Why does vvIBDV infection cause hemorrhages and mortality?

• High Cytolytic Capacity: vvIBDV strains are characterized by a high ability to induce cytolysis, leading to widespread lymphoid necrosis and vascular injury.
• Cytokine Storm: vvIBDV triggers an intense inflammatory response and a “cytokine storm” (such as TNF-α, IL-1β, IL-6), which contributes to vascular damage and coagulation disorders.
• Thrombocytopenia: Hemorrhages occur secondary to a decrease in platelet count (thrombocytopenia) and coagulopathy.
• Fatal Outcome: This vascular damage leads to hemorrhaging and clotting disorders, ultimately resulting in death.

Why does varIBDV infection NOT cause hemorrhages or mortality?

• Mild Inflammation: Variant IBDV (varIBDV) induces a milder immune response with limited inflammation.
• Targeted Replication: It replicates primarily in bursal lymphocytes but does not significantly damage or infect the blood vessels.
• Hemostatic Stability: Platelet counts and clotting times typically remain within normal ranges.
• Result: Consequently, no hemorrhages or mortality occur.

If so, why is the breakthrough level of the variant strain (varIBDV) higher than that of the very virulent strain (vvIBDV)?

• The Primary Reason: Immune Evasion.
• VP2 Mutations: Mutations in the VP2 protein mean that antibodies generated from vaccines cannot neutralize the virus.
• Vaccine Failure: Infection occurs despite having high levels of maternal or vaccine-induced antibodies.
• Pathogenicity vs. Breakthrough: Breakthrough capability does not equal pathogenicity. Variant strains may be less virulent (no mortality) but are more capable of escaping immunity.
• Outcome: No mortality, but severe immunosuppression and poor response to subsequent vaccinations due to the virus’s ability to evade the immune system.