Equine Herpesvirus, EHV-1, canEquid Herpesvirus, EHV-1, can cause detrimental neurological disease (Equid Herpesvirus-associated Myeloencephalopathy or EHM) in horses and abortion in late-gestational mares. EHM outbreaks remain rare or typically occur as single events that usually are confined to single horse operations during almost an ‘EHM season‘ that runs from October/November through April, with almost 90% of outbreaks during that period. For the last 20 years, we‘ve observed EHM outbreaks in the northern hemispheres most commonly at boarding facilities with single or mixed breeds. However, breeds like Thoroughbreds, Standardbreds, various Warmblood breeds and Quarter Horse/Paint/Appaloosa seem overrepresented in developing EHM during these outbreaks.

The EHV-1 outbreak at an international jumping competition in February located in Valencia, Spain, has been world news, not only in equestrian and equine media, including various platforms of social media, but has also been on national television and in newspapers in most of the participating countries. The following outbreak description does not claim to be complete or accurate for all numbers. This report is also biased, as not all involved have been heard. The information gathered here is based on phone/Zoom™ conversations with local veterinarians at the venue, a few returning participants and communication with several of the referral hospitals directly or indirectly involved in the outbreak. The interpretation of the collected data is based on this author’s previous experience with EHM outbreaks, as well as on current knowledge of EHV-1 pathogenesis and epidemiology.

The Valencia outbreak brings back memories of the 2011 EHV-1 outbreak that began at the National Cutting Horse Association’s Western National Championships in Ogden, Utah. Each year during the months of February and March, equestrian venues in the South of Spain, Portugal and along the Mediterranean coast open their doors for international jumping and dressage competition, under names like Sunshine Tour (Jérez, Spain), CES Valencia Tour and Mediterranean Equestrian Tour at Oliva Nova (Spain). Similar events are organized in Gorla Minore, Italy; Vilamoura, Portugal; and HUBSIDE Jumping, St. Tropez, France.

Each venue hosts competitive events for several weeks, usually around five to eight weeks, with weekly change of competitive levels. This allows horses and teams to move from one location to the next with reasonable transportation time and distance. So it is not uncommon for individual teams to see several venues during a single Spring tour, and each venue will host between 1,000 to 2,000 horses and 400 riders with their support team of grooms from up to 40 different countries, the majority from Central, Eastern and Northern Europe. Several facilities along the roads in France and Spain open their doors for traveling horses. All-in-all, it is a major event of training and competition after a long winter up north and in addition, preparation for European and world championships, as well as for the Olympics.

To set the tone, EHV-1 infection in its chronic-persistent form as a latent infection is endemic in Europe, meaning the virus persists as a sleeper, non-active infection in many if not all horses that became infected at young age as a foal or yearling. Furthermore, EHV-1/EHM is not a reportable disease in Europe, with few exceptions (Sweden, Thoroughbred breeding and racing), and it is the national equestrian organizations and not the FEI, that define core vaccinations for competing horses. Many countries in Europe recommend EHV-1 vaccination; however, it remains a recommendation.

The venue for the CES Valencia Tour is located north of the city of Valencia, a location that has hosted this event in the past (www.cesvalenciatour.com). According to its website, the location provides indoor and outdoor arenas, warm up areas, individually fenced paddocks and stabling for 80 horses in a permanent barn building, as well as another 300+ boxes in a large barn tent with at least two central barn aisles.

The first out of a total seven weeks of competition started on January 28. Typically, four days of competition were followed by three days of non-competing activities (travel days or training). Counts differ, but apparently in week two, there were between 150 and 250 horses on the grounds. It was reported that in week two, there was a first horse with fever, and five or more horses with a fever in week four. Nasal swabs submitted for PCR analysis in week four returned negative for EHV-1. As horses left for Doha, Qatar, during week three, and tested positive for EHV-1 on nasal swabs at the Doha venue, it is very likely that virus had already circulated during weeks two and three of competition in Valencia.

The trickiness of EHV-1 infection and recognition is in the timing. Phase 1 of EHV-1 infection is infection of the upper airways, where it replicates in masses, and is subsequently spread via droplets into other in-contact horses. During phase 1, virus moves towards the local lymph nodes where it also replicates and quickly enters various cells of the immune system, mainly lymphocytes and monocytes.

What follows is a phase 2 of infection, viremia, where virus enters the bloodstream, but inside these immune cells: a cell-associated viremia. While phase 2 does not occur in all phase 1 horses, this phase is a key necessity for the development of EHM as a phase 3. Here, virus-infected immune cells interact with small blood vessels of (mainly) the spinal cord, causing the vascular breakdown and thus inadequate support of the spinal cord with malfunction of neuronal circuits leading to ataxia, weakness and, potentially, recumbency (EHM).

A fever greater than 38.5°C/101.3°F is typically a fever of a viremic (phase 2) animal. Viral replication is most abundant during phase 1 and often tapers off during phase 2. However, phase 1 is typically a very subtle phase of the infection with regards to clinical signs, and early recognition of an infected animal remains difficult at best. This explains the explosive spread of the infection at the venue.
In week five, about 30 horses were reported with fever and most of the horses with EHM were noticed during this week. Several nasal swabs and EDTA whole blood samples (testing for viremia) were then positive for EHV-1. An N-variant, the variant less likely to cause EHM in horses, was identified. Severe cases were transferred to regional (university) hospitals, which cleared their barns first to be able to exclusively tend to these challenging patients. During this week, veterinarians from participating countries were brought in, quarantine stable tents were set up and slings were assembled.

In weeks four-five, quarantine of the entire facility was announced. By week six, several more horses were reported with a fever, as well as a few additional horses with EHM.

On March 29, (week nine), the last non-resident horse left the premises following an orchestrated lift of quarantine based on a rigid sampling and negative test result protocol.

By the end of the tour, Valencia reported that out of 250 to 300 horses on the venue at the beginning of the outbreak, a total of 100 horses developed fevers. Of those, 30 developed EHM, and for 17 of those 30, the disease was severe enough to recommend euthanasia. It is not clear whether the number included affected horses and casualties in the horses‘ home countries.

Back to week four and five…It would have been prudent that all horses should have stayed in quarantine. However, and against recommendations by local authorities, some owners and riders traveled home with both febrile (showing fever) and non-febrile horses. The virus was subsequently transported to the lay-up facilities (France) along the road back north, and into several home farms in France, Germany, Belgium and the Netherlands, with satellite outbreaks that included abortions and further EHM casualties in the home barns.

What went wrong?

1. In numbers, EHM is a rare disease among horses, despite the fact that we assume that many horses are carriers of EHV-1 in its silent, latent stage. First-time infection probably occurs at a young age without complications or harm, and many horse owners and riders are unfamiliar with the disease and its contagious nature. This virus is not perceived by riders and owners as a threat. Thus, there is generally no perceived risk, nor perception of need for vaccination.
    
2. An outbreak that leads to a subsequent (or satellite) outbreak is extremely rare, and is typically caused by horses being transported from affected facility to the yet unaffected. There are only two reports in peer-reviewed literature that describe subsequent satellite outbreaks in the home premises upon return from competition: Belgium 2003 and Ogden, Utah, 2011.

   Despite the ever-increasing competition schedule and the number of horses participating in events since Utah in 2011, EHM outbreaks starting at competition have not been reported, and Valencia apparently is a rare exception.

   Reactivation of virus from its latent location with a return to the respiratory tract, from where it can spread horizontally, is also likely a rare event. This assumption is based on the results of many studies where horses have been screened for virus during and after stressful events and the virus remained undetectable in the nasal passages. That the reoccurrence after reactivation, also called recrudescence, is rare is supported by the fact that the number of EHM outbreaks per year in endemic areas are incidental at best. Typically, an outbreak occurs at a single horse operation, a training or boarding facility -- often upon return of a horse from an event. If the returning horse is the source of the spread, it seems likely to assume that it takes a number of days for the virus to return to the respiratory tract and for the horse to become a productive spreader. These observations and assumptions would explain the usually isolated and incidental nature of EHM outbreaks.

   As horses were transported over long-distances to the Valencia venue, arriving at an unfamiliar and bustling competitive event, it can be assumed that circumstances for recrudescence in that one horse were met.
    

3. When one became many...Horses with different backgrounds were stabled in a large stable tent after strenuous transportation with possible negative effects on a horse’s immune system. Many of them were (reportedly) not current on their EHV-1 vaccination or had not been vaccinated. Although the closed sides of the stalls did not allow easy nose-to-nose contact, the riders and grooms from different parties often know each other, which also facilitates the contacts among horses. In addition, it is not uncommon that equipment (tack, but also, as we were told, nebulizers) is shared between parties. If one horse becomes the super spreader after either horizontal infection or recrudescence in a densely populated stable tent with suboptimal ventilation, it is easy to envision the rapid spread of the virus among this population.

   We also heard of non-compliance to simple isolation and precaution measures. Owners did not want their horses that were showing a fever to be moved into isolation, mainly because of a feared increased risk of injury. Owners left the venue with sick horses, transferring virus into satellite facilities and/or home barns. Relocating untested and unconfirmed ‘negative‘ horses from one barn to another has been shown to be a very effective way to transfer disease.
    

4. The perfect storm…multi-factorial disease: Horses at the Valencia venue were mostly Warmblood breeds, and thus in the high-risk group to develop EHM upon infection and viremia (phase 2). The isolated virus from a horse at the venue was an N-variant of EHV-1. The virus‘ genome has been sequenced, and it appeared very closely related to a Belgian strain that was once recovered from an aborted fetus. While EHM outbreaks associated with an N-variant have been associated with EHM-cases in the past, the number of EHM-cases were considered to be less than with infections caused by a D-variant. However, other factors such as crowding, opportunity for nose-to-nose contact and immune status due to transportation may have facilitated transmission and thus viral load/infectious dose that reached an individual animal.

   An observation by local veterinarians was that with an even gender distribution, female horses were overrepresented in the EHM group. Interestingly, this observation was also made during the Ogden outbreak in 2011; however, it is different from the various EHM outbreaks that occur at (mixed breed) premises considered home operations. Whether this was a coincidental finding or caused by differences in endocrinologic status at competition has to be determined.
    

5. How to prevent this from happening? Vaccination against EHV-1 should be required to enter the grounds and administered more than five weeks prior to an event. Additionally, owner/rider/groom education should be conducted; bedding materials completely removed, a box/stall cleaned and disinfected prior to use; an empty box/stall employed between horses of different teams; intake (physical) exams conducted that include temperature check; regular temperature check by owner and results displayed on the box/stall door; and standard operating procedures for horses with a fever (or other signs of possibly contagious disease). Additionally, there should be little as possible contacts with horses from other teams; no sharing of tack or equipment with other teams; one-way traffic in barn aisles; regular cleaning and disinfection of crosstie areas/ wash stalls between teams.

Lutz Goehring, DVM, PhD, is the Warren Wright, Sr. – Lucille Wright Markey Endowed Chair in Equine Infectious Diseases at the University of Kentucky’s Gluck Equine Research Center.