Above: A frog infected with Batrachochytrium dendrobatidis (Wikimedia Commons/Forrest Brem)
In the 1980s and 1990s, when many frog populations were declining, scientists desperately tried to discover the cause. Was it pollution? Damage from UV rays? An invasive species? The main culprit turned out to be a fungus that infects the skin of frogs and toads: Batrachochytrium dendrobatidis, or Bd for short.
Did you know? Clawed frogs were once used for pregnancy tests. The urine of a pregnant woman will cause female frogs to lay eggs.Now, European salamanders are being threatened by a similar threat introduced from Asia. Scientists are working to stop it from spreading to other parts of the world through a better understanding of the fungus itself and through stricter controls on the international trade in amphibians.
Bd caused massive numbers of adult frogs to die, especially in Australia and Central America. In eastern Australia, populations declined by about 90% over a period of 15 years. The fungus has been detected in more than 500 of the 5709 known frog species in the world. The effects of the disease have usually been stronger in cooler habitats that favour the growth of the fungus, such as mountain streams.
The fungus infects parts of the body that contain keratin, which is a major protein in your nails and common in the upper layers of skin. Bd harms frogs by causing their skin to thicken so much that they cannot breathe, regulate the transport of ions across or their skin, or properly regulate their body temperature. Infected frogs lose weight quickly, are slow to hop away from threats, and often have a build-up of shedding skin.
Origins of the fungus
Where did Bd come from? The fungus probably originated in Africa, where it was detected as early as 1934 in African clawed frogs. Until the 1970s, clawed frogs were regularly used for pregnancy tests. They were kept in hospitals around the world and this might be how the fungus could have spread to other continents. Another possibility is the international trade in American Bullfrogs, which are a popular food in different parts of the world.
Both African clawed frogs and bullfrogs can carry the fungus without suffering many of the symptoms that occur in other frogs. However, once they transmit the fungus to other species in water, the fungus can quickly cause large numbers of deaths.
Why frogs matter
Did you know? Batrachochytrium dendrobatidis, a type of chytrid fungus that infects the skin of frogs, has been detected in over 500 species since its discovery in the 1990s.An ecosystem refers to the different species that live in a particular area, as well as the non-living parts of the environment they interact with. If one part of the ecosystem disappears, such as all the frogs that live in a stream, it can affect all the other organisms in the system. It’s a bit like dropping a small pebble in a bathtub: the ripples quickly spread across the entire surface of the water.
Scientists have found that in places where frogs have disappeared because of Bd outbreaks, the ecosystem has changed in many ways, such as the amount of algae present and the number of insects that live there.
Salamanders now face a similar threat
Recently, European scientists discovered a related species of fungus that has been linked to a decline in the numbers of the endangered fire salamander. The new fungus, B. salamandrivorans, had existed for millions of years in Asia before being introduced to European salamanders, through imported amphibians being sold as pets or food. But unlike Asian salamanders, European salamanders have no natural resistance to the fungus.
Scientists hope that the early discovery of this new disease can help prevent its spread to continents other than Europe and Asia. It is particularly important to avoid a “salamander plague” in North America, where salamander diversity is the highest in the world. Measures that can be taken include treating imported animals and more closely controlling the international trade in amphibians.
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Although world travel and international trade in food and other goods have many benefits, they also make it possible for diseases to move around the globe and threaten wildlife in different parts of the world. It took many years and large numbers of scientists to discover that the Bd fungus was behind the dramatic decline in frog frog populations. Hopefully, the early discovery of the salamander fungus can lead to smart decisions that reduce the chances of it spreading any further and disrupting even more ecosystems populated by amphibians.
Chytrid Fungus (2015)
Allan Pessier, Amphibian Ark
General introduction to chytrid fungus and Bd, including a discussion of the threat they pose to amphibians and links to additional resources.
Amphibians of the World (no date)
Clinton N. Jenkins, Instituto de Pesquisas Ecológicas
Maps showing the global ranges of different types of amphibians.