Gastrointestinal Pathogenesis in Aquatic Animals - Aquaculture Nutrition: Gut Health, Probiotics and Prebiotics - page 63

3.3.2 Aeromonas hydrophila
Aeromonas hydrophila is a motile Gram-negative bacterium that may cause diseases of
both warm water and cold water fish. In general, A. hydrophila does not cause disease in
healthy well-conditioned fish. The bacterium has been reported to be a secondary pathogen
or a primary pathogen of fish under stressful situations such as low oxygen levels, thermal
fluctuations and spawning (Harikrishnan and Balasundaram 2005). As reported by Hazen
et al. (1982), isolates of A. hydrophila from fish lesions had a significantly higher chemotactic
index to surface mucus than isolates of A. hydrophila from water. In another recent study the
chemotaxis towards, adhesion to, and growth in common carp ( Cyprinus carpio ) gut mucus
of two A. hydrophila strains was examined (Van der Marel et al. 2008). Both strains did not
move towards the mucus but the more pathogenic strain of the two showed 13 times higher
adhesion to mucus and a higher growth rate in the mucus. As discussed by the authors, a
faster multiplication of bacteria in mucus may increase the pathogenicity since the flushing
of bacteria due to mucus erosion and turnover is reduced (Van der Marel et al. 2008). Despite
these interesting data, it has been suggested that the gills and skin are the main portals of
entries of A. hydrophila in carp (Chu and Lu 2008). Future studies on the pathogenicity of A.
hydrophila are recommended.
3.4 YERSINIA RUCKERI
Yersinia ruckeri is the causative agent of yersiniosis or enteric redmouth (ERM) disease
(Furones et al. 1993). The disease has been reported in salmonids, predominantly in rainbow
trout in North America, Europe, Australia and South Africa (Tobback et al. 2007). Y. ruckeri
belongs to the Enterobacteriaceae family and is a Gram-negative rod of variable motility.
The rods spread between fish by direct contact with infected animals or carriers. As many
as 25% of fish in a rainbow trout population were recognized to carry the bacterium in their
lower intestines (the intestinal region exhibiting an absorptive epithelium but with a degree
of mucosal folding lower than that of the upper gut) (Busch and Lingg 1975). The portal
of entry of Y. ruckeri was studied by Tobback et al. (2009) after experimental infection of
rainbow trout. These studies demonstrated that the gills may be an important portal of entry.
In addition the bacterium may gain access to the body by a variety of routes including the
skin and gut (Busch and Lingg 1975; Tobback et al. 2009). In a recent study the interactions
of Y. ruckeri with the gills and gut of rainbow trout were studied by perfusion techniques
(Tobback et al. 2010). Five Y. ruckeri strains were tested, two virulent and three avirulent.
All the strains adhered to both gill and gut tissues and there were no significant differences
in adhesion and invasion of the strains between the tissue types. Additionally, there were
no significant differences between virulent and avirulent Y. ruckeri strains with regard to
adherence to the tissues.
3.5 EDWARDSIELLA spp.
Members of the Edwardsiella genus are Gram-negative, facultatively anaerobic rod-shaped
bacteria. There are three species within the genus and two of them are pathogenic to fish:
Edwardsiella ictaluri and Edwardsiella tarda .