Wednesday, 25 July 2012

How trade is killing amphibians

The disease Chytridiomycosis has been found in 287 species of amphibians in 36 countries. It is responsible for the extinction of several species of amphibians and threatens the survival of many more. Chytridiomycosis is due to infection of the skin by a fungus, Batrachochytrium Dendrobatidis (Bd), and was first diagnosed in 1998.

Related fungi are fairly benign and feed on decayed plants rather than live animals. Bd, by contrast, has hundreds of genes that produce proteins that can digest amphibian skin. Genetic studies have shown that samples of Bd from around the world are very similar and that the most dangerous variety has existed for less than century. So Bd is a single, recently evolved, highly anomalous, variety that has spread rapidly.

This suggests two questions:
  1. How has BD spread so fast?
  2. How did a meat-eating fungus evolve from plant-eating ancestors?
The first question is the easier. Mathew Fisher, an epidemiologist at Imperial College, says "trade is getting this thing from continent to continent ... it doesn't survive in salt water, and, as far as we know, has no airborne stage". The most likely kind of trade is a trade in amphibians and there are two main possibilities - the African clawed frog and the North American bullfrog. Both can act as carriers because they are relatively resistant to Bd. The clawed frog is used in research whilst the bullfrog provides the frogs' legs used in some cuisines. Circumstantial evidence from the UK and the Phillippines points to the bullfrog.

So how did it arise? This is less clear but the the farming of bullfrogs creates high population densities that favour highly virulent varieties. With international trade bringing different varieties together in conditions that promote virulence the appearance of a virulent hybrid variety is no surprise - and has been seen in salamanders raised for bait.

So trade has spread this deadly disease and probably contributed to its existence. That, given the continued growth in world trade, is bad news. But there may be worse to come. The same factors that created and spread Chytridiomycosis are found in all trade in animals and plants. They may yet create new, highly virulent, diseases of chickens, pigs or even wheat.

No comments: