Effect of Sublethal Exposure to the Trematode <i>Bolbophorus</i> spp. on the Severity of Enteric Septicemia of Catfish inChannel Catfish Fingerlings

Abstract

Abstract The digenetic trematode Bolbophorus damnificus has been implicated as a cause of mortalities and reduced production in farmed channel catfish Ictalurus punctatus in northwestern Mississippi. Field observations indicate that infection can exacerbate losses from common bacterial diseases, such as enteric septicemia of catfish (ESC), which is caused by Edwardsiella ictaluri. The interaction of concurrent infections with Bolbophorus spp. and E. ictaluri was investigated using three replicates of four experimental treatments. Trematode infections were established by placing fingerlings for 24 h in triplicate static tanks containing marsh rams-horn snails Planorbella trivolvis shedding Bolbophorus cercariae at a rate of 770 ± 82 (mean ± SE) per 24-h period. Unexposed fish were maintained in three tanks under similar conditions. From each tank, trematode-infected or noninfected fish were transferred to six aquaria (30 fish/aquarium). Three aquaria from each replicate treatment tank received 7.5 × 105 colony-forming units (CFU) of E. ictaluri/mL of water for 30 min (Bolbophorus–ESC and ESC-only groups). Fish in the remaining three aquaria were not exposed to E. ictaluri and served as Bolbophorus-only and negative control groups. No mortalities were observed in the Bolbophorus-only and negative control groups. Twenty-one days after exposure to E. ictaluri, the percent cumulative mortalities were 84.1 ±16.2% in the Bolbophorus–ESC treatment and 45.9 ± 3.2% in the ESC-only treatment. Mortalities were significantly different between the two groups (P = 0.0161). In a second study, fingerlings were challenged with E. ictaluri 28 d after trematode exposure; 21 d later, no differences in cumulative percent mortalities were observed between trematode-infected (21.5 ± 1.7%) and trematode-negative (17.8 ± 4.01%) fish. These studies show that sublethal Bolbophorus infection increases mortalities from E. ictaluri when exposure occurs simultaneously but not when ESC infection occurs 28 d after trematode exposure.