Epizootiology Gram negative bacteria 267 investigated the possibility that the

Epizootiology gram negative bacteria 267 investigated

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Epizootiology: Gram-negative bacteria 267 investigated the possibility that the disease could be carried by salmon or sea trout smolts (previously infected in freshwater) when they migrated to the sea. The Fur- unculosis 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 confirmed Aer. salmonicida from four smolts (two salmon and two sea trout), and believed the findings to be significant as such fish would possibly develop the disease on exposure to suitable conditions or remain resistant, possibly transmitting the infection upon contact with healthy fish in sea or brackish waters. Lund (1967) could not offer a definitive reason for the results differing from those of Williamson and Anderson (see Mackie et ai, 1930), who examined 1,339 smolts taken from the Coquet without recovering any isolates of Aer. salmonicida. Certainly, mortalities attributed to Aer. salmonicida in anadromous fish in seawater and in trout grown on in seawater have been reported (Evelyn, 1971a; Hastein and Bullock, 1976; Novotny, 1978). However, it has not been determined whether the disease outbreaks resulted from stress experienced by fish carrying a latent infection initially contracted in freshwater, or whether they represented a case of lateral transmission of the pathogen via seawater. Smith (1962), for example, had established that Aer. salmonicida survived in seawater 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 fish 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 fish farms in Ireland, presenting evidence for the lateral transmission of Aer. salmonicida in seawater to a group offish not known to be carriers. They also provided data suggesting that, subsequent to the stocking in Spring 1978 and removal of carrier fish in Summer 1979 at a marine fish farm, the pathogen became established and persisted in the fish farm environment for at least 6 months after the removal of the carrier fish. 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 fish outside the cages or in the sediments under the cages. To lend support to a seawater transmission of furuncu- losis, Evelyn (1971a) has documented isolation of Aer. salmonicida from a strictly marine host, the sable fish, although probably the route of infection was by ingestion of moribund or dead salmonid carrier fish (Klontz and Wood, 1972). Obviously, Aer. salmonicida
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  • Spring '20
  • Bacteria, representative, gram-negative bacteria

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