A novel inactivated bivalent vaccine for chickens against emerging hepatitis-hydropericardium syndrome and infectious bursal disease

Genetic evolution of fowl adenovirus serotype 4 and its pathogenicity to Cherry Valley ducks in China

Hydropericardium hepatitis syndrome (HHS), a novel poultry disease, is caused by fowl adenovirus 4 (FAdV-4). It mainly infects 3-5-week-old broilers. In July 2015, the first outbreak of HHS occurred in the broilers in east China, which caused great economic losses to the poultry industry. In June 2019, infectious disease was detected with suspected HHS symptoms on a duck farm in Linyi City, Shandong Province. The main necropsy lesions included pericardial effusion and hepatitis. In this study, we isolated a strain of FAdV-4 from naturally infected ducks and named it SDLY190604, and the hexon gene sequence was amplified and analyzed using polymerase chain reaction (PCR). In order to study the effect of FAdV-4 on Cherry Valley ducks, we inoculated three-week-old ducks with 0.2 ml of FAdV-4 virus fluid (TCID₅₀ of 10^-6.3/0.1 ml) by orally, subcutaneously and intramuscularly. Clinical signs, gross lesions and histopathological changes, cytokines and viral load were detected and recorded within 15 days after infection. The results showed that ducks in the experimental groups exhibited typical symptoms of hydropericardium and hepatitis. The histopathological sections showed multiple-organ damage, including serious liver and kidney damage with elevated levels of inflammatory cytokines, probably due to the infection and innate immune response. Later, immunosuppression occurred, resulting in decreased levels of cytokines. The viral load indicated that the virus could be present in several organs of the ducks, with the highest viral DNA found in the liver, followed by the kidney. Compared to the subcutaneous and oral groups, the intramuscular group exhibited the highest viral load. In summary, this study can increase our understanding of the pathogenicity of FAdV-4 in ducks and provide a basis for further understanding of the virus, imparting new insights into disease research.

Probiotics Surface-Delivering Fiber2 Protein of Fowl Adenovirus 4 Stimulate Protective Immunity Against Hepatitis-Hydropericardium Syndrome in Chickens

Background and Objectives

Hepatitis-hydropericardium syndrome (HHS) caused by Fowl adenoviruses serotype 4 (FAdV-4) leads to severe economic losses to the poultry industry. Although various vaccines are available, vaccines that effectively stimulate intestinal mucosal immunity are still deficient. In the present study, novel probiotics that surface-deliver Fiber2 protein, the major virulence determiner and efficient immunogen for FAdV-4, were explored to prevent this fecal–oral-transmitted virus, and the induced protective immunity was evaluated after oral immunization.

Methods

The probiotic Enterococcus faecalis strain MDXEF-1 and Lactococcus lactis NZ9000 were used as host strains to deliver surface-anchoring Fiber2 protein of FAdV-4. Then the constructed live recombinant bacteria were orally vaccinated thrice with chickens at intervals of 2 weeks. Following each immunization, immunoglobulin G (IgG) in sera, secretory immunoglobulin A (sIgA) in jejunum lavage, immune-related cytokines, and T-cell proliferation were detected. Following challenge with the highly virulent FAdV-4, the protective effects of the probiotics surface-delivering Fiber2 protein were evaluated by verifying inflammatory factors, viral load, liver function, and survival rate.

Results

The results demonstrated that probiotics surface-delivering Fiber2 protein stimulated humoral and intestinal mucosal immune responses in chickens, shown by high levels of sIgA and IgG antibodies, substantial rise in mRNA levels of cytokines, increased proliferative ability of T cells in peripheral blood, improved liver function, and reduced viral load in liver. Accordingly, adequate protection against homologous challenges and a significant increase in the overall survival rate were observed. Notably, chickens orally immunized with E. faecalis/DCpep-Fiber2-CWA were completely protected from the FAdV-4 challenge, which is better than L. lactis/DCpep-Fiber2-CWA.

Conclusion

The recombinant probiotics surface-expressing Fiber2 protein could evoke remarkable humoral and cellular immune responses, relieve injury, and functionally damage target organs. The current study indicates a promising method used for preventing FAdV-4 infection in chickens.

Advances in Vaccine Development of the Emerging Novel Genotype Fowl Adenovirus 4

Fowl adenovirus (FAdV) was first reported in Angara Goth, Pakistan, in 1987. For this reason, it is also known as “Angara disease.” It was later reported in China, Japan, South Korea, India, the United States, Canada, and other countries and regions, causing huge economic losses in the poultry industry worldwide. Notably, since June 2015, a natural outbreak of severe hydropericardium hepatitis syndrome (HHS), associated with a hypervirulent novel genotype FAdV-4 infection, has emerged in most provinces of China. The novel virus FAdV-4 spread rapidly and induced a 30-100% mortality rate, causing huge economic losses and threatening the green and healthy poultry breeding industry. Vaccines against FAdV-4, especially the emerging novel genotype, play a critical role and will be the most efficient tool for preventing and controlling HHS. Various types of FAdV-4 vaccines have been developed and evaluated, such as inactivated, live-attenuated, subunit, and combined vaccines. They have made great contributions to the control of HHS, but the details of cross-protection within FAdVs and the immunogenicity of different vaccines require further investigation. This review highlights the recent advances in developing the FAdV-4 vaccine and promising new vaccines for future research.