Diagnosis in clinically infected fish

The boil-like lesions observed by Emmerich and Weibel (1894), from which classical furunculosis derives its name, are the exception in clinical cases, rather than the rule (Bernoth, 1997b) and are normally only observed during chronic infections in older fish. Table 10.4 summarizes the main clinical signs, gross pathology and histopathological features that have been described for the different forms of typical furunculosis (peracute, acute, subacute/chronic). The gross pathological signs of peracute and acute furunculosis in young fish are often indistinguishable from other bacterial septicaemias on preliminary examination. The inexperienced diagnostician may also have difficulty in differentiating the morphology and Gram-staining behaviour of A. salmonicida in fish tissue, as seen under a microscope, from other Gram-negative bacteria occurring in fish (Bernoth, 1997b). Thus, a firm diagnosis of clinical furunculosis requires isolation of the dominant infecting organism on agar media and identification by morphology, combined with either biochemical or serological tests, as A. salmonicida.

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