Control And Treatment Transmission

While the transportation of cultured salmonids is believed to have facilitated the dissemination of R. salmoninarum (Rohovec and Fryer, 1988), the natural route of infection in salmonids is not fully understood. There is evidence that the disease is transmitted both horizontally and vertically. Circumstantial evidence for natural horizontal transmission was obtained by Mitchum and Sherman (1981). They stocked rainbow, brook and brown trout that were negative in a river system enzootic with BKD. When stocked trout were examined, R. salmoninarum was identified in 32-45% of the fish, using the IFAT. Mortalities attributed to BKD were observed within 9 months of stocking. Unfortunately, a group of stocked fish was not kept as controls to determine if they had undetectable levels of R. salmoninarum prior to release. Horizontal transmission has been demonstrated under laboratory conditions (Bell et al., 1984; Murray et al., 1992); R. salmoninarum-inoculated fish were placed in the same tank as uninoculated fish and the uninoculated fish became infected.

The mechanism through which horizontal transmission occurs is not clear. Infection through contaminated water may be one route. Viable R. salmoninarum have been demonstrated in fresh and salt water using FAT and bacterial culture (Austin and Rayment, 1985; Elliott and Pascho, 1991), and infection is probably via ingestion of infected water. Wood and Wallis (1955) infected 100% of salmon by feeding them infected adult viscera. A natural source of infective material may be faeces of clinically or subclinically infected or carrier fish. Balfry et al. (1996) showed that oral intubation of infected faecal material, but not autoclaved material, resulted in IFAT-positive fish and subsequent mortality. They suggested that this mechanism may contribute significantly to the horizontal transmission of R. salmoninarum among salmonids reared in sea-water net-pens.

In addition, R. salmoninarum may be transmitted to offspring via the egg. Allison (1958) and Bullock et al. (1978) were the first to report circumstantial evidence that gametes from infected adults, transferred to historically disease-free locations, resulted in clinically infected progeny. Renibacterium salmoninarum has been identified on both the surface and the inside of eggs from a naturally infected female coho salmon. The fish harboured 4 x 109 colony-forming units (CFU) R. salmoninarum ml-1 of ovarian fluid (Evelyn et al., 1984). The bacterium was cultured from 15.1% of surface-disinfected eggs and observed also within the yolk of sectioned eggs. Under laboratory conditions, infection of unfertilized steelhead (Oncorhynchus mykiss), coho (Oncorhynchus kisutch) and chinook (O. tshawytscha) salmon eggs by immersion challenge has only been accomplished using high numbers of R. salmoninarum (1.4 x 109, 1.3 x 1012 and 1.7 x 105, respectively) (Evelyn et al., 1986a). Under these conditions, only 1-5.5% of the experimentally exposed eggs contained viable R. salmoninarum (Evelyn et al., 1986a; Lee and Evelyn, 1989). It is not clear why only a low percentage of eggs were infected or when they became infected. Evelyn et al. (1986a) postulated that eggs in nature became infected after ovulation while they were in contact with coelomic fluid. However, Bruno and Munro (1986b) observed the presence of R. salmoninarum in tissue sections of maturing oogonia of experimentally infected trout. This suggests that infection may result directly from ovarian tissue prior to ovulation.

Apparently, intraovum infection results in infected progeny. Lee and Evelyn (1989) demonstrated that smolts reared from the eggs of naturally infected adults, but not from uninfected adults, had subclinical levels of R. salmoninarum, detectable using FAT. Low levels of R. salmoninarum (23-113 cfu ml-1 in ovarian fluid) were associated with a 1-2% infections in smolts with subclinical disease. Experimental immersion challenges of eggs also correlated with the prevalence of subclinical infections in smolts. Interestingly, no mortality was attributed to BKD in subclinically infected offspring, even though the prevalence was 44% in the highest challenge group. Unfortunately, bacterial cells from FAT-positive fish were not cultured or enumerated. Certainly, the mechanism (s) of intraovum transmission need to be further studied as this represents a novel mode of transmission of bacterial pathogens in vertebrates.

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