The health of native Siluriformes in northern Australia

Native Australian catfishes are vital to the function of freshwater ecosystems and have the potential to be successful aquaculture species. The effective protection and management of wild catfish, and the commercial viability of their use in aquaculture, is reliant on the ability to understand, identify, and control the health issues and pathogens of these species. Significant gaps in our knowledge of Australian catfish species currently limit our ability to achieve these aims, including limited information on the presence and distribution of exotic fish pathogens in Australia, and the lack of reference data for common methods of health assessment. The primary aim of this study was to determine whether the exotic bacterium Edwardsiella ictaluri, the most important pathogen of farmed catfish in the United States of America, was present in wild Australian catfish. A secondary aim was to generate baseline data on the haematology, histology and parasite fauna of six species of freshwater catfish; Wet Tropics tandan Tandanus tropicanus, freshwater catfish Tandanus tandanus, Hyrtyl’s tandan Neosilurus hyrtlii, black catfish Neosilurus ater, blue catfish Neoarius graeffei and silver cobbler Neoarius midgleyorum, which are widely distributed in northern Australia. A scenario tree approach was used to develop a model for a targeted, risk-based survey of E. ictaluri in wild catfishes in northern Australia. The final sampling design, based on the model, involved a mean sample size of EF fish from EY sites, providing a probability of ZY%that wild catfish populations in northern Australia are free of E. ictaluri at an overall prevalence of E% (among-river and within-river design prevalences of ED% each), given negative survey results. Catfishes were sampled from these sites and tested for the bacterium by selective culturing and DNA testing. Edwardsiella ictaluri was detected in Wet Tropics tandan Tandanus tropicanus from the Tully River, Queensland, at a prevalence of D._D (ZY% CI D.CE-D.`E), indicating that wild fishes in Australia can no longer be assumed to be free from E. ictaluri. Phylogeographic data are currently inadequate to determine the possible origin of the bacterium. However, if the E. ictaluri detected in the Tully River catchment is the result of a recent introduction, the most likely source is via the release of infected exotic ornamental fishes. The eradication of E. ictaluri is probably not a viable option, and therefore management options that limit pathogen spread should be considered. This study also provides the first report on the normal histology and haematology of the six arid and plotosid species studied. Erythrocytes, thrombocytes, lymphocytes, monocytes and heterophils were observed in all catfish species studied and were similar to those reported in other Siluriformes. Leukocytes uncommonly reported in teleosts were also observed, including basophils in T. tropicanus and N. ater, eosinophils in N. graeffei, and periodic acid-Schiff positive granular leukocytes in N. graeffei, N. midgleyorum, N. ater and T. tandanus. The results of cytochemical stains that assist in the identification of leukocytes in peripheral blood smears are provided to assist in the application of haematology in assessments of fish health. The histology of all catfishes examined was largely similar to reports in other Siluriformes. However, significant differences in the structure of renal collecting and opisthonephric ducts occurred between Tandanus, Neosilurus and Neoarius species. The collecting and opisthonephric ducts of Tandanus tandanus and Tandanus tropicanus were lined by mucus cells, which may be related to the unique reproductive biology of these species. This finding is rarely reported in teleosts, and has not been reported in other species of Siluriformes. Parasites have the potential to seriously impact wild and farmed fishes, and host-parasite interactions are an important aspect of ecosystems and their food webs. The diversity and impact of haematozoa on native Australian freshwater fishes is largely unknown, and it is likely that a substantial number of protozoa in Australian fish have not been recorded. This study provides the first widespread survey of wild Australian catfishes for haematozoa. Haematozoa were observed only in fishes from Queensland, at an overall prevalence of D.EZ (ZY% CI D.Ea-D.C`). Intraerythrocytic haemogregarines were observed in N. graeffei from the Brisbane River at a prevalence of D.aY (D.EFE-D.Y`b). Trypanosomes were present in Tandanus species from four rivers, at prevalences ranging from D.EE (D.DC-D.aa) to E (D.`_-E), and in N. graeffei from one river at a prevalence of D.D` (D.DDa-D.aD). Tandanus species were significantly more likely to be infected, suggesting a high parasite-host specificity. Although a number of metazoan and protozoan parasite species have been reported from the tissues of native Australian catfishes, very few assess the pathology associated with parasitic infection. This study aimed to investigate the prevalence and spatial distribution of parasites, record the pathology of the infections, and investigate the influence of fish species and locality on the prevalence of parasitic infection and parasite community composition. Forty-three putative species from at least eight phyla were identified, including Acanthocephala, Arthropoda, Cnidaria, Nematoda, Platyhelminthes (Cestoda, Trematoda and Monogenea), Amoebozoa, Apicomplexa and non-assigned protozoans (observed as bradyzoites). In total, parasites were recorded from CDE (FF%) of catfishes. There was major variation in parasite species richness and community composition between fish species and locality. Parasites were most commonly associated with a mild to moderate inflammatory response, and in some cases appeared to be contributing to the size of pigmented macrophage aggregates. Although the majority of parasites observed appeared to have a minimal impact on their hosts, this study indicates that native catfishes can be host to a relatively large and diverse range of parasites. This study provides the first report on the comparative histology, and leukocyte morphology and cytochemistry of six catfish species common to northern Australia. It also provides the results of the first widespread survey of Australian freshwater catfishes for haematozoa, resulting in the first record of haemogregarines from Australian freshwater fishes, and the first record of trypanosomes from Neoarius graeffei and Tandanus tropicanus. In addition, this study also reports the results of the first widespread assessment of the pathology of parasitic infections in Australian freshwater catfishes, including the first report of acanthocephalans in Australian catfishes. Finally, this study provides the first report of the exotic bacterium Edwardsiella ictaluri in wild fishes in Australia, indicating that important exotic pathogens of fish may be entering Australia, and constitute a potential threat to freshwater biodiversity