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EHV-1 Vaccines May Limit Spread of EHM



The neurologic form of equine herpes virus 1 (EHV-1) has become a very important issue for the equine industry today. The ability of a vaccine to protect a horse from equine herpes myeloencephalopathy (EHM) is unknown, as this disease has occurred in even well-vaccinated horses.


The results of a pilot study to look at the efficacy of a commercially available, high antigen vaccine (Pneumabort K®, Zoetis Animal Health, Kalamazoo, MI) against EHM were recently reported. The project used 12 aged mares (>20 yrs old) that were divided into two groups. Three IM injections were administered at monthly intervals, with one group receiving saline (control group) and the other group receiving the high antigen vaccine (test group), followed by inoculation with a strain of EHV-1 that produces EHM. Each mare was then scored as to clinical presentation, and ataxia (difficulty with balance and coordination) and body temperatures were recorded. The results of this study showed that the control horses had more severe neurological signs (5/6 mares) when compared to the vaccinated group (1/6 mares). The body temperatures of the vaccinated group were lower after inoculation with the virus as compared to the control group. Clinical scores (median value) were 20% lower in the vaccinated group than in the control group.


Immunization of horses with a high antigen EHV-1 vaccine decreased clinical signs of infection when challenged with a neuropathogenic strain of EHV-1, although differences were not statistically significant. A larger number of horses would be needed for conclusive evidence of the ability of vaccination to protect horses from EHM.” What this pilot study did provide was enough positive evidence to justify a more extensive study utilizing a greater number of horses.


This study should not prompt all of us to begin vaccinating every horse with Pneumabort K® with the mistaken belief that we can prevent the neurological form of EHV from affecting our horses at this point in time. What it does do is give us hope that, with more research, an effective vaccine and vaccination protocol will be developed that will accomplish this very goal.


Scientific Article: Maxwell, L, et. al. “Prevention of Equine Herpes Myeloencephalopathy by Vaccination: A Pilot Study”. AAEP Proceedings, vo. 59, 2013, pp. 51-52.


Companion Article: Larson, E. “Can Vaccination Protect Horses from Neurologic EHV- 1?” theHorse.com, Dec. 30, 2013, Article#33119.



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