Equine Herpes Virus-1

What is EHV-1?

The acronym EHV-1 refers to Equine Herpes Virus -1 which is one of 4 varieties of the equine herpes virus complex (EHV-1, EHV-2, EHV-3, and EHV-4). EHV-4 is associated with upper respiratory disease in horses where as EHV-1 is associated with respiratory, neurologic, abortion, and foal death. EHV-3 is also known as coital exanthema and is a sexually transmitted disease in horses. This family of viruses is found in horses all over the world and it is unclear why some horses develop the neurologic form of this disease complex.

How is EHV-1 transmitted?

Transmission of the virus from one horse to another is dependent on 1: direct contact (nose to nose), 2: indirect contact via contaminated items and 3: aerosolized fluids (coughing or sneezing). Aerosolized fluids may travel up to 35 feet! The virus may survive for up to 30 days in the environment if the conditions are ideal. Once horses are infected they become latent carriers for the remainder of their life. They may become spontaneous "shedders" during periods of stress!

What are the clinical signs?

Incubation period is typically 6-8 days (time from exposure to onset of clinical signs) however it has been reported to be as long as 21 days!

Common clinical signs may include fever, depression, inappetance, upper respiratory infection, and abortion.
Neurologic signs range from temporary ataxia (in-coordination), urinary incontinence, rear limb weakness (dog sitting), complete paralysis and death. Death may occur within 24 hours of the onset of neurologic signs!!

How do you diagnose and treat horses with EHV-1?

Detection of EHV-1 in horses may be through PCR testing of nasal swab or blood, serologic testing, virus isolation and post-mortem exam.

Treatment is based supportive care which may include IV fluid therapy, anti-inflammatory medication and in some cases anti-viral drugs. There is no specific medication to treat EHV-1 in horses!!

Does vaccination protect horses from EHV-1?

There is no commercially available vaccine that prevents the disease! However there are several vaccines which are believed to reduce nasal shedding and hence limit the spread of disease. These include Rhinoimmune (Boehringer Ingelhein), Calvenza (BI), Pneumorabort-K (Pfizer) and Prodigy (Merck). Vaccination during an outbreak is recommended ONLY if there is a history of being vaccinated previously with these vaccines. Recommended to vaccinate every 3-6 months.

What should you do in the face of an outbreak?


Encourage barn personnel to disinfect clothing, shoes, and hand-wear at the entry and exit of all barn areas.

Monitor rectal temperature daily in horses exposed to known EHV-1 positive horses.

If your horse has been exposed to a horse known to be positive for EHV-1, a 21 day isolation protocol is necessary! Isolation area must consider the potential for a 35 foot range of aerosolized mucus.


Additional information may be viewed at the following sites:

AAEP and EHV-1

UF Veterinary Hospital and EHV-1

Department of Agriculture in Florida and EHV-1