Gastrointestinal Pathogenesis in Aquatic Animals - Aquaculture Nutrition: Gut Health, Probiotics and Prebiotics - page 53

3 Gastrointestinal Pathogenesis in Aquatic
Animals
Jarl Bøgwald and Roy Ambli Dalmo
Norwegian College of Fishery Science, UiT The Arctic University of Norway,
Tromsø, Norway
ABSTRACT
The mucosal surfaces are the first barrier against systemic infection of bacteria. Bacteria may
find their way to and through the skin, gills, eye and gastrointestinal (GI) tract. Some bacte-
ria may induce disease symptoms in the mucosal surfaces, while others form 'disease foci'
in internal organs and tissues. In this review emphasis has been put on GI tract infections
as this organ represents one of the most important portals of entry of numerous bacterial
species. Infection through the GI tract is multifactorial and requires several stages: potential
pathogens must be able to survive gastric transit, outcompete commensal gut microbiota, sur-
vive and colonize the intestinal mucus, and finally colonize and breach the intestinal brush
border. The precise mechanisms underpinning these processes for a number of pathogens
are partly described. Although we are becoming increasingly informed about the mucosal
immune responses of fish, there remains a paucity of information regarding production of
cytokines or expression of cytokine mRNAs in the GI tract during natural pathogen outbreaks.
In response to experimental bath challenges however it is evident that immune responses may
be stimulated, resulting in increased expression of certain pro-inflammatory cytokines (e.g.
IL-1β and TNF-α) in the oral mucosae. The use of prebiotics, and probiotics, offers a means
to fortify the gut microbiota, potentiate the intestinal immune response and reduce intestinal
pathogenesis.
3.1 INTRODUCTION
The main boundary between teleost fish tissues and the surrounding water is the mucus layer
which covers the epidermis, the gills and the GI tract (see Chapter 2 ). Attachment of bacte-
rial pathogens to one or more of these surfaces is important for initiation of infection. But the
mode of transmission and the route of infection of fish bacterial pathogens are still debated.