year.  The specific cause of these amphibian deaths was not determined, although chytrid almost certainly played its role in the massive die-offs, because even in areas where the habitat is protected die-offs still occur.
South America
            Manu National Park in Peru has the most diverse amphibian population in the world and has approximately 155 species of amphibians.  This park ranges from lowland Amazonian rainforests to high-altitude cloud forests along the eastern slope of the Andes.  Frogs in this park are being affected by chytrid.  The chytrid fungus is threatening the biodiversity present in this park and threatens amphibian populations all over the world. 
            In Chile the Darwin’s frog has been hit hard by chytrid.  One species of this frog has been pushed to probable extinction, while the other species has suffered a severe decline in population.  The decline of the Darwin’s frog is one of the first instances in which the fungus has been directly implicated in its disappearance.  The spread of chytrid to Chile may be from the introduction of the African clawed frog to the region in the mid-20th century.   
            A new chytrid species has been identified, Batrachochytrium salamandrivorans, “the salamander-eating fungus.”  Fire salamanders in Europe have been greatly affected by chytrid, and one of the last surviving populations is found in Bunderbos in the southern Netherlands.  In some cases, and in the case of the fire salamander, preemptive testing was done on rare and endangered amphibians, and early detection of chytrid allowed for the establishment of captive breeding populations.  In addition to the Netherlands, chytrid has also been discovered in the United Kingdom.  Not much is known about its distribution in the U.K, and studies are being done to map the presence of disease in order to understand how it spreads so that a plan of action can be made.
            American bullfrogs farmed in Southeast Asia, used in commercial trade, were found to test positive for chytrid in Singapore.  The American bullfrog is tolerant of chytrid infections, so it may just act as a carrier for spreading chytrid to a region when it is imported for commercial trade.  A co-author of the study investigating the presence of chytrid in Singapore, Assistant Professor David Bickford, from the Department of Biological Sciences at the NUS Facility of Science,  says “In light of the fact that this emerging infectious disease is now known to be spread by commercial trade, it is in everyone's best interest to eliminate it from the trade in live animals before both the native amphibian populations of Southeast Asia are affected and before it completely decimates the commercial trade and people are unable to make a living. This is not just about the frogs.”
Grim Future
            Amphibian populations continue to suffer massive population reductions all over the world due to chytrid.  The international trade of amphibians allows for the continuous spread of this disease.  Unless a treatment is developed or the international trade is stopped, amphibian populations all over the world will continue to decline and eventually face the threat of extinction. 

1 comment:

  1. If amphibians continue to be extinguished in many systems all over the world, what local species would benefit? I know that disease ecologists are looking for evidence of chytrid-resistant populations, but meanwhile Bd is spreading fast, as you show. Even more grateful for "our" spring peepers these last couple of weeks...