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The Problem
Bacterial kidney disease (BKD) is caused by Renibacterium salmoninarum and is considered to be the most difficult salmonid bacterial disease to control, in part because of its dual modes of transmission. This bacterium, unlike most other fish pathogens, is transmitted vertically from parent fish to progeny in association with the eggs, as well as horizontally from fish to fish. Certain types of intensive fishery management activities may enhance the horizontal and vertical transmission R. salmoninarum. For example, evidence from WFRC research indicates that fish marking procedures such as coded wire tagging may enhance the horizontal transmission of R. salmoninarum under certain conditions. Currently, more than 40 million salmonids per year are marked with coded wire tags for federal, state, and tribal fisheries management programs on the west coast of the US and Canada, but the contribution of these marking programs to disease transmission is unknown. Based on earlier research performed by the WFRC, federal, state, and tribal salmonid brood stock screening programs have been under development for several years to reduce the vertical transmission of R. salmoninarum in salmonid populations. Although such programs have shown promise as management tools, important questions remain regarding the optimization of the screening procedures for maximum effectiveness. This project is designed to study aspects of vertical and horizontal transmission of R. salmoninarum in order to develop a better understanding of the factors important for successful transmission, and to optimize procedures for interrupting or preventing transmission of the pathogen. Objectives The overall objective of this research is to improve understanding of the mechanisms of horizontal and vertical transmission of R. salmoninarum for the purpose of facilitating the development of better methods for prevention and control of BKD. The specific current objective of research on horizontal transmission is to investigate the possible roles of fish tagging procedures and skin injury (descaling) in the transmission of R. salmoninarum. Specific objectives of research on vertical transmission are to investigate: (1) the relation between ovarian fluid and kidney R. salmoninarum levels in adult female salmon and the success of transmission of the pathogen to progeny; (2) the role of the male parent in vertical transmission; and (3) the utility of various diagnostic methods for discerning those fish most likely to transmit the pathogen to progeny fish. Methodology Laboratory challenges (waterborne exposure or cohabitation with infected fish) and field studies of naturally infected fish at hatcheries are being used to investigate horizontal transmission of R. salmoninarum. Naturally infected female salmon and their progeny are being used to study vertical transmission of R. salmoninarum. Methods used for the detection and quantification of R. salmoninarum include the enzyme-linked immunosorbent assay (ELISA), the membrane filtration-
Highlights and Key Findings
Preliminary results of vertical transmission studies indicated that the use of male chinook salmon with high R. salmoninarum infection levels does not result in increased prevalence of R. salmoninarum in progeny fish. Because previous research at the WFRC and other laboratories has shown that female chinook salmon with very high numbers of R. salmoninarum in the ovarian fluid (the fluid surrounding the eggs at spawning) are likely to have some infected progeny, development of a quantitative PCR was begun for correlation of the numbers of bacteria present in the ovarian fluid of a female salmon with the percentage of her eggs that are infected, and the level of infection in those eggs. Such information would be extremely valuable for developing or modifying programs for screening and segregation or culling of spawning salmonids that would likely transmit the bacterium to progeny. Laboratory experiments conducted at the WFRC have indicated that R. salmoninarum infections can become established in sites of minor skin injury such as those that occur during handling of fish at hatcheries or dams. However, because the outer skin of fish is transparent, many of these injuries are not readily visible. Laboratory experiments at the WFRC showed that fast green, a food dye, can be used as a non-lethal stain to identify sites of recent minor skin injury that would not be visible to the naked eye but that could provide portals of entry for R. salmoninarum and other fish pathogens. Fast green staining is now being used in field studies to study the occurrence of skin injury and disease transmission during fish handling procedures.
Coded wire tags implanted in the snouts of fish are widely used for identification and management of stocks of migratory salmonid fishes. Research at the WFRC has shown that horizontal transmission of R. salmoninarum can be enhanced under certain conditions during coded-wire-tagging of chinook salmon. Studies were conducted at two Washington Department of Fish and Wildlife Hatcheries to compare the occurrence and severity of incidental injuries and the potential for R. salmoninarum transmission during coded-wire-tagging of fish in a conventional marking trailer and in a new automated trailer that minimizes handling of fish by humans. The conventional trailer uses a recirculating water system, which can result in the concentration of bacteria, whereas the automated trailer uses a flow-through water system. Preliminary results of field studies at the hatcheries showed no difference between the automated and conventional marking methods in the occurrence of incidental skin and fin injuries in fish. Both procedures were associated with significant incidental skin and fin injuries, which could be reduced by modifications to fish handling procedures. Preliminary analyses of water and fish tissue samples suggested that tagging by either trailer of progeny fish from female chinook salmon with moderate to high infection levels of R. salmoninarum can enhance the transmission of the bacterium. Where Are We Headed In 2003 For studies of R. salmoninarum horizontal transmission, analysis of samples from comparisons of conventional and automated marking systems will be completed. The samples include water samples for R. salmoninarum analysis, fish tissue samples for R. salmoninarum detection (histopathology and ELISA) and skin injury analysis (histopathology and digital photographs). Procedures will be developed for evaluation of skin injury severity by computer image analysis of digital photographs. For R. salmoninarum vertical transmission studies, testing of quantitative PCR procedures will continue for the purpose of developing a method for correlation of the prevalence and levels of R. salmoninarum in eggs with numbers of bacteria present in the ovarian fluid of the female parents. Project Contact Diane Elliott Email: diane_elliott@usgs.gov Publications |
