An aggressive strain of Equine Herpesvirus (EHV) has been confirmed by three major equine veterinary clinics in Texas, with cases traced to a recent event in Waco.
In response, here is what horse owners can do to monitor at-risk horses and reduce the spread of the disease.
The Horse&Rider team is closely monitoring the situation and will update as we receive official information. Our thoughts are with those affected by this disease outbreak.
What is EHV?
Equine herpesvirus—commonly referred to as EHV or equine rhinopneumonitis—is a group of highly contagious viruses found in horses worldwide. Of the nine identified equine herpesviruses, EHV-1, EHV-3 and EHV-4 pose the greatest disease risk in the United States. While these viruses can cause serious illness in horses, they do not infect humans.
Types of EHV and what they cause
- EHV-1 is the most concerning strain. It primarily causes upper respiratory disease and abortion or stillbirth in pregnant mares. In some cases, it can progress to equine herpes myeloencephalopathy (EHM)—a neurological form of the disease that can be fatal.
- EHV-3 causes coital exanthema, a venereal disease transmitted during breeding.
- EHV-4 is typically associated with respiratory disease in foals and can also trigger abortion in mares. Though less common, EHV-4 can produce neurological cases similar to EHM.
Most horses infected with EHV recover without long-term effects if they do not develop neurological complications.
EHV Outbreak Symptoms
Clinical signs of EHV can vary widely. Some horses may show no outward symptoms, while others develop severe respiratory or neurological disease. Common signs include:
- Fever (often the earliest and most consistent sign)
- Nasal discharge
- Cough
- Depression or lethargy
- Difficulty urinating
- Hind-limb weakness or incoordination
- Stumbling or ataxia that may progress to all four limbs
- Loss of tail tone
- Head tilt
- Recumbency (down and unable to rise)
- Abortion in pregnant mares
Because the early signs can be subtle, consistent temperature monitoring and prompt veterinary evaluation are critical during an outbreak.
Temperature Monitoring Encouraged
Owners are urged to take their horses’ temperatures twice daily, particularly if the horse attended the Waco event or any show within the past 14 days.
A rectal temperature of 101.5°F or higher is cause for concern, especially if paired with nasal discharge, coughing or neurologic signs. Owners should contact their veterinarian immediately if symptoms appear.
How EHV-1 Outbreaks Spread
Equine Herpesvirus-1 spreads efficiently through several routes:
Direct horse-to-horse contact:
The most common mode of transmission is nose-to-nose contact, where infected horses shed the virus in nasal secretions.
Aerosolized particles:
Coughing and sneezing can release virus-carrying droplets into the air, allowing nearby horses to inhale them.
Shared or contaminated equipment:
Feed tubs, water buckets, halters, grooming tools, tack and even thermometers can transfer the virus if used on multiple horses—a process known as fomite transmission.
Human transmission:
People may unknowingly carry the virus on their hands, clothing, jackets, boots or equipment and transfer it between horses.
Mare-to-foal transmission:
Certain forms of EHV-1 can be passed from infected pregnant mares to their unborn foals, leading to abortion or weak neonatal foals.
Silent Carriers
One of the most challenging features of EHV-1 is its ability to remain latent. Horses may carry the virus without showing clinical signs, and stress from travel, competition, weather changes or illness can reactivate viral shedding.
— H&R —