The role of the lethal extracellular cytolysin of Aeromonas salmonicida in the pathology of furunculosis

Abstract. The histopathological effects of intramuscular (i.m.) and intraperitoneal injections of the purified lethal cytolytic toxin of A. salmonicida in Atlantic salmon, Salmo salar L., were restricted to extensive degranulation of eosinophilic granular cells (EGCs) in the gills (both routes) and a limited coagulative necrosis of muscle fibres at the site of i.m. injection. The severity of these lesions did not apparently differ amongst fish that were moribund within 48 h and the surviving fish at 90 h. Previous studies indicate a maximum time to death caused by this toxin of 48 h. Thus, it is difficult to account for death of the fish from the toxin solely on the grounds of the histopathological features. It is suggested that a complex metabolic dysfunction results in death. The mechanism of 'furuncle' formation was demonstrated to result from a combined effect of the cytolysin and the purified 70-kDa protease present in the ECP. When i.m.-injected singly, the purified cytolysin produced a limited coagulative necrosis and the purified protease produced a limited liquefaction and haemorrhaging. In combination, they induced the extensive liquefactive haemorrhagic 'furuncle' characteristic of the whole ECP.