Systematic Veterinary Virology

Infectious bursal disease

INFECTIOUS BURSAL DISEASE (IBD)

Field diagnosis

• Based on clinical symptoms and lesions, vaccination history, prevalence of the infection in the area and age of the birds affected by the infection. A flock will show very high morbidity with severe depression in most birds lasting for 5–7 days. Mortality rises sharply for 2 days then declines rapidly over the next 2–3 days. Usually between 5% and 10% of birds die, but mortality can reach 30–40%.

Isolation and identification

• Clinical materials: Samples should be collected from sick or freshly dead birds.
• Fresh bursa and spleen for virus isolation.
• Samples of bursa, spleen, intestines, caecal tonsil, liver and kidney should be collected in neutral buffered formalin for histopathology
• Blood samples for serology.
• Isolation systems: Chickens, ECE and Chicken embryo fibroblast (follow as given under cultivation).

Direct identification of antigen from bursa

• Agar gel immunodiffusion test
• Immunofluorescense test
• Antigen capture ELISA
• RT-PCR

Strain differentiation

• IBDV serotypes are differentiated by cross neutralization virus neutralization test or by serological tests using serotype specific monoclonal antibodies.

Pathotype identification

• The terms ‘variant’, ‘classical’ and ‘very virulent’ have been used to qualify IBDV strains that exhibit a different pathogenicity. Based on the signs and lesions observed in White Leghorn SPF chickens during acute experimental IBD
• ‘Variant’ IBDVs induce little if any clinical signs and no mortality but marked bursal lesions,
• ‘Classical’ IBDVs induce approximately 10–50% mortality with typical signs and lesions
• ‘Very virulent’ IBDVs induce approximately 50–100% mortality with typical signs and lesions

Serological tests

• AGID: The AGID test is the most useful of the serological tests for the detection of specific antibodies in serum
• Quantitative AGID test: The AGID test is used to measure antibody levels by using dilutions of serum in the test wells and taking the titre as the highest dilution to produce a precipitin line. This can be very useful for measuring maternal or vaccinal antibodies and for deciding on the best time for vaccination.
• Virus neutralization test: VN tests are carried out in cell culture. The test is more laborious and expensive than the AGID test, but is more sensitive in detecting antibody. This test is also useful for evaluating vaccine responses or for differentiating between IBDV 1 and 2 serotypes
• ELISA: This is the preferred test for identification and quantification of antibodies against IBD.

Differential diagnosis

• IBD should be differentiated from Marek's disease Mycotoxicosis, Coccidiosis, Haemorrhagic syndrome, Avian adenovirus infection and Infectious bronchitis.