Essentially pasteurellosis is a septicaemia in which acute cases exhibit only a

Essentially pasteurellosis is a septicaemia in which

This preview shows page 62 - 64 out of 594 pages.

2006). Essentially, pasteurellosis is a septicaemia in which acute cases exhibit only a few pathological signs. Internally, granulomatous-Hke deposits, which have led to the coining of the descriptive name of pseudotuberculosis, may develop on the kidney and spleen. These deposits comprise many greyish-white bacterial colonies of 0.5- 1.0 mm^ in size (Kusuda and Yamaoka, 1972). Purulent material may accumulate in the abdominal cavity (Lewis et ai, 1970). Piscirickettsiaceae representative Piscirickettsia salmonis Although the pathogen was initially associated with salmon, there has been a spread to other groups, including sea bass (McCarthy et al, 2005; Arkush et al, 2005). Also, the geographical range has spread from Chile to North America and Europe, includ- ing Greece (McCarthy et ai, 2005) and Scotland (Birkbeck et ai, 2004). Infected fish gathered at the surface of cages, became sluggish and were inappetent. External signs included melanosis, epidermal indurations, and paleness of the gills, which was indicative of anaemia. The haemacrits fell to <27%. Internally, haemorrhages were evident on the abdominal walls, visceral fat and on the air bladder. A mottled liver and swollen spleen was apparent in heavily infected animals. The kidney was inflamed and swollen. The intestine was full of yellowish mucoid material (Schafer et al, 1990; Branson and Diaz-Munoz, 1991). Initially, the disease was not considered to occur during the freshwater stage of fish culture. Instead, mortalities have been noted to begin 6-12 weeks after transfer of fish from fresh to seawater (Fryer et al, 1992). However, in a later development the pathogen was recovered from the fresh- water stage of coho salmon and rainbow trout (Bravo, 1994; Gaggero et ai, 1995). Rickettsia-like organism (RLO) Chern and Chao (1994) reported that the RLO caused mass mortalities in tilapia, with disease signs including the presence of white nodules and microscopic granu- lomas on all the organs, and an enlarged spleen. Subsequently, Comps et al. (1996) described a small coccoid organism from the brain of juvenile sea bass obtained from the South of France. These were derived from a population which suffered 20% mortalities. Also, an RLO has been identified among grouper in Taiwan (Chen et al, 2000a), tilapia (Mauel et al, 2003) and white sea bass in the U.S.A. (Chen et al, 2000) and farmed Atlantic salmon in Tasmania and Canada (Cusack et al., 2002). Mor- ibund grouper displayed dark lesions, splenomegaly (with white nodules), necrosis in the liver, kidney and spleen, and the blood was thin (Chen et al, 2000a). However, it is difficult from the literature to determine the significance of the Tasmanian RLO to fish (Corbeil et al, 2005).
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Characteristics of the diseases 39 Pseudomonadaceae representatives Pseudomonas anguilli septic a Originally observed in eels, when the disease was referred to as Sekiton byo (= red spot), the pathogen has been identified in a wide range of other species, including Baltic herring (Clupea harengus membras) (Lonnstrom et ai, 1994), gilthead sea bream (Domenech et ai, 1999), black-spot sea bream (Pagellus bogaraveo) (Lopez- Romalde et ai, 2003), orange-spotted grouper
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