Bolbophorus confusus

From Fish
A channel catfish with small bumps and inflammation on its tailfin, which indicate the presence of Bolbophorus flatworms beneath the skin.

Bolbophorus confusus is a digenean trematode parasite that has recently been reported to cause mortality in channel catfish fingerlings in production ponds in Mississippi, Louisiana, and Alabama.

Life cycle

The definitive host of B confusus is the white pelican, and the first intermediate host is the ram’s horn snail (Heliosoma spp). Cercariae released from snails encyst in fish tissue, forming metacercariae in any tissue, but the majority are found in skin and skeletal muscle of the peduncle of juvenile channel catfish. Severe disease occurs when metacercariae encyst in visceral organs, particularly the posterior kidney and liver. Involvement of these organs can result in a presentation similar to enteric septicemia or channel virus disease, characterized by fluid accumulation in the abdomen and exophthalmia. Skin and muscle lesions typically result in raised bumps that are white to reddish in color. Visceral involvement can result in high mortality (95%) of small fish. Heavy infestation of older fish may result in anorexia, lethargy, and loss of condition. Digenea in skeletal muscle can result in condemnation of affected carcasses by processing plants[1].

Ponds at greatest risk to B confusus are those frequented by white pelicans. Pelicans are federally protected; however, assistance for control of nuisance wildlife is available through Wildlife Services of the USDA. Snail control is an important part of an overall control strategy and requires a mix of chemical, biological and aquatic plant control strategies.

Treatment

Copper sulfate is effective against snails but will not penetrate when they are buried in mud or sealed into their shells. Treatment is likely to be most effective in summer and early fall when snails are actively feeding. Nocturnal application of copper sulfate has been helpful in ornamental fish ponds, but care must be taken not to precipitate an oxygen depletion by killing plants and algae. Bayluscide® may be labeled in some states for control of aquatic snails. Chemical control will not eliminate snails, and efforts should be augmented by control of aquatic weeds. Snails climb emergent vegetation to lay eggs, so eliminating vegetation can decrease reproduction. Finally, biologic control may be attempted using black carp; however, these are an exotic species and stocking is prohibited in many geographic areas. Red-ear sunfish are also known to eat snails, but their potential impact on snail populations has not been tested. Due to the complexity of this problem, and rapid generation of new information, practitioners are urged to consult with extension and other aquaculture specialists.

References