External & Internal Signs

Clinical Diagnosis
Incubation period may be long, as environmental factors are likely to modulate the clinical expression of the disease.

The first sign of an outbreak in Salmonids fry is usually a sudden and progressive increase in daily mortalities in the hatchery, particularly in the faster-growing individuals. Other signs of infection include:

  • Corkscrewing/spiral/whirling swimming motion
  • Darkening pigmentation
  • Pronounced distended abdomen
  • Absence of food but presence of clear or milky mucus in stomach and anterior intestine is pathognomonic
  • Long thin whitish faecal casts
  • Mild to moderate exophthalmia
  • Gills typically pale
  • Haemorrhages sometimes present in ventral areas, including ventral fins
  • Spleen, kidney, liver and heart of fry are abnormally pale
  • Petechial haemorrhages on the pyloric caecae and anterior adipose tissue and the body cavity may contain ascitic fluid

Laboratory Tests
The OIE (Office International des Epizooties) stipulates that the diagnosis should be done by using standard histopathological analysis combined with the immunological demonstration of the IPNV antigen in infected tissues, followed by confirmation using isolation and immunological identification of the virus in tissue culture. However, other techniques, such as detection of specific antibodies in the blood of infected fish, RT-PCR techniques and in situ hybridisation have recently been developed and proven to be valuable diagnostic tools.

Virus re-isolation and cell culture:
appropriate fish cell line cultures can be inoculated with tissue homogenate from killed fish, sperm or ovarian fluid from broodstock fish, in order to observe for cytopathic effect (CPE). If this is positive, serological tests (neutralisation test, ELISA, etc.) using specific antibodies can be used for final identification of the virus.

Serological diagnosis:
specific antiserum (monoclonal or polyclonal antibodies) raised against the several serotypes can be used directly on fresh suspect material (tissue homogenates, sperm, ovarian fluid, blood, and faeces) in ELISA, immunoblot/dot assays or coagglutination tests. Fixed material can be evaluated using the antibodies in immunohistochemistry.

RT-PCR analysis and in situ hybridisation:
Infected cell cultures, infected fish samples or on formalin-fixed tissues can be further analysed for the presence of IPNV using DNA technology.

Post Mortal Diagnosis
Diagnosis is based on external and internal symptoms but varying according to fish species and age.

Internal Gross Lesions:

  • No food in the gastro-intestinal tract; instead filled with mucoid whitish material
  • Ascitic fluid in abdominal cavity
  • Pinprick haemorrhages on the viscera
  • Pale and enlarged liver and spleen

Paraffin-embedded tissue processed according to standard methodology and stained with haematoxylin-eosine.

The lesions are easily observable in the pyloric ceca and the pancreatic tissue.

  • Acute to subacute focal necrosis of the exocrine acinar cells with picnotic nuclei is seen.
  • The focal necrotic areas are later replaced by a loose fibrous network with fat degeneration.
  • The intestinal epithelium also shows necrosis to form an exudate that fills the lumen.
  • There is an increase in eosinophilic granular cells in the granulosa layer of the intestinal wall.
  • Necrotic foci can also be observed in the liver and kidney.

The virus is very resistant to various and adverse environmental conditions. Control of the microbiological water quality is therefore a critical point in the prevention of the disease in the hatchery.

Broodstock fish need to be carefully screened using the previously mentioned assays, to detect carriers of the virus.

Disinfection of eggs is advised as a routine practice.

Prevention during the grow-out, marine phase of the production cycle is best achieved through injection vaccination of healthy fish at the hatchery.

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