Morphologic, Pathologic, and Genetic Investigations of Bolbophorus Species Affecting Cultured Channel Catfish in the Mississippi Delta.

Trematodes belonging to the genus Bolbophorus have recently been reported as the cause of substantial morbidity and mortality in cultured channel catfish Ictalurus punctatus in Mississippi and Louisiana. Previous investigators identified only a single species, B. confusus. In this investigation, genetic techniques were used to identify all stages of the parasite in all of its hosts. The 18s rRNA genes from specimens collected in Mississippi were sequenced and compared; this analysis revealed that there are two distinct species, B. damnificus (previously identified as B. confusus) and another, undescribed species. (Phylogenetic analysis indicated that a third species, B. levantinus, is also closely related to the Mississippi species.) Species-specific polymerase chain reaction assays capable of identifying and differentiating between these two parasites were developed. Both species were found to infect the first intermediate host (the ram's horn snail Planorbella trivolvis) in commercial channel catfish ponds, but only B. damnificus was recovered from the fish themselves. The new, unidentified Bolbophorus species was determined to be highly pathogenic to a number of fish species. The contribution of B. damnificus to disease in cultured channel catfish remains undetermined. Future investigations of these parasites must now take into account the presence of two distinct species.

Genetic relationships among members of the Ichthyobodo necator complex: implications for the management of aquaculture stocks.

Abstract Ichthyobodo necator (costia) is a common and important flagellate parasite that infests the skin and gills of many freshwater and marine fish. Costia infestations are often fatal and cause significant aquaculture losses worldwide. Recently it has been demonstrated that Ichthyobodo is a multispecies complex with differing host preferences. Knowing if those species have broad or narrow host specificity has important implications for the management of costia. To address the question of host specificity, genomic DNA was isolated from Ichthyobodo trophonts collected from rainbow trout, Oncorhynchus mykiss, koi, Cyprinus carpio, mirror carp, C. carpio, goldfish, Carassius auratus, channel catfish, Ictalurus punctatus, swordtail, Xiphophorus helleri, and Japanese flounder, Paralichthys olivaceus. The small subunit ribosomal RNA (SSU rRNA) gene from each isolate was analysed with previously published Ichthyobodo sequences using Bayesian phylogenetic methods. The internal transcribed spacers (ITS) from six isolates were also PCR-amplified, cloned and sequenced. Both the SSU rRNA phylogenetic analysis and the ITS rRNA sequence data support grouping the 22 Ichthyobodo isolates examined into a complex of nine different species. Many of these species were frequently isolated from multiple hosts, indicating that exchange of infested fish from one region to another has a high potential for spreading the disease. In one instance, the same species was obtained from marine and freshwater fish, further suggesting that certain Ichthyobodo species may not be limited by salinity.

Morphologic, Pathologic, and Genetic Investigations of Bolbophorus Species Affecting Cultured Channel Catfish in the Mississippi Delta.

Trematodes belonging to the genus Bolbophorus have recently been reported as the cause of substantial morbidity and mortality in cultured channel catfish Ictalurus punctatus in Mississippi and Louisiana. Previous investigators identified only a single species, B. confusus. In this investigation, genetic techniques were used to identify all stages of the parasite in all of its hosts. The 18s rRNA genes from specimens collected in Mississippi were sequenced and compared; this analysis revealed that there are two distinct species, B. damnificus (previously identified as B. confusus) and another, undescribed species. (Phylogenetic analysis indicated that a third species, B. levantinus, is also closely related to the Mississippi species.) Species-specific polymerase chain reaction assays capable of identifying and differentiating between these two parasites were developed. Both species were found to infect the first intermediate host (the ram's horn snail Planorbella trivolvis) in commercial channel catfish ponds, but only B. damnificus was recovered from the fish themselves. The new, unidentified Bolbophorus species was determined to be highly pathogenic to a number of fish species. The contribution of B. damnificus to disease in cultured channel catfish remains undetermined. Future investigations of these parasites must now take into account the presence of two distinct species.