Bolbophorus confususInfections in Channel Catfish in Northwestern Mississippi and Effects of Water Temperature on Emergence of Cercariae from Infected Snails

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Abstract
The digenetic trematode tentatively identified as Bolbophorus confusus has recently been implicated as a cause of severe mortalities and reduced production in commercially raised catfish in the lower Mississippi Valley. Our study documents the prevalence of B. confusus infections in channel catfish Ictalurus punctatus from fish farms located in northwestern Mississippi. As of August 2000, 821 ponds had been sampled (approximately 20–40 fish per pond) from 32 farms in eight Mississippi counties. Of these ponds, 262 yielded at least one fish with metacercariae characteristic of B. confusus. The overall impact on farms and the industry cannot be fully evaluated from these data, but evidence suggests that infection with this trematode is becoming widespread, with the more severely affected farms being in close proximity to pelican roosting or resting sites. To evaluate some of the long‐term implications of this trematode for farm management, two laboratory studies were conducted to determine the relationship between temperature and the release of B. confusus cercariae from infected snails. In the first experiment, lowering the temperature from 25°C to 15°C significantly (P ≤ 0.05) decreased the number of cercariae shed from the snails, from 345 to 1.6 cercariae/mL. When these snails were reacclimated to 25°C, cercarial shedding returned to the rate initially observed at 25°C (330 cercariae/mL). There was no significant difference between the cercarial shedding rates of snails at 25°C and 35°C, but the shedding rate decreased significantly when the temperature was subsequently lowered from 35°C to 25°C. In a second experiment, infected snails maintained at 10°C for 7 and 14 d were shown to retain the ability to shed cercariae after temperatures were increased to 23°C. These results suggest that if infected snails survive the winter they may retain the ability to shed cercariae the following spring as the water temperature in ponds increases.