First description of atypical furunculosis in freshwater farmed Atlantic salmon, Salmo salar L., in Chile

Abstract

We report the first isolation, identification and characterization of a group of Chilean strains of atypical Aeromonas salmonicida isolated from freshwater farmed Atlantic salmon, Salmo salar. Affected fish showed superficial ulcers and pale liver with or without petechial haemorrhages. Outbreaks of the disease occurred in two farms in the south of Chile about 2200 km apart. Five strains were isolated in pure culture and identified by serological assays and immunofluorescence tests as belonging to Aeromonas salmonicida. Although the bacterial isolates were phenotypically homogeneous, minor differences with the reference strain A. salmonicida subsp. salmonicida ATCC 33658 were noted. Three specific primer sets and partial 16S rRNA gene sequencing allowed the identification of the Chilean isolates as atypical A. salmonicida, with A. salmonicida subsp. achromogenes and A. salmonicida subsp. masoucida as their closest relatives (100% sequence similarity). Molecular typing indicated that the atypical isolates belong to two genetic groups that were associated with the geographical origin.

Molecular Detection and Characterization of Furunculosis and Other Aeromonas Fish Infections

“…The mentioned systems normally tend to identify all Aeromonas strains as A. hydrophila (Figueras, 2005;Soler et al, 2003b). Some other authors have also reported poor results from the API systems and discrepancies with traditional tube testing when identifying Aeromonas strains isolated from fish (Godoy et al, 2010;Han et al, 2011;Joseph & Carnahan, 1994;and references therein). Some methods may even misidentify Aeromonas strains as Vibrio species (Park et al, 2003;Soler et al, 2003b).…”

“…salmonicida the similarity was 99.85%. In our view Godoy et al (2010) forgot to consider that the species A. bestiarum shows an identical 16S rRNA gene sequence to A. salmonicida subsp. achromogenes and A. salmonicida subsp.…”

“…In reality, furunculosis was the first fish infection for which the 'Koch postulates' were demonstrated more than a century ago (Austin & Austin, 2007;Bernoth et al, 1997). It was thought originally that the infection affected only salmonids, but it soon became evident that it was distributed worldwide and affected many other marine and freshwater fish species, such as the Atlantic cod (Gadus morhua), halibut (Hyppoglossus hyppoglossus), turbot (Scopthamus maximus), lamprey (Petromyzon marinus), carp (Cyprinus carpio), goldfish (Carassius auratus) and eel (Anguilla anguilla) among others (Austin & Austin, 2007;Bernoth et al, 1997;Godoy et al, 2010;Goldschmidt-Clermont et al, 2009;Noga, 2010;Wiklund & Dalsgaard, 1998).…”

“…We believe that this high variability could indicate that the group of atypical strains may in fact not only embrace other subspecies of A. salmonicida but also other misidentified Aeromonas species. This PCR method directed at atypical A. salmonicida strains was used recently by Godoy et al (2010) for characterising strains isolated from freshwater Atlantic salmon (Salmo salar) with successful results (Table 2), which we will discuss later.…”

“…Another study in which several molecular methods were used in parallel was the one performed by Godoy et al (2010) that aimed at re-identifying 5 biochemically characterised A. salmonicida strains isolated from a furunculosis outbreak that occurred in Chile in a freshwater farm of Atlantic salmon. The study tried to determine if the infection was produced by typical or atypical A. salmonicida strains, since atypical furunculosis had been detected previously in seawater in Chile.…”