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Parasite Threatens Mississippi Catfish
MISSISSIPPI STATE - Biologists are encouraging Mississippi catfish producers to control snails in ponds to combat a parasite that caused some severe fingerling losses last year.
1999 was the first year this internal parasite, a trematode tentatively identified as Bolbophorus confusus, was found in Mississippi Delta channel catfish. It is rarely fatal to large catfish, but it can kill young catfish, or fingerlings.
Mississippi Agricultural and Forestry Experiment Station biologists at the Thad Cochran National Warmwater Aquaculture Center in Stoneville monitored the problem when it first surfaced in the Delta region. A producer brought diseased fish in for diagnosis, and tests showed that 28 of his 32 ponds were infected with trematodes, 10 severely.
Further tests in six Delta counties found the problem in each county with 12 of the 14 farms examined testing positive for trematode infestations that varied greatly in severity. The trematode problem was first seen in Louisiana where it has caused severe fingerling mortality and farm production losses.
Jim Steeby, area aquaculture agent with Mississippi State University's Extension Service, said the parasite appears to have a life cycle that starts with the white pelican, moves into the ram's horn snail and from there it infects catfish in commercial production. The cycle is complete when the catfish are eaten by pelicans.
"In larger fish, it possibly diminishes some appetite, causing the fish to not grow as well," Steeby said. "But as the parasite develops under the skin of the smaller fish, it appears to cause liver and kidney damage, and kills many fingerlings."
MSU researchers with MAFES and the College of Veterinary Medicine are trying to find a way to break the parasite's life cycle and prevent catfish losses. To date, it appears that if catfish ponds can be cleared of snails, or their numbers greatly reduced, the trematode will be controlled.
MSU researchers are hoping to prevent a widespread problem in Mississippi. The trematode appears to infect fingerlings at much higher rates than it does larger catfish.
Dr. Lester Khoo, CVM veterinarian working in the fish diagnostic laboratory at the Thad Cochran National Warmwater Aquaculture Center, said the high numbers of deaths of trematode-infected catfish may indicate the catfish is an adopted host and not the true host.
"We're trying to understand why these fish are dying," Khoo said. "Normally you don't get such high mortality with a parasite as they usually are better adapted to the host."
Khoo is also trying to determine what affect sub-lethal doses of the trematode have on catfish and whether the parasite suppresses the catfishes' appetites as suspected.
Another area of interest is identifying the adult form of the trematode. The Extension aquaculture agent said this is harder than it sounds because adult birds, such as the white pelican, carry a variety of parasites in their body, including more than one type of trematode.
"We're trying to document the life cycle on this parasite and we're missing the adult stage," Steeby said.
Once the adult form is identified, researchers can determine if it is being spread by other fish-eating birds. Dr. Linda Pote is conducting this work at MSU's veterinary college with assistance from the U.S. Department of Agriculture.
"If you get more birds and snails infected, the fish-eating birds migrate and move around so much that pretty soon you could have the potential for large numbers of infected ponds," Steeby said.
MAFES aquaculture center researchers Drs. Jeff Terhune and David Wise are monitoring the parasite and its spread in Mississippi to date. They also are working on ways to rid ponds of snails. Hydrated lime and copper sulfate applications near the pond edges seem to offer the most promise.
"We've found an effective treatment is 50 pounds of hydrated lime per 75 feet of levee," Terhune said. "That treats just the pond margins, a swath about 2-feet wide around the pond."
Such a treatment uses about one ton of hydrated lime per 15-acre pond at a cost of about $200. Catfish producers should begin treatments if they see any pelican activity around their ponds and if they have significant snail numbers.
"If those two factors are in place, they need to start treating the ponds," Terhune said.
Steeby said snail control appears to be the current best line of defense.
"We're trying to raise the awareness of the farmers and get them into snail treatment programs for this spring so we don't have any severe fingerling losses," Steeby said.