By Audrey Hill, Contributor, Valley Ag Voice
An outbreak of Equine Herpesvirus (EHV-1), a highly contagious respiratory disease in horses, is sweeping the southern part of California. What is known about the virus, where has it been found, and how is it avoided?
What Is EHV-1?
EHV-1 is a virus that lives in the environment and is primarily transmitted via the respiratory tract. According to the American Association of Equine Practitioners (AAEP), “Almost all horses have been infected with the viruses and have most of the times no serious side effects” (Equine Herpesvirus, AAEP). Most horses will be infected at a young age and will recover easily. However, reactivation of the infection, often from high stress situations like traveling or from second exposure, can cause a horse to start shedding the virus, meaning they become contagious.
As stated by Dr. Craig Barnett, DVM, Senior Equine Technical Services at Intervet/Schering-Plough Animal Health, EHV-1 will reproduce on the back of the throat after entering a host horse via the nasal cavity, then travel into the lymph nodes, and eventually into the animals white blood cells (viremia). Because of this, the virus can hide from the animal’s immune system for long periods of time and is why many infected horses do not show signs of disease. If the virus starts to reproduce (through virus shedding), blood vessels are the first to come in contact. Inflammation of the blood vessels, known as vasculitis, can further cause inflammation of the placenta and nervous tissue. Subsequently, signs of infection like abortion, neonatal death and Equine Herpes Myeloencephalopathy (EHM), the neurological form of the disease, can appear and can be fatal.
If any of these symptoms do develop, they generally begin to appear within 2-10 days and viral shedding can last roughly between 7-10 days, according to the CDFA (Equine Herpes Virus, CDFA). It is important to note that delayed viral shedding is not uncommon—as seen frequently in reports made by the Desert International Horse Park (DIPP), an equestrian center with high exposure to EHV-1. The unpredictability of the virus makes it necessary to prepare for a high variability of symptomatic and contagious days.
Where Is the Outbreak?
The first case reported by the Equine Disease Communication Center was a single horse confirmed to have EHM in Alameda County on January 21, 2022. The horse started showing clinical signs on the twelfth of January and was reported to have exposed 27 other horses. Soon after, horses in San Mateo County, Riverside, more in Alameda, Orange, and LA counties became diagnosed. As of March 10th, Orange County had 41 cases (5 EHM, 36 fever-only), San Mateo had 38 cases of EHV-1 (5 EHM, 33 fever only), Riverside had 34 cases (3 EHM, 31 fever-only), and LA County had 1 EHM case. For frequent updates on EHV and other diseases affecting horses, visit the Equine Disease Communication Center’s Disease Alerts page.
Equine events, specifically English hunter and jumper events seem to be the most affected. One equine center, the Desert International Horse Park in Thermal, CA, has been under quarantine because of the virus since February 11th (roughly a month), when the first three cases were confirmed. Since that date, the equine park has seen around 13 cases of EHV-1, one euthanasia from EHM, and many horses tested and quarantined. Hannah Bolland, a hunter, jumper, and equitation rider that came to DIHP to compete in the Young Hunters and the National Hunter Derby in February, experienced this first-hand.
“Our barn went to DIHP for week 4 of the Desert Circuit. We found out about the virus a couple days before we were scheduled to head home to Arizona. They isolated and locked down the infected barn so that only specific staff could enter. No horses were allowed to leave that barn. In addition, barns that stayed longer could not leave the premises for the next couple of weeks and no new horses were allowed on the show grounds,” she states. Thankfully, she and the rest of her barn did not come in contact with EHV-1, and their horses are safe at home in Arizona. She comments that, “DIHP has made a lot of improvements to the show grounds, which has attracted many participants from all over the states. It is such a shame that the outbreak occurred, and I give my condolences to the equestrians whose horses were affected or lost to the virus.”
How Can EHV-1 Be Avoided?
The AAEP suggest two primary methods of prevention: vaccination and biosecurity. EHV’s primary transmission route is through horse-to-horse, horse-to-human-to-horse or indirectly through fomites, or disease carrying agents like buckets, people and other tack. The virus can also live outside a host for up to a month depending on the environmental conditions, and it is recommended that any tack thought to have come in contact with the virus should be washed and disinfected during this time period.
Vaccination is another primary defense against the virus. Many vaccines for EHV-1 exist and although not a 100% guarantee against infection, will prepare a horse’s immune system and increase resistance if they were to come in contact with the virus. Dr. Hector Gonzalez, DVM of Bakersfield Large Animal Hospital, Inc. pushes horse-owners to vaccinate and administer booster shots with urgency because of the Valley’s proximity to the Southern California outbreak. He also mentions that Bakersfield Large Animal Hospital offers a core vaccine that includes vaccination for EHV-1 and EHV-4. Updates that Dr. Gonzalez, DVM receives from the state veterinarian regarding EHV-1 push for postponing or canceling events, especially hunter jumper events, to reduce the spread.