By Linda Breazeale

MISSISSIPPI STATE -- Mississippi State University researchers are working to reduce the health challenges mass production of catfish can bring, which are similar to the issues faced by other food animals such as poultry and swine.

"Anytime you have intensive management situations, you can have various outbreaks of disease or parasite problems," said Dr. Linda Pote, parasitologist with MSU's College of Veterinary Medicine. "Unfortunately, if producers reduce the numbers of fish or animals, they also will reduce their chances for making a profit."

Pote and other researchers based in Starkville and at the Thad Cochran National Warmwater Aquaculture Center in Stoneville have been working to improve health for the state's catfish industry. The primary challenges are proliferative gill disease, commonly referred to as hamburger gill disease, and trematodes, the top two catfish production problems.

Proliferative Gill Disease...

PGD is one of the most commonly diagnosed problems of catfish in the southeastern United States. The disease causes severe gill damage leading to suffocation of the fish, with severe outbreaks often resulting in mortalities in excess of 50 percent.

Diagnosed cases of PGD have been on the rise in recent years, with the growth attributed to weather conditions and increased awareness.

"Our efforts to study PGD took seven to 10 years for us to figure out how the parasite gets into the gills," Pote said.

The cause of PGD is a myxozoan parasite, which requires an oligochaete worm as a host for part of its life cycle. Pote joined Dr. Larry Hanson, molecular biologist with MSU's CVM, to settle the life cycle questions.

"Once Dr. Larry Hanson and I established its life cycle using molecular techniques, we could start looking for ways to protect the fish and get PGD under control. We needed to determine the environmental susceptibility and the age of infection," Pote said. "One approach for reducing the disease would be to break the life cycle of the parasite by eliminating the worms from the ponds."

Problems can happen in a brand new pond within the first year or ponds can go for years with subclinical infections. Testing gills is the primary way PGD is detected.

David Wise, associate fisheries biologist in Stoneville, said researchers are focusing on management strategies to reduce losses associated with this disease.

"So far, chemical treatments have not been extremely effective in controlling the disease," Wise said. "Chemicals currently approved for aquaculture use do not appear to have much effect on the spores that cause PGD or on the Dero worms that harbor the spores."

Epidemiological studies conducted at the Thad Cochran center indicate that essentially all ponds have this disease, but vary in severity.

"Seasonal changes also appear to influence the severity and number of outbreaks, with the most severe cases being observed in May and June," Wise said. "Newly stocked fish are very susceptible to this disease and account for the majority of losses associated with PGD."

Wise said "sentinel fish" are used to assess the risk or potential for severe outbreaks. Adopting this type of monitoring program can reduce PGD-related fish deaths in newly stocked ponds.

Catfish's other nemesis, trematodes (Bolbophorus confusus), are not easy to manage either, but they are more manageable than PGD.

Trematodes...

Wise said the key to controlling this species of trematode is reducing ram's horn snail numbers and keeping fish-eating birds such as pelicans off ponds. Outbreaks of trematodes, which is "an emerging disease," were first documented in 1999 in the Delta and numbers are increasing.

The trematode life cycle is complex and involves several intermediate hosts. It begins when the final host, usually fish-eating birds, release eggs into ponds containing other intermediate hosts. Ram's horn snails serve as the next hosts and the larval trematodes they release straight into the water infect fish. Pelicans eat the infected fish and the life cycle continues.

"If you've got pelicans in the area or if the pond is remote and people are not available to keep pelicans away, you are at an increased risk," Wise said. "Weed management also is important. There are no easy solutions, but every little bit helps reduce the risk."

These trematodes cause massive damage to the excretory system of the kidneys and liver of infected fish. Smaller fish appear to suffer the heaviest losses, however larger fish that survive develop anorexia and poor growth habits making them unsuitable for market.

Grower's Perspective...

Austin Jones, a partner in Bear Creek Fisheries in Moorhead, said PGD is significantly more challenging to manage.

"There's not much managing you can do for PGD; your ponds either get it or they don't," he said. "You just need to keep the oxygen up and keep the fish as healthy as you can. With trematodes, it's different because you have some options."

Jones said trematodes were having a major economic impact last year when they wiped out about a third of Bear Creek's fingerling crop. A catfish producer since 1982, he said 2000 was his first year to suffer losses from trematodes.

"We're trying to monitor more closely for pelicans and do snail inventories. We've added triploid black carp, which eat the snails, and we've treated the banks with hydrated lime to significantly reduce snail populations."

While he didn't have a major loss to PGD, Jones said the disease tends to "nickle and dime" growers from their profits.

MSU researchers used cages to track the occurrence of PGD in Bear Creak's ponds. They recommended a change from stocking in the spring, a peak season for PGD, to stocking in December, January and February.

"Stocking before the peak months seems to improve the fish's ability to acclimate to the disease," Jones said.

Released: July 16, 2001
For more information, contact: Dr. Linda Pote, (662) 325-3432