Influential factors inducing suboptimal humoral response to vector-based influenza immunisation in Thoroughbred foals.

The research study explores the factors contributing to the inadequate immune response in Thoroughbred foals after equine influenza (EI) vaccination. The study found that the age at first immunisation and the presence of maternally-derived antibodies significantly influenced the effectiveness of the vaccine, leading to an improvement in response when certain adjustments were made to the vaccination schedule.

Study Objectives, Design and Process

• The primary objective of the research was to understand the substandard antibody response in Thoroughbred foals following EI vaccination.
• This field study was conducted over a two-year period and involved 174 Thoroughbred foals from three different stud farms.
• All foals were vaccinated with a commercial recombinant canarypox-based EI vaccine.
• Antibody response against a specific strain of EIV was assessed at various stages of vaccination, enabling identification of ‘poor-responders’ who didn’t develop a detectable antibody titre.

Initial Observations

• Unexpectedly high frequencies of ‘poor-responders’ were observed after the second vaccination, which further increased after the third vaccination.
• Even after three months following the third vaccination, there was a notable percentage of poor-responders.

Key Influential Factors

• There were two significant factors found to affect the foals’ antibody response – the age at first vaccination and the presence of maternally-derived antibodies (MDA) during the first vaccination.
• Both short-term and mid-term antibody levels were positively related to the age of the foals at the time of the first vaccination.
• But the presence of MDA at the time of first vaccination was negatively correlated to the antibody levels after the third vaccination.

Corrective Measures and Their Impact

• In response to the below-par antibody response at stud farm 1, the research team adjusted the vaccination schedule for the following year – the annual booster immunisation was brought forward and the initial vaccination was delayed.
• This adjustment led to a significant improvement in both short-term and mid-term antibody titres, indicating the potential benefit of modifying immunisation schedules based on specific circumstances.

Key Observations and Outcome

• The research concluded that the age and presence of MDAs at the time of first immunisation played a crucial role in determining the effectiveness of the canarypox-based EI vaccine.
• The study also emphasized the importance of serological surveillance to assess herd immunity and to implement appropriate vaccination measures.