May 2023
Equine Herpesvirus in Horses: 2023
By Dr. Elizabeth Davis, DVM, PhD, DACVIM
Equine herpesvirus-1 (EHV-1) is causing troublesome disease in the United States again in 2023. Although this is a virus that we have heard about for several years now, it continues to be a potentially serious pathogen that can cause devastating disease in horses. Most commonly EHV-1 and EHV-4 are associated with mild respiratory signs in horses. However, EHV-1 can also cause abortion and in rare instances EHV-1 can result in central nervous system signs = EHV-1 neurologic disease = myelitis (EHM).
In the past 30 days, EHV-1 neurologic disease cases have been diagnosed in: California, Maine, Oklahoma, Texas, Washington State and Wisconsin. These are the United States cases only, diagnosis has also been made in Canada.
EHV-1 is a very common virus among horses, estimates suggest that nearly all horses are latently infected with EHV. This means that they carry the virus in their system, but they are not sick from the virus. What most horse owners do know is that when EHV causes illness in horses, most common signs include respiratory disease. Upper respiratory signs most commonly include nasal discharge, fever and an occasional cough. Another important outcome of infection in pregnant mares is abortion, this is why pregnant mares should be well vaccinated against EHV-1 to prevent abortion. In rare instances some horses that develop illness associated with EHV-1 progress to develop neurologic signs. Although this is a very serious consequence of infection, when horses recover from infection many of them make a complete recovery and they are unlikely to get sick from the virus again in the future.
The most common scenario for disease outbreak is when large groups of horses gather for an event or show. Oftentimes horses are under stress under such circumstances, they are in an unfamiliar environment and in some cases they have traveled long distances to arrive at the event. Most commonly viral transmission results from horse to horse contact, so keeping horses away from other horses is an important part of managing horses to minimize spread. The virus can survive outside of the horse, so it is important to avoid sharing of grooming tools, tack or equipment. Keeping track of where your horse is housed and who they are in contact with will help to keep them healthy. EHV-1 is unlike other viruses such as equine influenza which can travel over longer distances.
When a horse is infected the initial stages of disease typically include a bout of lethargy, reduced appetite and fever. In horses that progress to show neurologic signs, it may take several days, possibly a week or more is possible. For these reasons when there is a high index of suspicion that EHV-1 disease may be present a quarantine for the suspect area (including all exposed horses) is recommended [state regulation in many areas] for 21-28 days after resolution of disease. This means that 28 days after complete disease resolution it is unlikely that the virus will continue to be shed. However, in a period of less than 28 days the virus may still be present and can cause illness in contaminated horses. The quarantine needs to include all horses that have been exposed to a sick horse. The goal is to avoid exposure of the sick horse [and any in contact horses] with other healthy horses.
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Dr. Lutz Goering Equine Neurology 2nd Ed. |
If your horse develops a fever and was recently around other horses, you should be aware of the potential that EHV-1 may be the culprit. Testing involves a nasal swab and a blood sample for EHV PCR testing. The Kansas State Veterinary Diagnostic Laboratory can perform EHV-1 PCR testing. The main advantage of testing early in the course of disease is to have a diagnosis, which will allow for proper treatment to be started quickly. An accurate diagnosis with proper treatment will provide your horse with the best prognosis for recovery. In addition, this will limit the chances that the virus is spread to other horses.
Although EHV-1 neurologic disease is a serious problem, with proper care there is a reasonable chance your horse will make a complete recovery. Particularly in mild cases where neurologic disease may include mild stumbling. Specific treatments that are used to treat EHV include anti-inflammatory therapy, supportive care such as intravenous fluids and correction of electrolytes. An important part of therapy, particularly when neurologic signs are present, may also include antiviral therapy to reduce the circulating viral load.
In more severe neurologic cases, when horses have trouble rising, a sling may be necessary for supportive care. In addition, some neurologic cases have difficulty with urination, which may necessitate placement of a urinary catheter to facilitate urine evacuation.
Important aspects of working to reduce EHV and specifically risk of EHM:
- Biosecurity: screen horses that come to the farm. Isolate new arrivals for 21 days to be sure they do not have respiratory disease. Monitor temperature daily on new arrivals twice daily to identify a fever spike. Fever is defined as temperature > 101.5 degrees Fahrenheit. Work with these horses after healthy horses, wash thoroughly before handling other horses on the farm. Avoid contact of new arrivals with individuals that have weakened immunity, young foals and elderly horses (>20 years of age) may be immunosuppressed.
- Infection control: when sick horses are present make every effort to minimize the chance of spread to healthy horses. Separate sick horses from healthy horses to reduce disease transmission.
- Herd immunity: work with your veterinarian to establish an optimal vaccine protocol that will keep the group of horses healthy. It is not helpful to only vaccinate some horses on a property. The goal of herd immunity is to have the entire group protected, which diminishes the likelihood for disease outbreak in the individual animal.
Vaccination against EHV-1: Among the AAEP risk-based vaccine recommendation guidelines EHV-1 is included in this group www.aaep.org/info/guidleines. The good news is that we have effective vaccines that protect broodmares from abortion and all horses from respiratory disease (rhinopneumonitis).
There is not a vaccine that protects horses from EHV-1 neurologic disease (myelitis). We do have recommendations from experts for horses in high risk settings which may be at greater risk for exposure to horses with EHV-1 neurologic disease. We therefore recommend that you communicate with the veterinarians at KSU VHC (785-532-5700) or your local veterinarian to determine if EHV-1 vaccination aimed at protection from neurologic disease is indicated for your horse and which vaccine is likely to provide the best protection.