EHV in Foals From Vaccinated Mares

The results suggest that certain mares have latent EHV-1, experience reactivation of the virus, and pass it on to their foals, allowing viral spread around the farm. Therefore, EHV-1 is continuing a silent cycle among foals even when the mares are vaccinated.
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Since 1997, a multivalent equine herpesvirus vaccine (EHV-1/EHV-4) has been available in Australia, but only in killed formulation. While this vaccine is considered safer for pregnant mares, it provides only short-term protection. Before the widespread use of this vaccine in Australia, some researchers had described the emergence of a "silent cycle" introduction, where EHV-1 circulates among unvaccinated animals in the absence of clinical disease. Recent advances in diagnostic testing have allowed differentiation of EHV-1 and EHV-4 DNA from nasal swabs. This has allowed researchers from the University of Macquarie, Australia, including James Gilkerson, BVSc, BScVet, PhD, to conduct a field study to investigate whether EHV-1 and EHV-4 can be detected in vaccinated mares and their unvaccinated foals, and whether silent cycling of both viruses occurs.

Over a four-week period during mid-breeding season in Australia, 237 mares and their foals had blood and nasal swab samples collected by Caroline Foote, MSc, as part of her PhD research. An EHV-1/EHV-4 vaccine was administered to 128 resident mares on the farm at five, seven, and nine months of pregnancy. Seventy-seven mares had an unknown vaccination history. A further eight mares had received at least two vaccinations, and 24 mares at least one vaccination. Tests were performed on mare and foal nasal swabs to identify EHV-1 and/or EHV-4 DNA, and seroprevalence (total number of samples with positive antibody levels to each virus) was calculated using an EHV-1/EHV-4 type specific ELISA. Gilkerson explains, "Seroprevalence gives us a pretty good measure of past exposure without having to isolate virus."

Results were positive for EHV-1 DNA in six foals and one mare with an unknown vaccination history. There were 44 foals and two mares with positive DNA samples for EHV-4. Both mares had been vaccinated during their previous gestation. Two additional foals tested positive for both EHV-1 and EHV-4. The seroprevalence of EHV-1 was 30.4% in mares and 27% in foals. For EHV-4, seroprevalence was 100% in mares and 97.9% in foals. Gilkerson points out that herpesviruses can become latent (inactive) and later reactivated, making it difficult to interpret seroprevalence in light of vaccination status.

To Gilkerson's knowledge, this was the first study to demonstrate EHV-1 and EHV-4 in nasal secretions of unweaned, unvaccinated foals born of vaccinated mares. The results suggest that certain mares have latent EHV-1, experience reactivation of the virus, and pass it on to their foals, allowing viral spread around the farm. Therefore, EHV-1 is continuing a silent cycle among foals even when the mares are vaccinated. This provides a source of latently infected fillies that will enter the broodmare herd. Gilkerson therefore suggests that alternative vaccination strategies are needed to combat these viruses

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Written by:

Susan Piscopo, DVM, PhD, is a free-lance writer in the biomedical sciences. She practiced veterinary medicine in North Carolina before accepting a fellowship to pursue a PhD in physiology at North Carolina State University. She lives in northern New Jersey with her husband and two sons.

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