The ram's horn snail,
Planorbella trivolvis, is
a carrier of the Bolbophorus
trematodes that infect farm-raised
catfish. Two snails are seen
here in their natural environment,
on aquatic vegetation under water.
The American white pelican will fly into the lower Mississippi River
Delta again this winter to feed on fish. Its bright orange bill and
a pouch that swells when it eats, combined with its short legs and typical
wingspan of more than 9 feet, make it quite a spectacle.
But its appearance isn't what makes an impression on catfish farmers.
The pelican also unwittingly harbors a small parasitic flatworm thatonce
it makes its way into channel catfishcan have a big financial
impact on the Delta catfish industry.
The flatworm, Bolbophorus confusus, lives in the bird's intestinal
tract. Eggs from the flatworm are shed into channel catfish ponds, where
they hatch and form larvae that infect an intermediate host, the ram's-horn
snail, Planorbella trivolvis. Once the larvae multiply and mature
inside the snail, they exit and find fish to infect. The cycle begins
again when a pelican eats an infected fish and the flatworm reaches
maturity inside the bird.
To kill parasite-harboring
snails, fishery biologist
Andrew Mitchell and maintenance
worker Robert Ideker (driving
tractor) apply a solution of
copper sulfate and citric acid
along the shoreline of an
experimental catfish pond.
"It's called a complex life cycle," says Andrew J. Mitchell,
a fishery biologist at the Harry K. Dupree Stuttgart National Aquaculture
Research Center in Stuttgart, Arkansas. "The flatworm must have
all three hosts to continue. It's interesting that if only one larva
penetrates a snail, it can multiply into tens of thousands of larvae
through asexual reproduction. It doesn't take a lot to get the cycle
A ram's-horn snail typically measures slightly more than one-third
of an inch across. As a carrier of this deadly fish parasite, it is
a strong link in a chain of events leading to extensive losses for catfish
farmers. Fish infected with B. confusus develop small cysts in
their flesh, often seen as bumps just below the skin. The disease can
kill smaller fish, and it lessens the appetites of larger fish not killed.
This leads to slower-than-normal growth, which makes them unsuitable
for market and susceptible to other diseases.
A channel catfish with small
bumps and inflammation on
its tailfin, which indicate
the presence of Bolbophorus
flatworms beneath the skin.
Breaking the Chain
Cases of the disease were first seen in the Delta in 1999. Currently,
there is no cure for infected fish. Losses for catfish producers are
estimated to be in the millions.
Mitchell determined that if he could reduce the snail population, he
could control the disease by breaking the parasite's life cycle midway
through. The flatworms cannot be transmitted from one fish to another.
Control of the pelican, a nationally protected bird, is not an option.
Mitchell developed a chemical combination to target the freshwater
snail. The treatment has proven very effective at reducing the threat
of the parasite to farm-raised channel catfish nationwide, including
those in Mississippi, Louisiana, Arkansas, and most recently California.
Channel catfish are the most commonly raised food fish species in the
United States. Fingerling, or juvenile catfish, deaths can exceed 90
percent in ponds affected by the flatworm. Mitchell says several farms
have lost entire ponds of fry (baby catfish) and fingerlings because
of severe infections.
Andrew Mitchell uses a dissection
scope to examine snails (in the
petri dish) for infection with
larval Bolbophorus flatworms.
A single larva shows up on the
monitor behind him.
| Stopping Snails in Their Tracks
Mitchell's treatment is to apply a formula of copper sulfate and citric
acid to the waters at the edge of production ponds. The snails live
and breed in these shallow waters, where they graze on the microscopic
life coating aquatic vegetation, small rocks, and hardened mud surfaces.
The concentrated treatment kills the snails, then quickly dilutes as
it disperses throughout the water, according to Mitchell. The troublesome
flatworms die when their host snails die. Catfish naturally avoid the
treated areas, and by the time the chemical combination spreads throughout
the entire pond, it won't harm them.
The formula consists of 10 pounds of copper sulfate plus 1 pound of
citric acid, mixed with at least 70 gallons of water, for each 250 feet
of shoreline. In experimental trials, the treatment was carried in a
110-gallon tank mounted on a tractor and sprayed through a hose. A plastic
pipe at the end of the hose distributes the formula over a strip of
land and water near the pond's edge. Mitchell says that farmers in the
field use 1,000-gallon tanks mounted on trailers to disperse the treatment.
Ninety-five to 100 percent of snails were killed in studies when the
water temperature was between 73.4 °F and 86 °F. Treatment effectiveness
and fish safety also depend on variables such as alkalinity and temperature
of the water. Wind speed is a factor, too, because it may stir up the
water so much that the formula is not retained long enough to be effective.
Fish size as well as pond size, shape, and depth also play roles in
the treatment's effectiveness.
"Copper sulfate had been commonly used in the past to curb the
growth of troublesome algae blooms in fish ponds," Mitchell says.
"The copper sulfate-citric acid treatment was approved for use
against snails by the Environmental Protection Agency, and it is already
being widely used in Arkansas and Mississippi for this purpose."
As a result, Mitchell says, the state of Arkansas has not had a serious
snail infestation since late in the summer of 1999. He says credit can
also be given to Asian black carp, which have been used to eat ram's-horn
snails. But black carp might be restricted because of fears that it
will escape fish ponds, enter natural waters, and consume threatened
or endangered snails and mussels.
A lime treatment developed under a cooperative research program at
the Thad Cochran National Warmwater Aquaculture Center in Stoneville,
Mississippi, has also had considerable success as a snail treatment
and is being further evaluated.By Jim
Core, Agricultural Research Service Information Staff.
This research is part of Aquaculture, an ARS National Program (#106)
described on the World Wide Web at http://www.nps.ars.usda.gov.
Andrew J. Mitchell
is at the USDA-ARS Harry K.
Dupree Stuttgart National Aquaculture Research Center, 2955 Highway
130 East, Stuttgart, AR 72160; phone (870) 673-4483, fax (870) 673-7710.