Exposed: Practical Lessons from a Real-Life Equine Infectious Disease Outbreak

An inside look at the 2018 EHV-1 outbreak that impacted hundreds of horses at a California show and how to control the disease.

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After an exhaustive retracing of the infected horse’s steps since his arrival on the grounds, Dr. Katie Flynn and her team determined that only one barn of 56 horses had been exposed to him. They placed that barn’s inhabitants under immediate strict quarantine. | Photo: Courtesy The Capistrano Dispatch

What if you were stuck in the thick of an equine infectious disease outbreak?

Infectious microbes can hit horse populations almost anywhere, causing illness and ruining horse owners’ best-laid plans. Whether you keep a couple of horses on your farm to ride in your spare time or travel with your horse to competitions every weekend, no equid on any operation is 100% safe from infectious disease.

Veterinarians ranging from private practitioners to public-health-centered professionals stay alert for infectious disease outbreaks because they can be a significant economic hit for horse owners and entire industries alike. What, however qualifies as a full-blown outbreak? Josie-Traub Dargatz, DVM, MS, Dipl. ACVIM, emeritus professor of equine medicine at Colorado State University’s College of Veterinary Medicine & Biomedical Sciences, in Fort Collins, says it’s any disease occurring at a level above what is average or expected.

“The designation of a disease situation as an outbreak is disease-agent- and location-dependent,” she says.

For example, Florida veterinarians diagnose an expected number of Eastern equine encephalitis (EEE) cases annually.

“Having EEE cases in Florida is not unusual, and they do occur every year,” says Traub-Dargatz. “Seeing those cases in the Western U.S., however, would be abnormal and, therefore, could be considered an outbreak.”

In this article we’ll take an in-depth look at the ­chronology of an outbreak that occurred in early 2018: a rash of equine herpesvirus-1 (EHV-1) cases impacting hundreds of horses competing at an equine event in Orange County, California. We’ll share how the monthlong roller coaster affected these horses; their riders, trainers, grooms, and veterinarians; and event and state officials. Our sources will also explain the steps required to control disease and protect horses going forward.

Practical Lessons from a Real-Life Equine Infectious Disease Outbreak
Dr. Katie Flynn and her team placed the barn that had been exposed to the index case and its inhabitatns under immediate quarantine. | Photo: Courtesy Dr. Katie Flynn

A Brief Overview of the Outbreak

On Tuesday, April 10, a 17-year-old Thoroughbred gelding began showing signs of neurologic disease, including fever, ataxia (incoordination), and depression, while stabled at an Orange County equine facility for an event. His owner immediately reported these signs to the horse’s veterinarian, who examined the horse and contacted event officials and state veterinarians.

Enter Katie Flynn, BVMS, MRCVS, equine staff veterinarian for the California Department of Food and Agriculture (CDFA). One of her responsibilities involves responding in such situations to contain disease and minimize its impact.

Working closely with facility management and owners, Flynn and CDFA colleagues made sure the first febrile, neurologic horse (i.e., the “index case”) was immediately isolated at an off-site facility, and they collected appropriate samples for laboratory testing. On April 13 they confirmed a diagnosis of equine herpesvirus myeloencephalopathy (EHM), the neurologic manifestation of EHV-1.

After an exhaustive retracing of the infected horse’s steps since his arrival on the grounds, Flynn and her team determined that only one barn of 56 horses had been exposed to him. They placed that barn’s inhabitants under immediate strict quarantine.

“This means that … the owners, trainers, grooms, and veterinarians were instructed to use appropriate infection control strategies, including foot baths (upon entering and exiting the barn), twice-daily temperature monitoring (of the horses), strict movement controls within the quarantined area, which included a quarantined arena to exercise,” says Flynn. “Access to the quarantined area was restricted to personnel associated with the exposed horses, and those personnel were not permitted in other parts of the premises. … Access to quarantined positive horses required use of disposable coveralls, gloves, and boot covers.”

In addition, CDFA staff members were on site daily to ensure caretakers followed biosecurity protocols and that any exposed horses displaying clinical signs would be promptly identified, samples collected and tested, and suspect cases isolated immediately until test results confirmed health status, she says.

Flynn and her team recommended that caretakers of the hundreds of other horses on the property use enhanced biosecurity measures, including twice-daily temperature-monitoring.

“Any horses in any barn on the property with fevers were sampled to ensure the disease had not spread to those other barns,” says Flynn. “At the point of quarantine issuance, there was no evidence of disease spread, and only one case—the index case—confirmed.”

All was quiet until five days later, April 18, when veterinarians confirmed two new EHV-1 cases in the quarantined barn. They relocated those horses to a separate isolation area on the property. All horses in the quarantined barn remained there, and the veterinarians continued monitoring all other horses on the property closely.

Again, and just as everyone was beginning to regain hope for a swift end to this viral scourge, vets confirmed two new cases of EHV-1 on April 23. Those horses originated from the same barn as the preceding three cases.

Veterinarians confirmed three additional cases of EHV-1 over the next two days, all from that barn, and viral transmission continued. By April 29, 16 days after confirmation of the first case, veterinarians had identified 10 more in the quarantine barn, including seven “classic” cases with fevers and three with EHM.

Flynn says this type of waxing and waning pattern is not unusual during outbreaks. Thus, she has a seven-day rule: She doesn’t relax and consider the worst of the outbreak behind her until at least seven days after detection of the last confirmed case. She also urges owners not to become lax on biosecurity protocols until at least 14 days after the last case diagnosis or until the quarantine is lifted.

“Nonetheless, thanks to the efforts of the owners, trainers, facility workers, and management, the virus was quickly contained and no new cases were subsequently diagnosed,” says Flynn. “The quarantine was finally lifted and all horses released alive on May 15.”

Practical Lessons from a Real-Life Equine Infectious Disease Outbreak
Access to the quarantined barn was restricted to people caring for the exposed horses, who were required to use protective clothing and disinfectant foot baths. | Photo: Courtesy Katie Hatch

From the Inside Out

If you’ve never been involved in an outbreak, you might not fully appreciate all the logistical aspects involved. Consider those 56 horses in the quarantined barn, some potentially brewing an infection that could easily spread down the aisle to other apparently healthy horses. All those horses still need to be fed and watered at least twice daily, handled for basic chores, and their stalls mucked by their respective owners, trainers, or grooms. If the owners or trainers have multiple horses at the event, they could have been traveling between quarantined and nonquarantined barns daily. And California’s geography certainly did not help.

“With limited available space in this region of California, finding an appropriate isolation facility and location for the quarantined horses was challenging, to say the least,” Flynn says. 

In sum, it was a veritable logistical nightmare, especially considering that:

  1. Each horse in isolation had at least one person entering the quarantined barn several times a day, increasing the risk of disease spread.
  2. Veterinarians and event and state officials also entered the quarantine barn several times daily to make sure caregivers were following their ­recommendations.
  3. Feed was being delivered to the quarantine barn and waste being removed.
  4. The owners/trainers/grooms could not abandon the event location until the quarantine was lifted, because someone needed to care for each horse. This affected other events an owner might have had planned, the lives of everyone “stuck” at the outbreak location, and in many cases the day-to-day running of the operation back home.

“This is why strict protocols were implemented to ensure personnel or supplies entering the quarantined area did not introduce or further spread the virus,” Flynn says.

Practical Lessons from a Real-Life Equine Infectious Disease Outbreak
Like many farms, the Orange County facility had limited space for isolating infeted horses, so veterinarians placed positive horses in temporary pipe corral pens in an arena. | Photo: Courtesy Katie Hatch

Code of Conduct

To minimize outbreak situations such as this, the American Association of Equine Practitioners (AAEP) has committed time and money to research and publish guidelines on both biosecurity and infectious disease control, which were updated this year (available at Biosecurity encompasses all practices designed to prevent the introduction and minimize the spread of infectious disease agents in equine populations.

The AAEP guideline’s authors emphasize that “many people focus on the ‘outbreak management’ aspect of biosecurity, but arguably more important are the day-to-day biosecurity practices that minimize the likelihood of a disease outbreak in the first place or make it easier to quickly contain an outbreak with minimal disruption and expense.”

Traub-Dargatz says the updated guidelines include more details on biosecurity actions and protocols, a resource table for where to obtain supplies needed to implement the recommendations therein, and a worksheet practitioners can use to ­determine what risks to ­address.

“Infection control efforts should begin with actions taken as soon as there is a suspect case,” she says. “Once a diagnosis is made in an outbreak situation, the infection control plans can be more focused based on what is known about a specific disease agent. The infection control guidelines developed by the AAEP assist veterinarians in responding to a suspect case of an infectious disease and provide guidance once a diagnosis is made through the specific recommendations under each disease heading.”   

Regardless of whether farms are following preventive biosecurity steps or dealing with disease control, the key to success in an outbreak is isolation, which is often easier said than done.

“Isolation is critical to quick control of disease spread,” says Flynn. “Unfortunately, most facilities have limited or no area to isolate. In the Orange County case we used an arena and put up pipe corral pens because temporary tent stabling was not available. This is why pre-planning for the worst-case scenario is so ­important.”

Communication: Key to Success

Both Flynn and Traub-Dargatz attest that communication also plays an integral role in any outbreak situation.

Managing a Horse Disease Outbreak (Infographic)
RELATED CONTENT: Managing a Horse Disease Outbreak (Infographic)

​“Communication is essential during a disease incident,” Flynn says. “Facilities with e-mail distribution lists for all individuals on the premises are able to quickly provide accurate information and recommendations for disease control measures to implement. Lack of e-mail or social media mechanisms to communicate hinders timely notification of individuals.”

“Communication can be a challenge when people hear rumors and are concerned about their horse,” adds Traub-Dargatz. “Having someone designated to oversee communication and being consistent in messaging can help put the outbreak situation into perspective.”

In her experience managing outbreaks Traub-Dargatz has learned a few important strategies:

  1. Never announce confirmation of an infectious disease without also outlining a plan for dealing with it and a time frame for providing updates;
  2. Relay the key points of the plan for dealing with the situation; and
  3. Keep your messaging straightforward and consistent to avoid ­confusion.

Excellent communication during an outbreak also reminds and motivates those involved to continue practicing the recommended biosecurity measures.

Plans for Prevention

“Control of disease takes effort on everyone’s part,” says Flynn. ­“Implementing biosecurity measures to prevent an outbreak takes time and effort but is absolutely essential at any equine event. In my experience, any delay in implementation of disease control efforts leads to continued disease spread and longer quarantines for regulatory diseases.”

She says the four most important steps to take at an event to minimize the chances of an outbreak include:

  1. Limiting horse-to-horse and horse-to-human contact;
  2. Avoiding communal water;
  3. Not sharing equipment unless it has been cleaned and disinfected between uses; and
  4. Monitoring the horse’s health with twice-daily temperature recordings and reporting any temperature over 101.5 degrees F to a veterinarian.

General recommendations Traub-­Dargatz says owners should follow, regardless of the scenario, include:

  1. Having their veterinarian vaccinate horses against preventable diseases based on their risk level for exposure;
  2. Helping horses fend off disease by making transport as low-stress as possible, providing well-ventilated—and, ideally, temperature-controlled—­housing at events and at home, and offering optimal nutrition;
  3. Taking steps to minimize the risk of disease introduction; and
  4. Acting quickly in cases of suspected infectious disease, with a plan for how to minimize spread.

In sum, Traub-Dargatz says, “Owners care about their horses and want to do what is best for them. If they have good everyday hygiene practices, it reduces risk of exposure. Plus, having necessary supplies and a pre-made ‘worst-case scenario’ plan already in place will make it easier to control an outbreak in a timely manner. Any actions we take to avoid a problem will help, especially considering everybody involved in an outbreak already has a full-time job doing the everyday things.”


Written by:

Stacey Oke, MSc, DVM, is a practicing veterinarian and freelance medical writer and editor. She is interested in both large and small animals, as well as complementary and alternative medicine. Since 2005, she’s worked as a research consultant for nutritional supplement companies, assisted physicians and veterinarians in publishing research articles and textbooks, and written for a number of educational magazines and websites.

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