A BACTERIUM causing an epidemic among catfish farms in the southeastern U.S. is closely related to organisms found in diseased grass carp in China, according to researchers at Auburn University in Alabama and three other institutions.
The study, published in mBio, suggests that the epidemic emerged from an Asian source.
Since 2009, catfish farming in Alabama, Mississippi and Arkansas has been seriously affected by an emerging and virulent strain of Aeromonas hydrophila, which causes aeromonas septicemia in catfish. Aeromonas septicemia is a serious infection that can cause death in as little as 12 hours; its clinical signs include skin lesions and blood loss.
Normally, A. hydrophila, which can be found in both fresh and brackish water, only affects fish that are stressed or injured. However, the newer strain has affected even apparently healthy fish with no obvious signs of being under duress, according to study senior author Mark Liles, an associate professor in the department of biological sciences at Auburn.
When initial tests of the diseased fish showed that A. hydrophila was responsible, Liles said the scientists "didn't believe it at first, because the signs didn't match the more typical opportunistic infections in stressed fish that we associate with A. hydrophila."
To date, disease outbreaks have been responsible for an estimated loss of more than $12 million in catfish aquaculture operations in the southeastern U.S., he said.
Liles and his colleagues studied the molecular epidemiology of the epidemic-causing A. hydrophila to try to trace its evolution. They compared samples of the bacteria to 264 known aeromonas strains in an international database. Only one virulent strain came close to matching the one sampled from Alabama: ZC1, isolated from a diseased grass carp in China's Guangdong province.
The researchers also identified a less-aggressive but related A. hydrophila strain called S04-690, a sample of which was taken in 2004 from a diseased catfish in a commercial aquaculture pond in Mississippi.
It's not clear how the bacterium was introduced in America, Liles said. It could have happened from importing Asian carp to America for aquatic weed control or from transporting ornamental fish or contaminated processed seafood products from Asia.
The spread of disease among farms also is not fully understood but could result from birds that move from pond to pond eating catfish or from harvesting equipment that may be insufficiently sanitized between uses.
Liles and other researchers are investigating means to control the spread of illness, including developing vaccines against the bacteria and using medicated feed and/or probiotics.
Meanwhile, he said, U.S. farmed catfish are safe to eat and pose no disease threat to humans due to strict standards regarding the harvesting and processing of sick animals.