Traditionally Aer salmonicida has been known as the causative agent of

Traditionally aer salmonicida has been known as the

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Traditionally, Aer. salmonicida has been known as the causative agent of furunculosis. However, it has become apparent that the pathogen manifests itself with other conditions, notably ulcerative dermatitis (Brocklebank, 1998), and ulcerations, especially in non-salmonids, e.g. in cod (Magnadottir et al., 2002). Furunculosis, named because of the sub-acute or chronic form of the disease, is recognised by the presence of lesions resembUng boils, i.e. furuncles, in the muscu- lature. In fact, the term furunculosis is a misnomer, because the lesions do not resemble those found in the similarly named condition of human beings (McCarthy, 1975a). The name has, however, become estabhshed in the fisheries literature, so that
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26 Bacterial Fish Pathogens it has been retained for convenience and to avoid the confusion which could result from a new name. The sub-acute or chronic form of furunculosis, which is more common in older fish, is characterised by lethargy, slight exophthalmia, blood-shot fins, bloody dis- charge from the nares and vent, and multiple haemorrhages in the muscle and other tissues. Internally, haemorrhaging in the liver, swelling of the spleen and kidney necrosis may occur (Snieszko, 1958a; McCarthy and Roberts, 1980). This form of the disease usually causes low rates of mortality, and fish may survive, although survivors have scar tissue in the vicinity of the furuncles (McCarthy, 1975a). Oddly enough, the chronic form of the disease is not the most frequently occurring, nor is the presence of furuncles the most typical symptom of the disease (Snieszko, 1958a). The acute form of furunculosis, which is most common particularly in growing fish and adults, is manifested by a general septicaemia accompanied by melanosis, inappetence, lethargy and small haemorrhages at the base of the fins. This form of the disease is of short duration, insofar as the fish usually die in 2-3 days, and causes high mortalities. The bacteria occur in the blood, disseminated throughout the tissues, and in the lesions. Internally, haemorrhaging occurs over the abdominal walls, viscera and heart. The spleen may appear enlarged. The acute disease is of sudden onset, with few, if any, external signs (McCarthy, 1975a). McCarthy and Roberts (1980) discussed a third cHnical form of furunculosis, termed peracute furunculosis, which is confined to fingerling fish. The infected animals darken in colour, and may quickly die with only slight external symptoms, such as mild exophthalmia. Haemorrhages may occur at the base of the pectoral fin, if the fish manage to survive for long-enough periods. Losses in farmed stock may be extremely high (Davis, 1946). Yet another form of furunculosis was discussed by Amlacher (1961), i.e. intest- inal furunculosis. The symptoms were described as inflammation of the intestine, and anal inversion. This description is similar to a report by Herman (1968) of chronic furunculosis, i.e. low, relatively constant rate of mortality with intestinal inflamma- tion and variable haemorrhages.
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