Gastrointestinal Pathogenesis in Aquatic Animals - Aquaculture Nutrition: Gut Health, Probiotics and Prebiotics - page 62

As
Lp
E
L
Fig. 3.5 Immunohistochemical staining, using rabbit anti- A. salmonicida LPS antiserum diluted 1:1000,
biotinylated goat anti-rabbit Ig and streptavidin biotinylated alkaline phosphatase, and Fast red/Naphthol
AS-Mx as the substrate, in analysis of the spotted wolffish intestine from fish infected with waterborne
atypical A. salmonicida . Large numbers of bacteria (As) were observed in the lamina propria (Lp). Sections
obtained from non-infected controls were negative for bacteria. Key: E, epithelium; L, lumen. Scale bar =
10 μ m. (Source: Dag Vollstad.) For colour detail see Plate 7.
The results of this study showed that A. salmonicida infection may occur both by ingestion
of infected fish and after epidermal contact. Jutfelt and colleagues studied the translocation of
viable A. salmonicida across the intestine of rainbow trout (Jutfelt et al. 2006; 2008). An 'Uss-
ing chamber' was used in this study and the results demonstrated translocation of the bacteria
across the intestinal epithelium, suggesting the intestine as a possible route of infection. The
bacterium seemed to use several mechanisms for this translocation including surface proteins
and secreted proteins (Jutfelt et al. 2008; Dacanay et al. 2010). Ringø et al. (2004) observed
substantial cell damage in Atlantic salmon after A. salmonicida infection. Enterocytes were
clearly detached from the anterior intestinal region while less damage was observed in the
posterior intestinal region.
In a study by Hodgkinson et al. (1987), rainbow trout were immersed in a suspension of
A. salmonicida for 2 min. The pathogen was present on the skin/mucus, on the gills and in the
mouth for up to 24 h after challenge. Small numbers of bacteria were also recovered from the
upper and lower regions of the intestine and in faecal samples. By immersion of the tail region,
A. salmonicida were recovered from the hindgut for up to 24 h. It is likely, however, that the
uptake of the pathogen occurred through all natural openings and/or the skin and gills. It has
been reported that A. salmonicida are capable of inducing the expression of pro-inflammatory
cytokines in the rainbow trout intestine (Mulder et al. 2007). Modulated IL-1β,TNF-α,TGF-β
and IFN-α mRNA expression was reported following experimental bath challenge. In another
study, isolated intraepithelial cells (IEC) responded to live A.salmonicida by increased expres-
sion of the pro-inflammatory cytokines TNF-α2 and IL-1β (Komatsu et al. 2009).