Effects of Equine Herpesvirus (EHV) Vaccination Frequency on Host Immunity to EHV-1

Principal Investigator: Bettina Wagner

Department of Population Medicine and Diagnostic Sciences
Sponsor: Harry M. Zweig Memorial Fund for Equine Research
Title: Effects of Equine Herpesvirus (EHV) Vaccination Frequency on Host Immunity to EHV-1
Project Amount: $78,997
Project Period: January 2017 to December 2017

DESCRIPTION (provided by applicant): 

Equine herpesvirus type 1 (EHV-1) continues to cause severe outbreaks of abortions or myeloencephalopathy in horses despite widely used vaccination. Current EHV vaccines have decreased abortion storms but neurological EHV outbreaks have continued to occur in recent years. Equine industry and the equine veterinary community have responded with a tendency to increase vaccination frequency. The American Association of Equine Practitioners (AAEP) recommends EHV vaccination every 6 months for competing horses. Some vaccine suppliers recommend vaccinating every 3 months. In addition, equine events may require frequent EHV-1 vaccination with documentation that the horse has been vaccinated within a certain time frame preceding the event (often less than 30 days). However, data to support that frequent administration of EHV vaccines improves immunity and protection from disease are widely missing. We have previously vaccinated 15 pregnant mares in short intervals of 60-90 days between vaccinations as currently recommended by the AAEP EHV vaccination guidelines. Because of the EHV-1-free status of our Cornell Icelandic herd, all horses have been monitored monthly for EHV-1-specific antibodies and cellular immunity. This revealed a surprising result: Frequent EHV vaccination with an inactivated commercial EHV vaccine resulted in declining antibody values and cellular immunity at several time points after vaccination and immune parameters declined despite additional vaccine boosts. This observation is summarized here as ‘adverse immunity to EHV vaccination’. Our preliminary data further suggested that adverse immunity occurred more often if vaccine intervals of 60 days were used. We hypothesize that frequent EHV vaccination in short intervals can also result in adverse immunity in non-pregnant horses and that intervals of 60 days or less increase the risk of adverse immunity.The goal of this project is to identify if frequent vaccination of non-pregnant horses with a commercial inactivated EHV vaccine also results in adverse immunity.