Treatment Reduces Fish Disease in Commercial Production Ponds
By Jim Core
September 4, 2002
A new treatment against a
freshwater snail that carries a deadly fish parasite reduces the threat of the
parasite to farm-raised channel catfish nationwide.
Infected fish develop small cysts in their flesh, often seen as bumps just
below the skin. The disease can kill smaller fish and reduces growth among
infected fish that survive. Currently, there is no cure. Andrew J. Mitchell, a
fishery biologist at the Agricultural Research Service's
Harry K. Dupree Stuttgart National
Aquaculture Research Center in Stuttgart, Ark., found that if he could
reduce the vector, he could control the disease.
Mitchell's treatment is a shoreline application of copper sulfate and citric
acid to production ponds. The formula is applied in the waters around the
pond's edge, where the snails live. The concentrated treatment kills the
snails, but dilutes as it disperses throughout the pond so it does not harm
The parasite is Bolbophorus confusus, a flatworm found in the
intestinal tract of the American white pelican, a migratory, fish-eating bird
found throughout the United States and in the lower Mississippi River Delta
during winter months. Flatworm eggs are released into catfish ponds, where they
hatch and form larvae that infect an intermediate host, the rams-horn snail,
Planorbella trivolus. When the larvae mature in the snail, they are
released as larval trematodes called cercaria that infect fish. The cycle
begins again when the fish are eaten by pelicans.
Reducing the rams-horn snail population breaks the parasite's life cycle
midway through its development. The parasites cannot be transmitted from one
fish to another.
More than 90 percent of snails were killed in studies. The shoreline
treatment is approved for use by the Environmental
Protection Agency, and it is being widely used in the Mississippi Delta
area. Copper sulfate is already commonly used to curb the growth of nuisance
algae in fish ponds.
ARS is the U.S. Department of
Agriculture's chief scientific research agency.
More information on the research is in the
September 2002 issue
of Agricultural Research magazine.