External & Internal Signs

Clinical Diagnosis
Photobacterium damsela causes a disease sometimes referred to as Pseudotuberculosis or Fish Pasteurellosis. It can cause massive losses in fish populations, both cultured and in the wild.

The disease is characterized by numerous, white bacterial colonies throughout the internal viscera, especially the kidneys and spleen.

The following external and internal signs and symptoms of Pasteurellosis can occur with the two forms of the disease.

Acute septicaemia:

  • Dark coloration
  • Focal gill necrosis
  • Dark enlarged spleen and congestion

Chronic form:

  • Granulomata on kidney and spleen with greyish-white bacterial colonies of 0.5-1.0mm2
  • Purulent material in abdominal cavity

The pigmentation of the skin is usually altered (becomes darker or lighter). Slight bleeding can be observed in the fin and head areas, while the gills become paler.

Laboratory Tests
Isolation on NaCl-supplemented media shows the growth of small, shiny, translucent colonies after 48 to 72 hours when incubated at 26° C. It does not grow on TCBS. When using the API-20 E identification kit, a specific code (2005004) is obtained. It is sensitive to vibriostatic compound O/129.

Gram staining reveals bipolar staining.

Commercial test kits, e.g., those based on agglutination with specific antibodies, are available and easy to use (not requiring a specialised lab), using either infected fish tissue or purified colonies. Specific antibodies can also be used in immunohistochemistry, ELISA, immunofluores-cence, etc., but these techniques require specialised lab equipment.

Post Mortal Diagnosis
Internal organs such as the spleen and kidney are often enlarged and paler. Haemorrhages are sometimes observed on the liver. In the chronic stage of the disease, white nodules typically appear on an enlarged spleen.

Histopathological examination shows an inflammatory, necrotic reaction caused by bacterial septicaemia, eventually leading to granuloma formation. The bacteria are easily observed in macrophages.

Disease reprinted courtesy of OIE Diagnostic Manual for Aquatic Animal Diseases, OIE (World Organisation for Animal Health), Paris, France.