JPC SYSTEMIC PATHOLOGY
DIGESTIVE SYSTEM
December 2021
D-V26
Signalment (JPC #1380446): Fingerling trout
HISTORY: None
HISTOPATHOLOGIC DESCRIPTION: Midline sagittal section, fingerling trout: There is diffuse pancreatic lytic necrosis characterized by loss of normal architecture and replacement with eosinophilic cellular and karyorrhectic debris, and infiltration by few lymphocytes, plasma cells, and granulocytes. There is rare saponification of adjacent adipocytes. Within the pyloric ceca there is extensive sloughing and necrosis of the mucosa including multifocal single cell death of the epithelium (McKnight cells). The cecal lumen contains abundant cellular debris and scant basophilic flocculent material (mucus).
MORPHOLOGIC DIAGNOSIS:
- Pancreas: Necrosis, lytic, acute, diffuse, severe, with diffuse acinar loss, trout (Salmon sp.), piscine.
- Pyloric ceca, mucosa: Necrosis, acute, diffuse, moderate, with McKnight cells.
ETIOLOGIC DIAGNOSIS: Birnaviral pancreatic necrosis
CAUSE: Infectious pancreatic necrosis virus (IPNV, an aquabirnavirus)
CONDITION: Infectious pancreatic necrosis (IPN)
GENERAL DISCUSSION:
- Family Birnaviridae, genus Aquabirnavirus, non-enveloped, icosahedral, dsRNA virus; significant antigenic diversity and spectrum of virulence
- Major cause of mortality in young salmonids (<6 months old) in freshwater in the United States, Canada, Chile, Japan, Taiwan, Korea, and Europe; brook and rainbow trout are most susceptible
- High mortality (10-90% depending on strain, host, and environmental factors) in farmed freshwater fry after they begin feeding and in smolts after transfer to salt water cages
- Older fish can be infected, becoming chronic carriers; survivors are also chronically infected and may shed virus for over two years
- IPN and related marine aquabirnaviruses have been isolated from many fish and shellfish species worldwide; usually not clinical (except yellowtail, eels, etc)
- OIE reportable disease due to significant economic impact on the farmed salmon industry with potential ecological impact
PATHOGENESIS:
- Acute, fatal syndrome (young) or persistent, chronic infection (old) fish
- Clinical signs appear on day 3-5 (fry) or day 8-10 (fingerlings); peak mortality occurs on day 12-18 after exposure; however, these timeframes and the severity of disease vary with age, species, temperature, etc
- Fish that survive become chronic carriers and continue to shed virus
- Virus shed in feces, urine, and sex products (fecal pseudocast major source)
- Highly contagious; horizontal transmission by contact and ingestion of infected tissue or vertical transmission in reproductive fluids or on/in the egg
- Target organs: Exocrine pancreas, gastrointestinal mucosal epithelial cells, interstitial cells and macrophages of the kidney (salmonids), intestine, kidney, and liver
TYPICAL CLINICAL FINDINGS:
- Usually acute increase in mortality of fry or fingerling trout
- Dorsal darkening
- Pale gills
- Petechial hemorrhage on ventrum
- Exophthalmia
- Abdominal distention
- Mucoid white fecal pseudocasts trailing from the vent
- Erratic, whirling, or corkscrew spiral swimming that alternates with prostration
TYPICAL GROSS FINDINGS:
- Dark skin, swollen abdomen, exophthalmos
- Pale liver, spleen, kidney, gills
- Lack of food in the GI tract with white catarrhal/mucoid stomach exudate and fecal casts
- Petechial hemorrhage of visceral organs (especially in fingerlings), especially the pancreas
TYPICAL LIGHT MICROSCOPIC FINDINGS:
- Pancreatic necrosis, primarily of acinar cells, with minimal inflammation
- +/- basophilic cytoplasmic inclusions (cell breakdown products, not virus)
- McKnight cells: Apoptosis of pyloric ceca epithelium (swollen cells with fragmented nucleus in mucosa)
- Necrosis and sloughing of the intestinal mucosa
- Mucus in intestinal lumen (catarrhal enteritis)
- May have necrosis in kidneys, liver, and adipose tissue adjacent to pancreas
ULTRASTRUCTURAL FINDINGS:
- Non-enveloped, 60 nm diameter, icosahedral virions in cytoplasm in paracrystalline array
ADDITIONAL DIAGNOSTIC TESTS:
- Virus isolation
- IHC
- ELISA
- PCR
- Immunofluorescence
DIFFERENTIAL DIAGNOSIS:
- Infectious hematopoietic necrosis (IHN, rhabdovirus): Similar species, clinical signs & gross lesions; severe necrosis of the eosinophilic granular cells of the intestinal wall is pathognomonic; focal hemorrhage and degeneration of hematopoietic tissue; some pancreatic hemorrhage & necrosis
- Viral hemorrhagic septicemia (rhabdovirus): Kidney & liver are major target organs (relative lack of pancreatic necrosis); edema & hemorrhage of intestinal mucosa; endotheliotropic-vascular damage; hemorrhage of gills, muscle & abdominal organs; affects older fish; previously confined to Europe
- Herpesvirus salmonis disease (HVS, alphaherpesvirus): Pancreatic acinar cell syncytia (pathognomonic) without necrosis; heart, liver, kidney & intestine with edema, necrosis and hemorrhage
- Acute toxemia: Diffuse sloughing of mucosa
- Bacterial septicemia (Aeromonas , Pseudomonas fluorescens, Renibacterium salmonarum): Ascites, hemorrhage, skin lesions
COMPARATIVE PATHOLOGY:
- Other aquabirnaviruses: Aquabirnaviruses (IPN-like) have been isolated from many fish and aquatic invertebrate species, including some marine species
- Japanese eels: Muscle spasms, retracted abdomen, congestion of anal fin +/- congestion of abdomen and gills; kidney hypertrophy with exudative glomerulonephritis and necrosis of the liver and spleen
- Yellowtail (yellowtail ascites virus): Acute pancreatic-hepatic necrosis with ascites; occurs in epidemics in May-June
- Other selected birnaviruses:
- Genus Avibirnavirus: Infectious bursal disease virus of chickens (Gumboro disease)
- Blotched snakehead virus
- Genus Entomobirnavirus: Drosophila X virus in fruit fly
REFERENCES:
- Frasca S, Wolf JC, Kinsel MJ, Camus AC, Lombardini ED. Osteichthyes. In: Terio KA, McAloose D, St. Leger J, eds. Pathology of Wildlife and Zoo Animals. San Diego, CA: Elsevier; 2018: 968-970.
- Noga EJ. Fish Disease: Diagnosis and Treatment. 2nd ed. Ames, IA: Wiley-Blackwell; 2010: 271-282, 300.
