Immune Defences of Teleost Fish - Aquaculture Nutrition: Gut Health, Probiotics and Prebiotics - page 37

antigen non-specific and do not involve memory. It is very much a future goal to identify
specific responses to pathogens and to harness these mechanisms for the benefit of immune
protection to the host, conferring clear health benefits to the host living in this antigen-rich
environment. To be able to harness this specific immunity, it is essential that fish pathogens
and the resulting pathological mechanisms are clearly understood. There are several issues
that must be considered which include: types of pathogens challenging teleost fish (parasites,
bacteria, viruses), size of pathogen, route of entry into host, location once inside the host, and
pathogen escape mechanisms.
The aquatic environment is a pathogen-rich environment through which the teleost fish
must navigate. They are exposed to a wide range of pathogenic organisms which include bac-
teria, viruses, fungi, protozoa and helminths. The predominant routes by which they infect
fish are via the mucosal surfaces of the skin, gills, eyes and the GIT. The immune battles that
ensue are determined by the route of entry, the molecules expressed by the pathogen, and
the relative size of the pathogen compared to host cells. The first immune battles initiated
will be non-specific innate defences which are likely triggered through recognition of PAMPs
by their respective PRRs, initiating destructive inflammatory responses mediated via a range
of cells (basophils, eosinophils, macrophages) and a wide array of their effector molecules
(chemokines, pro-inflammatory cytokines, proteases, antimicrobial peptides, lysozyme, com-
plement factors) (see Section 2.2). The second-line defences are generally slower to develop
but involve antigen-specific adaptive responses and develop memory to secondary exposure
with the specific pathogen. The effectiveness of this later specific response is, however, depen-
dent on the size and location of the pathogen. In general, large pathogens, that is larger than
host cells (helminth worms, fungi and some bacteria), reside in an extracellular environment,
open to a plethora of immune cells and their secreted effector proteins; the relevant humoral
immune response to these pathogens utilizes antibodies and other secreted effectors. In an
attempt to avoid these defences, smaller pathogens (such as viruses and some bacteria) will
shelter from these humoral effectors and will escape inside the host's cells. The appropriate
host defence to these pathogens would be to kill the host cell in which these infective microbes
reside; this response requires the killing capability of the cell-mediated immunity arm of the
host's defences (Tc and DTH).
There are a multitude of infectious pathogens causing a wide range of pathologies in
teleost fish. Such pathologies include: (1) bacterial pathologies, including furunculosis
caused by the bacterial pathogen Aeromonas salmonicida , enteric redmouth by Yersinia
ruckeri , emphysematous putrefactive disease by Edwardsiella tarda , and mycobacteriosis by
Mycobacterium marinum ; (2) viral pathologies, including infectious pancreatic necrosis by
IPNV, viral haematopoietic septicaemia by VHSV, and infectious haematopoietic necrosis by
IHNV; (3) fungal pathologies (reviewed in Ramaiah 2006), including dermatocystidiosis by
Dermatocystidium ; (4) protozoal pathologies, including ichthyobodosis by Ichthyophthirius
multifilis and trichodinosis by Trichodina sp.; and (5) helminth pathologies, including
intestinal nodules by Pomphorhynchus laevis . (For an extensive review of bacterial fish
pathogens, particularly of the intestinal tract, refer to Chapter 3 .)
Intracellular-resident pathogens such as IHNV and the bacterium Mycobacterium mar-
inum are barely visible to humoral defences as they evade these responses by living inside
host cells. It is vital that the teleost immune system is capable of detecting these pathogens
inside host cells. As discussed earlier, the innate immune system is capable of recognizing