Gastrointestinal Pathogenesis in Aquatic Animals - Aquaculture Nutrition: Gut Health, Probiotics and Prebiotics - page 65

1991). Mortalities ranged from 88% to 100% in both species. Moribund fish were anaemic,
the gills and liver were pale, the spleen was enlarged, and many fish had petechial haemor-
rhages on the pyloric caeca. The digestive tract was fluid filled and devoid of food. Horizontal
transmission did not occur between injected and non-injected individuals (Cvitanich et al.
1991). Experimental infection with P. salmonis via the intraperitoneal, oral and gill routes
were compared. The results revealed that the relative risks of mortalities by intraperitoneal
and gill infections were significantly higher than fish challenged by the oral route. After oral
challenge the pattern of mortality was delayed relative to the intraperitoneal and gill routes,
which indicates that infection via the GI tract may occur but the GI is not as important an
infection route as the gills.
3.7 PSEUDOMONAS ANGUILLISEPTICA
Pseudomonas anguilliseptica is an opportunistic Gram-negative fish pathogen first described
in Japanese eels suffering from haemorrhagic septicaemia. In Europe numerous fish species
have suffered from this disease: gilthead sea bream, black sea bream ( Acanthopagrus schlegeli
Bleeker), European sea bass and turbot (Doménech et al. 1997). P. anguilliseptica was also
isolated from diseased salmonids in Finland (Wiklund and Bylund 1990). Turbot have been
shown to be sensitive to intraperitoneal infection by a virulent strain from diseased turbot,
while bath exposure gave no mortalities (Magi et al. 2009). Clinical signs of naturally infected
gilthead sea bream were the presence of ascitic fluid in the peritoneal cavity, haemorrhages on
the kidney and liver, and a congestive intestine containing yellowish exudate (Doménech et al.
1997). Seven days after intraperitoneal challenge of turbot with P. anguilliseptica an enlarged
spleen, a friable liver and a congestive and dilated gut with yellow exudate were observed
(Magi et al. 2009). Immunohistochemically positive bacteria were detected in the spleen, gut,
liver, kidney, muscle and brain. Histopathological changes were seen in the gut four days after
injection including hypertrophia of epithelial cells, and seven days after infection necrosis and
oedema of the gut mucosa were observed.
3.8 PHOTOBACTERIUM DAMSELA subsp. PISCICIDA
(PASTEURELLA PISCICIDA)
Photobacterium damsela subsp. piscicida ,formerly Pasteurella piscicida , is a non-motile
pleomorphic Gram-negative bacterium and is the aetiological agent of pasteurellosis. The
disease causes the septicaemia known as 'pseudotuberculosis' due to the fact that diseased
fish show whitish tubercles in internal organs. Examples of affected species are European
sea bass, sea bream, turbot, yellowtail ( Seriola quinqueradiata Temminck and Schlegel) and
Japanese flounder (Magarinos et al. 1996a). Serologically the bacterium constitutes a highly
homogeneous group. Reports suggest that Photobacterium damsela subsp. piscicida can
be transmitted orally, since the bacterium was isolated from the freshwater fish snakehead
( Channa maculate Lacépède) fed infected marine trash fish. Following gastric intubation
of high doses of Photobacterium the authors reported the presence of this bacterium in the
peripheral blood within 72 h (Wakabayashi et al. 1977; Tung et al. 1985). Also, it was found
that P. damsela subsp. piscicida infected yellowtail either through the gills or via the GI tract