Evaluation of red clover isoflavone extract as a vaccine adjuvant for piglets against Haemophilus parasuis

Glässer’s disease of swine caused by Haemophilus parasuis (H. parasuis) is one of the major bacterial diseases affecting pig farms worldwide. Vaccination is a crucial measure for controlling the H. parasuis infection. Adjuvants are employed to enhance the immunity effects of inactivated vaccines or subunit vaccines. In the present study, a red clover isoflavone extract (RCIE) was investigated as an adjuvant for the H. parasuis inactivated vaccine. Thirty colostrum-deprived (CD) piglets (mixed-breed: Large White × Landrace) aged 15 days were vaccinated on days 0 and 14 with an inactivated H. parasuis vaccine with or without an adjuvant. The adjuvant groups’ vaccines were mixed with a high-dose RCIE (20 mg/ml), a middle-dose RCIE (10 mg/ml), a low-dose RCIE (5 mg/ml), or with Montanide Gel 01 (10%, v/v). Phosphate buffer saline (PBS) was also given as a blank control. Fourteen days after the booster immunisation, the piglets were challenged with H. parasuis LY02 (serotype 5). The IgG antibody, cytokines, T lymphocyte subpopulations, and clinical and pathological signs of the piglets were evaluated. The results showed that the RCIE enhanced the H. parasuis vaccine and elicited strong antibody levels as well as the cytokines IL-2, IL-4, and IFN-γ in serum, and the levels depended on the RCIE dose. Moreover, the piglets vaccinated with the inactivated LY02 containing the Middle-dose RCIE had a higher survival rate in the challenge experiments. In conclusion, RCIE can enhance the H. parasuis vaccine immunity by promoting titres of IgG antibody and by improving the Th1-type cellular immunity.