101
(Olson 1978 ). Of course, it is also possible that the media are defi cient in certain
essential nutrients, necessary for the replication of Renibacterium. A long lag phase,
which has been suggested by Embley ( 1983 ), would be necessary for the organism
to adjust to the new environment of the laboratory medium, prior to replication. Any
or all these possibilities could apply to Renibacterium. Careful thought is necessary
to unravel many of the mysteries still surrounding the biology of this pathogen.
Pathogenicity
Pathogenicity experiments have met with varying degrees of success. Mackie et al.
( 1933 , 1935 ) succeeded in transmitting ‘Dee disease’ to brown trout by subcutaneous and i.m. injections of emulsifi ed spleen from Atlantic salmon. In these experiments, death followed in 5 weeks, although typical lesions, as found in fi eld
situations, did not occur. A similar observation was made by Belding and Merrill
( 1935 ), injected, intramuscularly, brook trout with purulent material collected from
kidney abscesses in the same species. Death followed in 18–25 days, but characteristic BKD lesions did not occur. This was, however, achieved by Earp ( 1950 ) following the injection of chinook salmon with a pure culture of the BKD organism.
Koch’s postulates were fi nally satisfi ed by Ordal and Earp ( 1956 ) following the
establishment of BKD in chinook salmon after i.p. injection of an organism obtained
from sockeye salmon. Mortalities started after 12 days, and continued until day 23,
when all the fi sh were dead. At this point, the organism was re-isolated. Sakai et al.
( 1989b ) found mortalities began 17 days after rainbow trout were injected with
4 × 10
8 cells. In comparison, carp ( Cyprinus carpio) were markedly resistant. Failure
greeted the attempt by Snieszko and Griffi n ( 1955 ) to transmit BKD to brook trout
by co-habiting with diseased fi sh for 21 days, followed by feeding with infected
viscera. However using feeding, success was achieved by Wood and Wallis ( 1955 )
with 100 % infection of 993 chinook salmon fi ngerlings. Later, Wolf and Dunbar
( 1959 ) achieved success by immersing experimentally wounded brook trout into a
suspension of the pathogen. Murray et al. ( 1992 ) succeeded in inducing BKD in
chinook salmon by immersion (10
4 –10
6 cells/ml for 15–30 min) and co-habitation
with other experimentally infected fi sh. However, the time to death was much longer than in most experimental models. By co-habitation and immersion, the average
periods leading to mortalities were 145 and 203 days, respectively. Transmission
from wild to cultured fi sh has been reported (Mitchum and Sherman 1981 ) and vice
versa (Frantsi et al. 1975 ). Prior infection with Ren. salmoninarum may well contribute to the poor survival of coho salmon upon transfer from fresh to sea water
(Moles 1997 ).
Evidence has pointed to the ability of Ren. salmoninarum becoming internalised
within non-phagocytic cells (González et al. 1999 ) and macrophages in which putative virulence factors are produced (McIntosh et al. 1997 ). Fish cell lines coupled
with iFAT were used to study the internalisation of the pathogen with results revealing that Ren. salmoninarum became localised in the vacuoles of CHSE-214 and
Renibacterium salmoninarum
(Olson 1978 ). Of course, it is also possible that the media are defi cient in certain
essential nutrients, necessary for the replication of Renibacterium. A long lag phase,
which has been suggested by Embley ( 1983 ), would be necessary for the organism
to adjust to the new environment of the laboratory medium, prior to replication. Any
or all these possibilities could apply to Renibacterium. Careful thought is necessary
to unravel many of the mysteries still surrounding the biology of this pathogen.
Pathogenicity
Pathogenicity experiments have met with varying degrees of success. Mackie et al.
( 1933 , 1935 ) succeeded in transmitting ‘Dee disease’ to brown trout by subcutaneous and i.m. injections of emulsifi ed spleen from Atlantic salmon. In these experiments, death followed in 5 weeks, although typical lesions, as found in fi eld
situations, did not occur. A similar observation was made by Belding and Merrill
( 1935 ), injected, intramuscularly, brook trout with purulent material collected from
kidney abscesses in the same species. Death followed in 18–25 days, but characteristic BKD lesions did not occur. This was, however, achieved by Earp ( 1950 ) following the injection of chinook salmon with a pure culture of the BKD organism.
Koch’s postulates were fi nally satisfi ed by Ordal and Earp ( 1956 ) following the
establishment of BKD in chinook salmon after i.p. injection of an organism obtained
from sockeye salmon. Mortalities started after 12 days, and continued until day 23,
when all the fi sh were dead. At this point, the organism was re-isolated. Sakai et al.
( 1989b ) found mortalities began 17 days after rainbow trout were injected with
4 × 10
8 cells. In comparison, carp ( Cyprinus carpio) were markedly resistant. Failure
greeted the attempt by Snieszko and Griffi n ( 1955 ) to transmit BKD to brook trout
by co-habiting with diseased fi sh for 21 days, followed by feeding with infected
viscera. However using feeding, success was achieved by Wood and Wallis ( 1955 )
with 100 % infection of 993 chinook salmon fi ngerlings. Later, Wolf and Dunbar
( 1959 ) achieved success by immersing experimentally wounded brook trout into a
suspension of the pathogen. Murray et al. ( 1992 ) succeeded in inducing BKD in
chinook salmon by immersion (10
4 –10
6 cells/ml for 15–30 min) and co-habitation
with other experimentally infected fi sh. However, the time to death was much longer than in most experimental models. By co-habitation and immersion, the average
periods leading to mortalities were 145 and 203 days, respectively. Transmission
from wild to cultured fi sh has been reported (Mitchum and Sherman 1981 ) and vice
versa (Frantsi et al. 1975 ). Prior infection with Ren. salmoninarum may well contribute to the poor survival of coho salmon upon transfer from fresh to sea water
(Moles 1997 ).
Evidence has pointed to the ability of Ren. salmoninarum becoming internalised
within non-phagocytic cells (González et al. 1999 ) and macrophages in which putative virulence factors are produced (McIntosh et al. 1997 ). Fish cell lines coupled
with iFAT were used to study the internalisation of the pathogen with results revealing that Ren. salmoninarum became localised in the vacuoles of CHSE-214 and
Renibacterium salmoninarum