JPC SYSTEMIC PATHOLOGY
Signalment: Young chicken
HISTOPATHOLOGIC DESCRIPTION: Bursa of Fabricius: There is atrophy of bursal follicles and diffusely within the cortex and the medulla, there is marked lymphoid depletion. Undifferentiated epithelial cells within the medulla are prominent. Multifocally, follicular medullas often contain eosinophilic cellular and karyorrhectic debris (lymphocytolysis) admixed with macrophages that occasionally contain phagocytized cellular debris. Multifocally, lymphoid follicles contain dilated and congested vessels, mild hemorrhage and low to moderate numbers of infiltrating heterophils. Diffusely the interfollicular connective tissue, and to lesser degree follicular cortical areas are markedly expanded by clear space and lymphatics are multifocally ectatic (edema). Scattered heterophils transmigrate the surface epithelium, which is multifocally mildly attenuated.
MORPHOLOGIC DIAGNOSIS: Bursa of Fabricius: Lymphoid depletion, acute, diffuse, moderate, with lymphocytolysis and mild heterophilic bursitis, breed unspecified, chicken, avian.
ETIOLOGIC DIAGNOSIS: Avian birnaviral bursitis
CAUSE: Infectious bursal disease virus
CONDITION: Infectious bursal disease (IBD)
SYNONYMS: Gumboro disease, avian infectious bursitis
- IBD is a highly contagious, lymphocytolytic, immunosuppressive disease of predominantly young chickens (3-6 weeks), with predilection for differentiating B-lymphocytes in the bursa of fabricius
- IBDv is a non-enveloped, icosahedral, 60-70 nm, double-stranded RNA virus of the Avibirnavirus genus and the family Birnaviridae
- There are two serotypes (I and II) of IBDV; only type I is pathogenic
- Serotype 1 has antigenic variation due to angenic drift resulting in emergence of new strains with implication for vaccines
- Random homologus recombination also possible
- Serotypes 1 and 2 affect ducks and turkeys but do not cause disease in these species
- Serotype 2 does not protect against Serotype 1 infection
- IBD occurs worldwide, and is the most important disease of concentrated broiler producing areas of the US today; few commercial flocks are free of the virus
- Non-enveloped virus resists heat and many disinfectants; allows virus to survive months in the environment
- Very virulent IBD (vvIBD) is a highly pathogenic strain present in Europe, Asia, S. America and Africa
- Transmission is via oral or direct contact with contaminated feed, water, droppings and litter
- Infected chicks excrete the virus for 10 to 14 days; it is very stable remaining infectious for many months in the environment
- Reservoir hosts are the meal worm (Alphitobious diapesinus) and the litter mite
- Oral inoculation > viral infection and replication within gut associated macrophages and lymphoid tissue> liver> primary viremia > spreads through bloodstream to other tissues include BF > bursal infection >secondary viremia
- Binds N-glycosylated membrane protein expressed in IgM-bearing immature B lymphocytes, causing acquired B cell deficiency
- Only serotype 1 of the virus has tropism for bursal lymphoid cells (pre-B lymphocytes), which results in cell lysis and release of large amounts of virus into the circulation (secondary viremia)
- Other organs (spleen, Harderian gland and thymus) are later infected, and opportunistic infections develop because of immunosuppression
- Bursal tropism accounts for young age of animals infected (3-6 wks)
- Birds that survive initial infection are immune suppressed due to decreased humoral immunity; T-cells are resistant, but IBD still causes thymic necrosis/atrophy and impaired cell-mediated immunity; phagocytosis is also impaired
TYPICAL CLINICAL FINDINGS:
- Severity of signs dependent on age, breed (Leghorns especially susceptible), antibody level in the chick, and virulence of the strain
- Birds 1-14 days of age are less sensitive because of maternal antibodies
- Young chicken, especially egg laying birds, 3-6 weeks of age are most susceptible with IBDV newly introduced into the flock, morbidity approaches 100% and mortality 90%
- Tremors and unsteadiness
- Depression, anorexia, ruffled feathers
- Diarrhea and dehydration
- Birds older than 6 weeks rarely develop signs of the disease, although they do produce antibodies
- Infection/challenge with less virulent pathotypes may result in less severe disease when also challenged with very virulent IBD strains
- Inoculation of SPF chicken vvIBVD (rB strain) (very virulent IBD strain) resulted in 100% mortality in both the 4- (10/10) and 6-week-old (5/5) inoculated birds
- There was a significant reduction in mortality observed when the birds were co-challenged with rB (very virulent strain) and the Del-E, T1 (sub-clinical Delaware E, variant T1), and OH viruses (Ohio strain) (less virulent strains)
TYPICAL GROSS FINDINGS:
- Bursa of fabricius is consistently swollen and covered by colorless to yellow, gelatinous material
- Mucosal folds are swollen and edematous and diffusely or focally hemorrhagic
- Hemorrhage of the thigh and pectoral muscles
- Increased mucous in the intestine
- The spleen and liver may be enlarged with small gray foci on the surface
- The kidney may be swollen because of accumulation of urates (dehydration)
TYPICAL LIGHT MICROSCOPIC FINDINGS:
- Acute: Enlarged and edematous
- Degeneration and necrosis of lymphocytes
- Heterophilic infiltrate
- Hyperplastic reticuloendothelial cells
- Frequent hemorrhage
- Chronic: Grossly shrunken
- Cystic cavitations in the medullary region
- Necrosis and phagocytosis of heterophils with interfollicular fibroplasia
- Hyperplasia of the bursal epithelium with formation of glandular structures
- Acute: Enlarged and edematous
- Spleen: Acute: Hyperplasia of reticuloendothelial cells around the adenoid sheath; Chronic: multifocal lymphoid necrosis in the white pulp
- Cecal tonsil and thymus: Lesions similar to spleen
- Kidney: Acute nephrosis characterized by large tubular casts composed of homogeneous material infiltrated by heterophils (secondary to dehydration)
- Double-stranded RNA genome encapsulated within an enveloped, single shelled icosahedral particles of 60 – 70 nm in diameter
ADDITIONAL DIAGNOSTIC TESTS:
- PCR, virus isolation, ELISA, AGP or virus neutralization
- Bursal atrophy:
- Physiologic atrophy
- Chronic Marek’s disease (Alphaherpesvirus)
- Chick anemia virus (Circovirus): Bursal lesions are less severe than IBD; lymphocyte depletion and small necrotic foci with infolded epithelium, with no inflammation and no intracytoplasmic inclusion bodies; thymus is most severely affected organ in CAV with severe lymphoid depletion; intranuclear inclusion bodies have been reported in thymus and bone marrow
- Lymphoid necrosis in the bursa:
- Newcastle’s disease virus (Paramyxovirus, genus Rubulavirus)
- Duck plague virus (Alphaherpesvirus)
- Psittacine beak and feather disease (Circovirus): Only in Psittacines; bursal necrosis with intracytoplasmic inclusions in macrophages
- Epithelial changes:
- Vitamin A deficiency: Squamous metaplasia of the epithelium
- Cryptosporidium bailleyi: Cryptosporidia cover the surface epithelium, epithelial hyperplasia
- Sero-positive conversion in wild and pen-reared ring-necked pheasants in central valley of California
Other viruses in family Birnaviridae:
- Novel Birnaviridae - Chicken proventricular necrosis virus (R11/3 virus), cause of transmissible viral proventriculitis in broiler chickens; recently identified as a Birnaviridae
- Microscopic lesions: Proventriculus: glandular epithelial necrosis, ductal epithelial hyperplasia, expansion of the glandular interstitium by lymphocytes and replacement of glandular epithelium by hyperplastic ductal epithelium
- Gross lesions: Fragility and rupture of the proventriculus at processing
- Genus Aquabirnavirus - Infectious pancreatic necrosis virus (IPNV): fish, mollusks, and crustaceans; Lethal in salmonids: necrotic foci in acini and islets in the pancreas
- Genus Entomobirnavirus - Drosophila X virus (DXV) of insects
- Genus Blosnavirus – Blotched snakehead virus (BSNV)
- Likely genus: Tellina virus (TV-1) of bivalve mollusks
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