Bacterial kidney disease ( BKD ) caused by Renibacterium salmoninarum occurs in most parts of the world where salmonid fish are cultured or occur in the wild ( 25, 26 ) . Salmonids vary in their susceptibility to BKD, and Pacific salmon species of the genus Oncorhynchus are generally considered to be the most susceptible ( 21, 26, 49, 52 ) . BKD can cause serious mortality in juvenile salmonids in both fresh water and seawater, and also in prespawning adults. The disease has been described throughout North America and in many countries in Europe, as well as Japan, Chile, and Iceland ( 2, 21, 25, 27 ) . Most recorded outbreaks of BKD have occurred in fish culture facilities, and the spread of BKD has followed the expansion of salmonid culture ( 25 ) . Clinical BKD has also been reported in feral fish ( 5, 36, 39, 45 ) , including naturally spawning populations that have never been supplemented with hatchery fish ( 22, 51 ) . Whereas the chronic nature of the disease has hindered accurate estimates of fish losses, particularly in feral fish populations, BKD remains one of the most important bacterial diseases affecting cultured salmonids. Losses as high as 80% in stocks of Pacific salmon and 40% in stocks of Atlantic salmon (Salmo salar) have been reported ( 25 ) .
Renibacterium salmoninarum is a small ( 0.3-0.1 µm by 1.0-1.5 µm ) , nonmotile, nonspore-forming, nonacid-fast, Gram-positive diplobacillus ( 27 ) . It typically causes a slowly progressing systemic infection, with overt disease rarely evident until fish are 6-12 months old ( 21 ). Fish with severe R. salmoninarum infections may show no obvious external signs, or may exhibit one or more of the following: lethargy; skin darkening; abdominal distension due to ascites; pale gills associated with anaemia; exophthalmos; haemorrhages around the vent; and cystic cavities in the skeletal muscle. Internal examination usually reveals the presence of focal to multifocal grayish-white nodular lesions in the kidney, and sometimes in the spleen and liver. In addition, there may be turbid fluid in the abdominal cavity, haemorrhages on the abdominal wall and in the viscera, and a diffuse white membranous layer ( pseudomembrane ) on one or more of the internal organs. In tissue sections of BKD lesions, R. salmoninarum is frequently observed within phagocytic cells, particularly macrophages. The bacterium appears to survive and perhaps replicate within these cells ( 4, 10, 31, 57 ) .
The kidney disease bacterium can be transmitted both horizontally from infected fish sharing the water supply ( 6, 38 ) , and vertically in association with eggs from infected parents ( 23, 43 ) . As with other infectious diseases of salmonids that are difficult or impossible to treat, avoidance is recommended for the control of BKD in cultured salmonid stocks ( 1, 21 ) . Because R. salmoninarum is often enzootic in wild salmonid populations ( 9, 22, 34 ) , measures to control losses from BKD may be defeated by constant exposure of hatchery fish to waterborne bacteria shed into the water supply by wild fish residing upstream from the hatchery ( 33, 38 ) . Salmonids reared in seawater present special problems because it is difficult to ensure adequate separation of groups of fish to prevent horizontal transmission, and because of the possibility that other marine species might serve as reservoirs for R. salmoninarum( 7, 14, 50, 54 ) .
To reduce the probability of vertical transmission of R. salmoninarum in cultured salmonids, brood stock segregation or culling is now used to select egg lots to retain as a source of juvenile fish for hatchery rearing ( 30, 43, 55 ) The selection process is aimed at rearing egg lots from mating pairs with undetectable or very low levels of R. salmoninarum . This requires the use of sensitive BKD detection methods for testing the prevalence and levels of R. salmoninarum in each parent fish. Elliott & Barila ( 16 ) believed that the membrane-filtration fluorescent antibody test ( MF-FAT ) would provide the sensitivity and quantification necessary to investigate the relationship between the levels of R. salmoninarum in the female parent and the probability of transmitting the disease to the progeny. Pascho et al.( 43 ) later demonstrated the usefulness of brood stock segregation for controlling losses from BKD by using the MF-FAT in conjunction with the enzyme-linked immunosorbent assay to segregate egg lots from chinook salmon parents infected with either very low or very high levels of R. salmoninarum. These researchers reported that the losses from BKD among the progeny of parents with very low levels of R. salmoninarum were significantly less than those among the progeny of parents with very high infection levels. The aquaculturist must be aware, however, that brood stock segregation may not completely eliminate BKD from an affected population. Because the broodstock used for commercial fish farming should be free of the kidney disease bacterium, it may be necessary to repopulate a contaminated facility with brood fish from a BKD-free population.
Crucial to the success of any BKD control programme is the application of reliable diagnostic methods that can detect low levels of R. salmoninarum in a variety of sample types. For that reason, fish health specialists and researchers have long been interested in developing methods for more rapid and reliable detection of R. salmoninarum infections ( 29, 41, 42, 48, 56 ) . As each new test has been developed, however, there has been a tendency to reject older techniques. Nevertheless, no single ideal diagnostic test has yet been developed for the evaluation of multiple samples for the presence of BKD.