Agriculture Reference
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Mycobacteriumhaemophilum , a recognized pathogen of children, has also been reported as
a pathogen in fish (Kent et al. 2004). Two severe outbreaks from different zebrafish facilities
on different continents (Europe and North America) occurred with mortalities over a period of
several weeks. Zebrafish appear to be particularly vulnerable to M. haemophilum and a recent
study showed that experimentally infected (intraperitoneal injection of M. haemophilum fish
exhibited severe mycobacteriosis characterized by multiple granulomas and diffuse chronic
inflammation (Whipps et al. 2007). Essentially all organs were involved: the kidney, spleen,
liver, pancreas, heart, muscle and intestine exhibited particularly heavy infections in most fish.
Mycobacterium infections have also been studied in wild fish associated with sea cages.
Bacterial transmission may occur between wild and caged fish if local wild fish communities
gain access to cages, if wild animals escape from sea cages into the surrounding ecosystem, or
if there is contamination through the water column. Diamant et al. (2000) studied the infection
patterns of M. marinum in wild rabbitfish ( Siganus rivulatus Forsskål) associated with com-
mercial mariculture cages along the Israeli Red Sea coastline. Two M. marinum isolates from
rabbitfish captured at Eilat harbour and the coral reef were shown to be identical to isolates
from rabbitfish trapped inside the mariculture cages as well as isolates from locally cultured
European sea bass. Further study of the transmission pathways is clearly warranted.
While there is scarce information on mucosal immune response following M. marinum
infection it has been shown that M. marinum infections induce expression of intestinal ISG15
(Interferon Stimulated Gene 15) which may indicate an ongoing cellular defence process (Liu
et al. 2002). Some further information is available at the transcriptomic level - especially in
zebrafish (Meijer et al. 2005). Of particular interest is the down-regulation of MHC class I
genes that may be deleterious since M. marinum is an intracellular pathogen that would better
be eradicated by effective MHC class I antigen presentation and CD8+ activation.
3.12 CONCLUSION
The bacterial pathogens of major importance for disease in the GI tract of fish have been
summarized in this chapter. As shown, the pathogens are a diverse group, of which many are
Gram-negative. Some of the bacteria may live for a long period outside the host, and some may
live and multiply within their host without causing disease. Some are intracellular and so the
bacterium's survival and spread depend on the ability to replicate inside eukaryotic cells and
on the host immune responses. There are a growing number of studies in the literature dealing
with the mechanism of GI tract infections, particularly with respect to V. anguillarum and A.
salmonicida , but still the mechanisms of establishment, cytotoxicity and translocation of GI
tract infections are debated and only partly described. This should be a topic of future focus,
as well as comprehensive studies of the endogenous gut microbiome and the applications of
probiotics and prebiotics in order to stabilize beneficial bacterial populations and reduce the
presence of, and incidence of infection caused by, GI based pathogens.
REFERENCES
Almendras, F.E., Fuentealba, I.C., Jones, S.R.M., Markham, F. and Spangler, E. (1997) Experimental infection
and horizontal transmission of Piscirickettsia salmonis in freshwater-raised Atlantic salmon, Salmo salar L.
Journal of Fish Diseases 20 , 409-418.
 
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