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.

Preliminary data: 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.

Hypothesis: 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 hypothesis will be tested by two Specific Aims.

Specific Aims: 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. In Aim 1, we will vaccinate non-pregnant mares and geldings with the same vaccination schedule (60 or 90 day intervals) as used previously for the pregnant mares to identify if pregnancy status or gender influences adverse immunity to frequent vaccination. In Aim 2, we will test if short vaccination intervals of 30 or 45 days will consistently induce adverse immunity in horses. For both aims, we will evaluate local and systemic EHV-1 specific antibody and cellular immune responses induced by the different vaccination protocols.

Relevancy to equine health and the racing industry: The outcomes of this study may not be in line with current EHV vaccination recommendations and may challenge current EHV vaccination practices. A detailed understanding of immunity resulting from frequent EHV vaccination will help to optimize current EHV vaccination protocols for existing vaccines, to maximize immunity against EHV-1 and to improve protection of the US horse population against EHV-1 outbreaks.