Challenges of Controlling Inclusion Body Hepatitis (IBH) and Vaccination in Poultry
Dr. Majed Hamed Al Saegh / poultry pathologist / Australia
Controlling Inclusion Body Hepatitis (IBH) and Hydropericardium Syndrome (HPS) in poultry remains a complex global challenge. These diseases are primarily caused by Fowl Adenoviruses (FAdVs), particularly serotypes 4, 11, and 8b. Despite various vaccination strategies, numerous challenges persist that undermine control efforts.
- Genetic and Antigenic Diversity
The high genetic and antigenic variability of FAdVs presents a significant hurdle in developing effective vaccines. The co-circulation of multiple serotypes, especially FAdV-D (serotypes 2 and 11) and FAdV-E (serotypes 8a and 8b), complicates vaccine formulation. Cross-protection between serotypes is generally poor, and vaccines targeting specific serotypes may not provide immunity against other circulating strains. Moreover, the continuous emergence of new strains further complicates vaccination efforts.
- Inadequate Cross-Protection
Commercially available vaccines, especially inactivated ones, are generally developed from specific serotypes. However, the immunity provided by these vaccines is often limited to homologous strains, with minimal cross-protection against heterologous serotypes. This limitation is particularly problematic in regions where multiple serotypes coexist, such as Egypt, where serotypes 2, 3, 8a, 8b, and 11 are predominant.
- Incomplete Understanding of Pathogenesis
The pathogenesis of IBH and HPS remains incompletely understood, particularly the role of immunosuppression. While some studies suggest FAdVs act as opportunistic pathogens exacerbated by immunosuppressive agents like Infectious Bursal Disease Virus (IBDV) and Chicken Anemia Virus (CAV), others identify certain FAdVs as primary pathogens. This complexity complicates vaccine development and efficacy, as immunosuppressed birds often have suboptimal immune responses to vaccination.
- Lack of Standardized Vaccination Protocols
In many countries, there is a lack of standardized vaccination protocols for controlling IBH and HPS. Vaccination practices vary widely based on regional serotype prevalence, vaccine availability, and farming practices. Such inconsistency in vaccination protocols contributes to inadequate immunity and continued outbreaks, even among vaccinated flocks.
- Vaccine Production Limitations
Producing vaccines for IBH presents technical and economic challenges. While live attenuated vaccines have demonstrated efficacy, concerns about reversion to virulence limit their application. Inactivated vaccines are safer but often less immunogenic, requiring booster doses to achieve adequate protection. Furthermore, producing vaccines via cell culture or embryonated eggs is expensive and inefficient, especially when targeting a broad range of circulating serotypes.
- Vertical Transmission
Vertical transmission of FAdVs from parent flocks to offspring is a major concern in controlling IBH. The virus can be transmitted through eggs, resulting in early infection in chicks and complicating vaccination strategies. In areas with high serotype diversity, this transmission route presents a persistent challenge.
- Co-Infection and Environmental Stress Factors
Co-infections with immunosuppressive agents such as IBDV and CAV, along with environmental stressors like aflatoxins, enhance the virulence of FAdVs. This weakens the birds’ immune systems, reducing the effectiveness of vaccination. Additionally, co-infections can worsen clinical symptoms and increase mortality, leading to substantial economic losses.
- Future Research and Recommendations
Future research should prioritize the development of multivalent vaccines capable of providing broad cross-protection against prevalent serotypes. Additionally, a better understanding of the molecular mechanisms underlying immunosuppression and pathogenesis is essential for creating improved vaccines. Enhanced diagnostic capabilities and standardized vaccination protocols tailored to regional serotype distribution are necessary. Finally, implementing robust biosecurity measures to minimize virus transmission and optimize vaccine efficacy is crucial for successful control of IBH and HPS.
