Gastrointestinal Pathogenesis in Aquatic Animals - Aquaculture Nutrition: Gut Health, Probiotics and Prebiotics - page 61

barrier (Figure 3.4). The importance of this intestinal damage should be assessed in future in
vivo studies and potential bacterial translocation through the intestinal epithelium should be
assessed.
3.3 AEROMONAS spp.
Aeromonas spp. are Gram-negative facultative anaerobic rods which are ubiquitous in fresh
and brackish water. With the exception of Aeromonas salmonicida , aeromonads are motile.
A. salmonicida , an obligate pathogen, is the aetiological agent of furunculosis which affects
salmonid fish and a number of other marine fish species. Of the motile group of aeromonads,
Aeromonas hydrophila , Aeromonas caviae and Aeromonas sobria are the most important with
relevance to fish diseases.
3.3.1 Aeromonas salmonicida
Aeromonas salmonicida subsp. salmonicida is the causative agent of 'typical' furunculosis.
The bacterium is a non-motile Gram-negative biochemically, antigenically and genetically
homogeneous bacterium (Birkbeck and Ringø 2005; Toranzo et al. 2005). A. salmonicida
subsp. salmonicida produces many extracellular proteases and toxins which have cytolytic
and cytotoxic effects on various fish cell types (for a review see Ellis 1991). Typical furuncu-
losis affects both salmonids and non-salmonid fish (Bergh etal. 1997; Toranzo etal. 2005) and
although the route of entry of this bacterium is still debated, there are some reports describing
intestinal presence (O'Brien et al. 1994; Hiney et al. 1994; Lødemel et al. 2001). 'Atypical'
furunculosis forms a very heterogeneous group of bacteria affecting both non-salmonids as
well as salmonids (Wiklund and Dalsgaard 1998). Examples include A. salmonicida subsp.
achromogenes , subsp. masoucida , subsp. pectinolytica and subsp. smithia . Atypical strains
demonstrate weak, slow or non-pigment production, catalase and oxidase negativity, and slow
growth, and are pathogenic to fish other than salmonids (Austin 2011). Very often atypical
strains give rise to superficial skin ulcerations.
Since 1994 a probe has been used for detection of A. salmonicida in effluent, water,
faecal and sediment samples from Atlantic salmon hatcheries (O'Brien et al. 1994). By this
method O'Brien et al. (1994) detected a positive correlation between pathogen detection and
clinical disease. Standard bacteriology and enzyme-linked immunosorbent assay (ELISA)
were employed to detect A. salmonicida in samples of kidney, intestine and mucus from
Atlantic salmon with stress-inducible infections (Hiney et al. 1994). These authors suggest
that the intestine may be a primary location of A. salmonicida in addition to mucus, fins and
gills (Figure 3.5).
In a study on Arctic charr ( Salvelinusalpinus L.), cohabitants with intraperitoneally infected
fish showed variable mortality (Lødemel et al. 2001). Infected fish were analysed for pathol-
ogy, detection of A.salmonicida bacteria and antigens by immunohistochemistry and immuno-
gold labelling. The midgut epithelium of infected fish had a higher number of goblet cells
(mucus producing cells), and bacteria were seen in close association with microvilli. In addi-
tion, A. salmonicida antigens were detected in the submucosa of pyloric caeca, midgut and
hindgut (Lødemel etal. 2001). The conclusion drawn was that the GI tract could be an infection
route of A.salmonicida in Arctic charr. In another study the infectivity routes were investigated
in Atlantic salmon after bath and intragastric challenge with A.salmonicida (Rose etal. 1989).