261
Aeromonas salmonicida – Transmission in Seawater A remaining aspect of the
epizootiology of Aer. salmonicida diseases which requires consideration is the
transmission of the infection in sea water. This is an important topic for the aquaculture industry, as salmonids are not infrequently placed in seawater for on-growing.
In addition, early in the study of the pathogen, the possibility that migratory
Salmonidae could spread the infection was considered. Lund ( 1967 ) investigated
the possibility that the disease could be carried by salmon or sea trout smolts (previously infected in fresh water) when they migrated to the sea. The Furunculosis
Committee had not agreed with this theory because examination of large numbers
of smolts taken from the River Coquet in 1928 and 1929 had given no evidence for
the presence of Aer. salmonicida, although the reverse process, i.e. salmon or sea
trout contracting the infection upon migration into rivers containing infected trout,
had become generally accepted. In an examination of 234 smolts from the River
Coquet, Lund ( 1967 ) isolated and confi rmed Aer. salmonicida from four smolts
(two salmon and two sea trout), and believed the fi ndings to be signifi cant as such
fi sh would possibly develop the disease on exposure to suitable conditions or remain
resistant, possibly transmitting the infection upon contact with healthy fi sh in sea or
brackish waters. Lund ( 1967 ) could not offer a defi nitive reason for the results differing from those of Williamson and Anderson (see Mackie et al. 1930 ), who examined 1339 smolts taken from the Coquet without recovering any isolates of Aer.
salmonicida. Certainly, mortalities attributed to Aer. salmonicida in anadromous
fi sh in sea water and in trout grown-on in sea water have been reported (Evelyn
1971a ; Håstein and Bullock 1976 ; Novotny 1978 ). However, it has not been determined whether the disease outbreaks resulted from stress experienced by fi sh carrying a latent infection initially contracted in fresh water, or whether they represented
a case of lateral transmission of the pathogen via sea water. Smith ( 1962 ), for example, had established that Aer. salmonicida survived in sea water for a prolonged
period of time. It had also been demonstrated that Aer. salmonicida is capable of
infecting sea and brown trout by contact with infected fi sh in sea and brackish
waters (Scott 1968 ). She found that the infection was transmitted between salinities
of 2.54 and 3.31 % (w/v) at water temperatures ranging from 5.6 to 14.5 °C. Smith
et al. ( 1982 ) reported on mortalities of Atlantic salmon from two marine fi sh farms
in Ireland, presenting evidence for the lateral transmission of Aer. salmonicida in
sea water to a group of fi sh not known to be carriers. They also provided data suggesting that subsequent to the stocking in spring 1978, and removal of carrier fi sh in
summer 1979, at a marine fi sh farm, the pathogen became established and persisted
in the fi sh farm environment for at least 6 months after the removal of the carrier
fi sh. Thus, a carrier-free population placed on the site in the spring of 1980 was
infected. Unfortunately, it was not determined whether the pathogen persisted in
feral fi sh outside the cages or in the sediments under the cages. To lend support to a
sea water transmission of furunculosis, Evelyn ( 1971 ) has documented isolation of
Aer. salmonicida from a strictly marine host, the sable fi sh, although probably the
route of infection was by ingestion of moribund or dead salmonid carrier fi sh
(Klontz and Wood 1972 ). Obviously, Aer. salmonicida has wider potential for causing disease problems than has been hitherto suspected. The study of the epizootiolAeromonas salmonicida
Aeromonas salmonicida – Transmission in Seawater A remaining aspect of the
epizootiology of Aer. salmonicida diseases which requires consideration is the
transmission of the infection in sea water. This is an important topic for the aquaculture industry, as salmonids are not infrequently placed in seawater for on-growing.
In addition, early in the study of the pathogen, the possibility that migratory
Salmonidae could spread the infection was considered. Lund ( 1967 ) investigated
the possibility that the disease could be carried by salmon or sea trout smolts (previously infected in fresh water) when they migrated to the sea. The Furunculosis
Committee had not agreed with this theory because examination of large numbers
of smolts taken from the River Coquet in 1928 and 1929 had given no evidence for
the presence of Aer. salmonicida, although the reverse process, i.e. salmon or sea
trout contracting the infection upon migration into rivers containing infected trout,
had become generally accepted. In an examination of 234 smolts from the River
Coquet, Lund ( 1967 ) isolated and confi rmed Aer. salmonicida from four smolts
(two salmon and two sea trout), and believed the fi ndings to be signifi cant as such
fi sh would possibly develop the disease on exposure to suitable conditions or remain
resistant, possibly transmitting the infection upon contact with healthy fi sh in sea or
brackish waters. Lund ( 1967 ) could not offer a defi nitive reason for the results differing from those of Williamson and Anderson (see Mackie et al. 1930 ), who examined 1339 smolts taken from the Coquet without recovering any isolates of Aer.
salmonicida. Certainly, mortalities attributed to Aer. salmonicida in anadromous
fi sh in sea water and in trout grown-on in sea water have been reported (Evelyn
1971a ; Håstein and Bullock 1976 ; Novotny 1978 ). However, it has not been determined whether the disease outbreaks resulted from stress experienced by fi sh carrying a latent infection initially contracted in fresh water, or whether they represented
a case of lateral transmission of the pathogen via sea water. Smith ( 1962 ), for example, had established that Aer. salmonicida survived in sea water for a prolonged
period of time. It had also been demonstrated that Aer. salmonicida is capable of
infecting sea and brown trout by contact with infected fi sh in sea and brackish
waters (Scott 1968 ). She found that the infection was transmitted between salinities
of 2.54 and 3.31 % (w/v) at water temperatures ranging from 5.6 to 14.5 °C. Smith
et al. ( 1982 ) reported on mortalities of Atlantic salmon from two marine fi sh farms
in Ireland, presenting evidence for the lateral transmission of Aer. salmonicida in
sea water to a group of fi sh not known to be carriers. They also provided data suggesting that subsequent to the stocking in spring 1978, and removal of carrier fi sh in
summer 1979, at a marine fi sh farm, the pathogen became established and persisted
in the fi sh farm environment for at least 6 months after the removal of the carrier
fi sh. Thus, a carrier-free population placed on the site in the spring of 1980 was
infected. Unfortunately, it was not determined whether the pathogen persisted in
feral fi sh outside the cages or in the sediments under the cages. To lend support to a
sea water transmission of furunculosis, Evelyn ( 1971 ) has documented isolation of
Aer. salmonicida from a strictly marine host, the sable fi sh, although probably the
route of infection was by ingestion of moribund or dead salmonid carrier fi sh
(Klontz and Wood 1972 ). Obviously, Aer. salmonicida has wider potential for causing disease problems than has been hitherto suspected. The study of the epizootiolAeromonas salmonicida