Deadly diseases threaten crowded fish farms

By Mark Schleifstein,
Staff writer

It took only three weeks last May for a little-understood disease to sweep through three shrimp farms in south Texas and send shudders through the nation’s aquaculture and shrimp fishing industries.
  The Taura syndrome, named for a shrimp-farming province in Ecuador where it was identified, wiped out as much as 90 percent of the farms’ crop, worth between $10 million and $15 million.
  Many feared it might spread into the wild shrimp population, threatening a $460 million industry along the U.S. coast of the Gulf of Mexico. The farms, at the southern tip of the Texas coast not far from South Padre Island, discharge waste water into the lower Laguna Madre, a crucial nursing habitat for shrimp in the Gulf of Mexico.
  The virus is harmless to humans and, so far, appears to be contained from infecting wild shrimp. But the outbreak and other environmental concerns highlight the risks involved in this fast-growing industry.
  Scientist think Taura virus attacks the shrimp just beneath its shell, infecting the flesh and eventually killing the shrimp.
  Taura and several similar diseases cause significant losses in aquaculture operations each year, but there’s little money available for research into their causes and potential cures, Texas state fisheries pathologist Ken Johnson said.
  For starters, researchers don’t know where the virus that hit the Texas farms came from: a Hawaii hatchery, packaging plants and warehouses, birds flying from Ecuador, human waste leaking into the ponds, or some other source.
  And chances are they will never know, Johnson said.
  ‘‘You’re just not going to see Dustin Hoffman and a crew of scientists flying into Texas on jet planes for a shrimp farm problem,’’ Johnson said, referring to the motion picture ‘‘Outbreak,’’ in which Hoffman played a scientist fighting a killer virus in humans.
  Ironically, the Taura outbreaks may be the result of genetic efforts to develop a virus-free shrimp.
  The farm-raised species — a white shrimp known as Penaeus vannamei to scientists but known as Vanna White shrimp to farmers — was bioengineered to be resistant to a killer virus that plagued shrimp farms previously.
  But because all the shrimp come from the same genetic stock, they are equally susceptible to any virus that kills any of them.
  ‘‘When the Taura virus problem arose, nearly all the stock used in the United States came from that single line,’’ Johnson said. ‘‘As a consequence of that, people now think that particular strain was sensitive to the Taura virus.’’
  But while scientists go back to the drawing board to develop a new version that will resist the Taura virus, fishers in Texas and all along the Gulf worry that the farms are a threat to wild shrimp populations.
  Michael Ray, director of inland fisheries for the Texas Department of Parks and Wildlife, said the agency moved quickly to quarantine the affected shrimp farms, prohibiting them from disposing of water until it was treated to kill the virus.
  Johnson said he’s found no indication that any wild shrimp have been affected by the Taura virus.
  Ray said it’s not surprising that the shrimp farms have occasionally run into trouble with viruses. The farms were developed at a time when there was virtually no regulatory oversight of aquaculture.
  ‘‘Soon after the beginning of their operations, it became pretty obvious that there were a lot of potential problems,’’ Ray said. ‘‘I wouldn’t say they didn’t know what they were doing, but it’s realistic to say that nobody knew what effects these shrimp farms would have locally.’’
  Viruses aren’t the only concern facing aquaculture. Scientists are raising disturbing questions about the effects of drugs and chemicals used to fight disease and pests.
  The biggest concern is over the use of feed spiked with antibiotics. Congress’ recently dismantled Office of Technology Assessment and the United Nations Food and Agriculture Organization have raised warnings about the practice after discovering that many strains of disease-causing bacteria are becoming resistant to antibiotics.
  There also is little to keep the antibiotics given to fish in ocean pens from escaping to be ingested by wild fish.
  When humans eat fish containing the antibiotics, the drugs can trigger resistant strains of bacteria that can cause illness in humans as well.
  Scientists also are concerned that the use of toxic chemicals to kill bacteria and parasites in fish pens could cause the same sorts of problems already resulting from the overuse of similar chemicals on agricultural lands.
  ‘‘The positive aspect is that aquaculture is relatively young in this country and can learn from the mistakes made with chemical-based agriculture,’’ said Rebecca Goldburg, senior scientist with the Environmental Defense Fund.