EHV-1: What We Know and What We Learned (AAEP 2011)

The multistate equine herpesvirus outbreak in May 2011 illustrated the importance of infectious disease control in the equine community. At the 2011 American Association of Equine Practitioners convention, held Nov. 18-22 in San Antonio, Texas, Jerry Black, DVM, summarized the outbreak in a group table topic discussion that he moderated with Josie Traub-Dargatz, DVM, MS, Dipl. ACVIM. Both are part of the faculty at Colorado State University, in Fort Collins.

He explained that horses had dispersed from an Ogden, Utah, cutting horse event just prior to recognition of the first confirmed cases. The veterinarian examining the initial two sick horses recognized how serious the situation was and reported the cases immediately to the Colorado state veterinarian. This communication was a critical factor in limiting the extent of viral spread.

Black explained that show officials made immediate decisions to cancel cutting horse shows in California, Nevada, and Oklahoma. Horses coming in from the Ogden Cutting Horse Championship were turned around before entering these show grounds, and future cutting horse shows were canceled for three weeks. Financial losses for the show organizers, traveling owners, and trainers were significant, he noted.

By the end of the first weekend, additional cases arose among some horses that had attended the Ogden event. Black reported that social media grossly exaggerated reports of the number of sick horses, noting, "There was a need for reliable industry sources of information to disseminate accurate facts rather than rumors." Calls to the American Horse Council, the American Association of Equine Practitioners (AAEP), and contact with the USDA Animal and Plant Health Inspection Service's Veterinary Services (USDA:APHIS:VS) initiated surveillance and monitoring of the outbreak as well as accurate reporting.

Josie Traub-Dargatz, DVM, MS, Dipl. ACVIM added, "This outbreak illustrates the cooperation between the American Cutting Horse Association and state officials as well as USDA:APHIS:VS. Everyone involved played a role in timely responses and communications."

Following the multistate outbreak related to Ogden show horses, the recommendation for quarantining premises with affected horses was 28 days after the last observed clinical signs (fever or neurologic signs) were seen. Testing of exposed and clinical cases with real-time PCR (polymerase chain reaction laboratory test) after resolution of all clinical signs in affected horses could potentially revise this guideline to a shortened period in the future, Traub-Dargatz noted.

She reported that USDA:APHIS:VS is conducting a study in collaboration with state animal health officials to better describe the Ogden outbreak, including collecting information on affected animals' vaccination histories. None of the currently licensed vaccines for equine herpesvirus-1 carry a claim for preventing equine herpes myeloencephalopathy (EHM). "However, at a population level," stressed Traub-Dargatz, "there is the potential for less viral shedding in vaccinates (horses that have been vaccinated fully) when immunization is given prior to exposure."

She noted that the best immune response appears to be associated with the use of vaccines that only contain the herpesvirus antigen and are not combined with other immunizing antigens (i.e., combo vaccines). Products containing high immune-responsive levels of antigen appear to be the most optimal to use. 

Brad Bentz, DVM, added from the audience that equine herpesvirus will never be eliminated entirely, noting that the neurologic strain has been found quiescent (present, but not causing clinical signs prior to death) at necropsy in horses that died from unrelated causes. It is also possible that stressed horses might shed as inapparent carriers. A study by Paul Lunn, BVSc, PhD, MRCVS, Dipl. ACVIM, while at Colorado State University identified that 3.8% of horses at shows and sales shed EHV-1, based on testing of nasal swabs. The subtype of EHV-1 was not determined in this study.

"Biosecurity is as important as vaccination to control disease outbreak," urged Traub-Dargatz. She recommends segregating populations of horses that move off the farm from resident horses that don't leave the premises. Owners and caretakers should check horses' rectal temperatures while at events and for at least a week after returning home. Equine herpesvirus-1 infection begins with an initial fever spike as it invades the respiratory tract. The second fever spike occurring days later is consistent with viremia (virus circulating in the bloodstream), which is a prerequisite for developing neurologic disease. The viremia is considered to be a prerequisite for developing neurologic disease but not all horses that develop viremia go on to develop neurologic disease. There are additional factors (host and environment), the specifics of which are yet to be fully understood, that predispose viremic horses to developing the neurologic form of EHV.

Monitoring rectal temperatures allows farm owners and veterinarians to promptly recognize and isolate febrile horses; this reduces the scope of disease spread. "Preplanning on and off the farm is critical to control and biosecurity, particularly as risk assessment of infectious disease has now become more intensive," stressed Traub-Dargatz.

About the Author

Nancy S. Loving, DVM

Nancy S. Loving, DVM, owns Loving Equine Clinic in Boulder, Colorado, and has a special interest in managing the care of sport horses. Her book, All Horse Systems Go, is a comprehensive veterinary care and conditioning resource in full color that covers all facets of horse care. She has also authored the books Go the Distance as a resource for endurance horse owners, Conformation and Performance, and First Aid for Horse and Rider in addition to many veterinary articles for both horse owner and professional audiences.

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