Catherine Brahic, New Scientist 7 May 09;
Millions of frogs are shifted around the world each year for sale as pets and food. Now research shows, for the first time, that this global trade is spreading two severe diseases – one of which is blamed for driving amphibians towards extinction.
Last year the World Organisation for Animal Health (OIE) took a step towards monitoring both diseases by making them "notifiable", but as yet there are no regulations to prevent the trade of infected frogs.
"This is a major issue," says Peter Daszak, president of the Wildlife Trust, and an expert on amphibian diseases. "Over a million bullfrogs a year come into the USA for food. If only five per cent are infected, that's 50,000 infected animals."
Importing disease
Daszak and colleagues surveyed frogs that were imported through three major US ports: Los Angeles, San Francisco and New York. In each city, they visited market stalls and stores selling live imported bullfrogs or frog parts, purchased samples, and took them back to the lab where they were tested for the fungus Batrachochytrium dendrobatidis (Bd) and for ranaviruses – viruses specific to amphibians.
Bd causes devastating chytrid disease, first identified about a decade ago. In the past few years, a number of studies have shown that it is an important factor in the recent drop-off in amphibian numbers. Worldwide, chytrid disease has caused extinctions and declines in 200 species of frogs. Ranaviruses also cause large die-offs, but scientists do not yet know whether they can contribute to extinctions.
Just over eight per cent of the frogs the researchers sampled had ranaviral diseases, whereas two thirds carried the chytrid fungus.
'Major factor'
"Considering the devastating impact Bd has had on global amphibian populations and the millions of animals being traded on an annual basis, this number is especially alarming," says Lisa Schloegel of the Wildlife Trust who led the work. "We may never completely know the extent to which trade has contributed to the global spread of amphibian diseases, but it does appear to be a major contributing factor."
Studies of chytrid fungus show its genetic makeup is very similar around the world, suggesting it is a relatively new pathogen, says Trenton Garner, a herpetologist at the Institute of Zoology in London. "The only way we can think of it getting this wide distribution without any evidence of it being an old pathogen is that it must have been moved around an awful lot, and recently."
The two main factors that move frogs around on a global scale are the pet and food trade. To get some idea of these movements, Daszak and colleagues obtained import data from the US Fish and Wildlife Service under the US Freedom of Information Act.
They found that between 1 January 2000 and 31 December 2005, over 7 million kilograms of amphibians and amphibian parts were imported into the US. That comes to more than 28 million animals in five years, or about 5 million animals a year.
'Important step'
Yet, although many nations, including the US, Australia and the European Union, have strong research programmes into the origins and spread of chytrid, none of the researchers New Scientist spoke to had heard of any national regulations to control infections in imported animals.
Animals are examined by vets when they enter the US, but both Bd and ranaviruses are easily missed in physical examinations. Skin swabs need to be taken and analysed in a laboratory to detect both conditions.
Ideally, says Garner, exporting nations would be required to certify that their animals are disease-free and importing nations should check this upon arrival.
The OIE's decision to make Bd and ranaviral diseases notifiable means that all 172 member countries must report on the status of the disease within their borders every six months. "The OIE measures are the first step, and a very significant one," says Daszak.
Journal reference: Biological Conservation (DOI: 10.1016/jbiocon.2009.02.007)
