Advantages of a Novel Bursa of Fabricius Subunit Vaccine in the Control and Prevention of Infectious Bursal Disease in Chickens

Infectious bursal disease (IBD) is an acute, highly contagious, lymphocytolytic infectious disease of young chickens caused by the infectious bursal disease virus (IBDV), a member of the genus Birnavirus within the family Birnaviridae. The viral genome consists of two segments, A and B; segment A encodes four proteins—VP5, VP2, VP4, and VP3—in that order, while segment B encodes the VP1 protein. Among these, VP2 is the major structural protein and a host‑protective antigen of IBDV. Studies have shown that mutations in the hypervariable region of VP2 are among the key factors underlying changes in viral antigenicity and virulence. IBDV primarily targets the bursa of Fabricius, inducing degeneration and necrosis of B‑lymphocytes in this organ, thereby causing varying degrees of immunosuppression. This results in an inability to mount a normal immune response to vaccines, leading to vaccine failure for multiple pathogens—including Newcastle disease and avian influenza—and increasing susceptibility to secondary infections such as coccidiosis, Escherichia coli, Salmonella, and adenovirus. Vaccination is one of the critical measures for controlling infectious bursal disease; however, the wide variety of IBD vaccines—ranging from strains of differing virulence to live vaccines, whole‑virus inactivated vaccines, and genetically engineered vaccines—often leaves poultry producers uncertain about their choices, prompting unscientific vaccination practices and ultimately resulting in vaccine failure. This article reviews the economic and health impacts of infectious bursal disease, the causes of vaccine failure, the characteristics of different types of bursal vaccines, and recommended immunization protocols, providing guidance for poultry producers.

1. The Harm Caused by Infectious Bursal Disease in Chickens

With the emergence of vvIBDV and the widespread use of vaccines, both the virus’s virulence and antigenicity have undergone mutations, giving rise to new characteristics of IBD: (1) Highly virulent bursal disease virus strains can induce typical clinical signs and post‑mortem lesions in chickens; (2) Infection with variant bursal disease virus strains primarily causes immunosuppression, resulting in poor immune responses to vaccination and facilitating secondary infections by multiple pathogens; (3) Vaccine failure is severe, as IBDV has increased resistance to physicochemical factors. Moreover, vIBDV and vvIBDV can evade antibodies generated in flocks vaccinated with conventional strains, leading to disease even in birds previously immunized with classical strains.

2. Causes of Immunization Failure in Infectious Bursal Disease in Chickens

There are many reasons for the failure of vaccination against infectious bursal disease in chickens, ranging from inappropriate selection of vaccines (strains) to issues related to immunization… If the method is incorrect, the immune response will be unreliable; moreover, intrinsic factors of the chickens themselves—such as the presence of immunosuppressive diseases and the level of maternally derived antibodies in the flock—also play a role. (Variable) These are all factors that contribute to vaccine failure; however, at the root of the problem, the primary causes of immune failure in chickens vaccinated against infectious bursal disease are… If it is in the following two aspects:

2.1 Low or uneven levels of maternal antibodies in chicks: The level of maternal antibodies in chicks directly determines and influences the flock’s ability to resist diseases. Fowl bursal disease virus infection; maternal antibody levels vary widely, and chickens with low maternal antibody titers may remain persistently infected and shed the virus long after clinical onset. Toxin causes illness throughout the entire flock.

2.2 Highly virulent infections or live vaccines with high pathogenicity can cause bursa atrophy, leading to severe immunosuppression and resulting in vaccine failure. In practice… In production, if a poultry farm consistently uses live bursal disease vaccines with high virulence, or if the flock becomes infected with highly virulent strains of infectious bursal disease, it will inevitably lead to bursal disease in chickens. Atrophy of the bursa of Fabricius reduces the chicken’s immune response to vaccines, leading to vaccine failure.

3. Characteristics of Different Bursa of Fabricius Vaccines

3.1 Live Vaccine

Live vaccines—ranging from highly virulent strains to moderately virulent or attenuated strains—are diverse in type and composition; however, live attenuated vaccines face challenges in terms of mutation. Although live vaccines against moderately virulent and highly virulent strains can overcome maternal antibodies, they may cause irreversible damage to the bursa of Fabricius, induce widespread immunosuppression in poultry flocks, and trigger respiratory diseases, making it difficult to strike a balance between preventing infection by extremely virulent strains and ensuring flock safety.

3.2 Inactivated whole-virus vaccines: They have the drawbacks of a slow antibody response and a prolonged immune gap.

3.3 Traditional VP2 subunit vaccines: Their antigen is recombinant VP2 protein, which has a simple structure. Its antigenic determinants are conformation-dependent; detergents and heat readily disrupt its conformation, thereby abolishing its ability to bind neutralizing antibodies. Furthermore, these vaccines elicit only humoral immunity and fail to provide robust protection for poultry flocks.

3.4 Novel Genetic Engineering Subunit Vaccine: This vaccine uses purified VP2 protein as the immunogen. The VP2 protein can self-assemble into virus-like particles (VLPs), which fully activate the chicken immune system. VLPs closely resemble natural viruses in both morphology and antigenicity, maintaining their native spatial conformation. They elicit robust humoral immunity while also inducing a strong cellular immune response, providing robust protection against challenge with highly virulent and variant strains of infectious bursal disease virus. Moreover, the larger size of the antigenic particle (VLPs) compared to r‑VP2 accelerates recognition by macrophages and shortens the time to antibody production relative to VP2 alone. As virus-like particles, VLPs lack viral nucleic acids, preclude viral replication, and eliminate the risk of viral shedding, making them particularly suitable for use in farms heavily contaminated with bursal disease virus and for virus clearance programs.

4. Advantages of the bursa of Fabricius subunit vaccine from Huizhong Biotechnology:

4.1 Strain Characteristics: The purified VP2 protein is used as the vaccine antigen; this VP2 protein can self-assemble into virus-like particles (VLPs), fully activating the chicken immune system and providing robust protection against challenge with highly virulent and variant strains of infectious bursal disease. Protection is more suitable for use in areas with severe epidemics.

4.2 Process Characteristics: Optimization of the protein purification process, including cell disruption and protein purification steps, enables the VP2 protein purity to reach 40% (standard). The protein purity is approximately 8%. Endotoxin removal process: endotoxins are removed via aqueous two-phase extraction, ensuring that each dose contains no more than 20 EU of endotoxin, thereby effectively preventing adverse reactions.

4.3 The vaccine elicits a rapid antibody response and provides long-lasting protection. Ten days after injection, it confers 60% protection against challenge with a highly virulent strain of infectious bursal disease, while by day 14, protection exceeds 90%. It is particularly well suited for broilers and breeding chickens. For chicks aged 7 to 14 days, administer 0.3 mL per bird; the immune period lasts approximately 4 months. For birds older than 14 days, administer 0.5 mL per bird; the immune period extends up to 6 months.

4.4 Safety: Immunization of laying hens at twice the recommended dose and of 7-day-old chicks at three times the recommended dose did not result in any adverse effects on the production performance of the laying hens or the growth performance of the chicks, and the vaccine was well absorbed.

5. Control and Prevention Strategies for Infectious Bursal Disease in Chickens

The foregoing analysis has identified the primary factors contributing to bursa‑of‑Fabricius–mediated immune failure. In light of the characteristics of various bursa‑of‑Fabricius vaccines and Huizhong Bio’s recombinant subunit vaccine against bursa‑of‑Fabricius, the following preventive and control strategies are hereby proposed:

5.1 To ensure that the flock acquires high levels of bursa‑related antibodies or maternally derived antibodies at an early stage and thereby achieves robust protection, it is recommended to vaccinate day-old chicks with 0.15 mL per bird of Huizhong Bio’s New‑Strain Trivalent Inactivated Vaccine, or to administer 0.3 mL per hen to breeder hens at 40 weeks of age and 0.15 mL per chick at 7–10 days of age. This vaccine is unaffected by maternal antibodies and can counteract immune suppression caused by bursal atrophy, helping to elicit active immunity more rapidly.

5.2 For broiler hens and chicks, it is recommended to use Huizhong Bio’s New Liu‑Fa Sanlian Inactivated Vaccine: If a poultry farm has been using live bursal disease vaccines—especially those containing highly virulent strains—for an extended period, the virus’s long survival outside the host and its persistence make it possible for bursal disease virus to remain in the flock and infect birds with low antibody levels, leading to chronic, ongoing infection. The VP2 subunit of bursal disease virus included in Huizhong Bio’s New Liu‑Fa Sanlian Inactivated Vaccine is a viral capsid‑like particle vaccine that fully activates the host immune system, eliciting both cellular and humoral immunity. Moreover, because it contains no viral nucleic acids and does not support viral replication, there is no risk of viral shedding, making it particularly suitable for farms severely contaminated with bursal disease virus or undergoing virus clearance.