Attempts to immunoprotect adult horses, specifically pregnant mares, with commercial vaccines against clinical disease induced by equine herpesvirus-1.

This research article chronicles a four-year-long investigation into immunizing more than 300 Lipizzan horses from Equine Herpesvirus-1 (EHV-1), with a specific focus on adult horses including pregnant mares. Key findings involve two vaccines with differing levels of efficacy and suggested changes to their typical intervals of administration.

Methodology and Target Group

• Over the course of four years, more than 300 Lipizzan horses, with around 180 of them being adults, were methodically vaccinated against EHV-1.
• These horses’ antibody responses were assessed periodically through serological tests, forming representative groups for control.

Types of Vaccines Tested

• One vaccine tested was a live virus vaccine, which was found to be ineffective if humoral antibodies were already present in the horses.
• The antiviral immunogenicity of an oil-adjuvanted vaccine was high; however, it caused severe local reactions so frequently that its use for adult horses had to be discontinued.
• The research team suspects that the fetal calf serum existing in the cell cultures used for viral propagation — and not removed from the product before it hit the market — might be responsible for the adverse reactions.
• A mixed virus vaccine with an inactivated EHV-1 component presented a weaker antigenicity, making it unsatisfactory for primary immunization but adequate for maintenance of acceptable antibody levels, especially in horses primed with the oil-adjuvanted vaccine.

Key Observations and Recommendations

• The team discovered that the best results in terms of humoral immunity and clinical acceptability were achieved when two different vaccines were used in combination: one for the induction of immunity (the oil-adjuvanted vaccine) and one for the maintenance of protection (the inactivated mixed virus vaccine).
• Vaccination intervals different from those recommended on the packaging of the mixed vaccine were found to be more effective, thus prompting a change in the suggested schedule of administration.

This research offers significant insight into the most effective vaccination methods for adult horses, particularly pregnant mares, against EHV-1. It suggests that a sequential use of different vaccines could be the most promising approach, providing better disease control and horse safety. However, it also underlines the need for improvements and more rigorous control in vaccine production processes to avoid potential adverse reactions.