Equine Herpesvirus-1

The equine herpesvirus-1 (EHV-1) is one of a large group of viruses in the family Alpha herpesviridae that causes potentially serious disease in horses.

EHV is common in horse populations and this can lead to sporadic outbreaks.

EHV-1 infection may occur subclinically or be manifest in three clinical forms that can occur independently or concurrently. These forms include reproductive, respiratory and neurologic disease.

EHV-4, or the equine rhinopneumonitis virus, is most common among foals and yearlings. It mainly causes respiratory disease, but it can also cause abortion and neurologic disease.

Transmission

Transmission occurs when infected and uninfected horses come in either direct (nose to nose) or indirect (through contaminated buckets, clothing and blankets) contact with nasal discharges of infected horses. 

The virus can travel via aerosol (in the air) for short distances. 

The virus may also be transmitted by contact with aborted fetuses, placental fluids or placentas from infected horses. 

Also, following infection, horses may become latent carriers of EHV and the virus may be reactivated after stress or high doses of corticosteroids.

Clinical Signs

The incubation period (period of time from exposure to development of first clinical signs) ranges from two to 10 days. 

Respiratory signs for EHV-1 and EHV-4 include fever of 102 to 107 degrees F lasting one to seven days, coughing, depression, inappetence (going off feed) and nasal discharge. 

Abortion usually occurs between months seven and 11 of gestation, about two to 12 weeks after infection. 

There is no evidence that the mare's reproductive tract is damaged, and it does not affect her ability to conceive. 

Signs of neurologic disease for EHV-1 and EHV-4 include mild incoordination, hind limb paralysis, recumbency (lying down and being unable to get up), loss of bladder and tail function, and loss of sensation to the skin around the tail and hind limb areas. 

Diagnosis

Upon detection of clinical signs suggestive of EHV, the veterinarian may choose to take a nasopharyngeal (nose and throat) swab of the horse, blood sample or tissue from the aborted fetus for detection of virus in the tissues. Paired blood samples for detection of antibody titers (levels) may also be taken.

Treatment

Treatment involves supportive care and treatment of the symptoms. Non-steroidal anti-inflammatory drugs are commonly used to reduce fever, pain and inflammation. 

In uncomplicated cases, complete recovery will occur in a few weeks. 

Horses with neurological disease have variable recovery rates depending on the severity of the clinical signs. The prognosis is poor if the horse is recumbent for an extended period of time. 

The horse should rest until fully recovered and then gradually return to work. 

Prevention

There are two types of vaccines available for use in the horse for prevention of the disease, but their use remains controversial. 

Vaccination may reduce the severity and duration of disease, but will not totally prevent the disease. 

Your equine veterinarian should be consulted regarding the most appropriate use of vaccination in your particular circumstance. 

Since latent infection is still a problem, vaccination must go hand-in-hand with the use of best management practices.

There are both modified live virus and killed virus vaccines available. The modified live virus vaccine contains virus that has been altered to make it unlikely to cause disease but still able to reproduce in the body cells and stimulate immunity. The killed vaccine contains virus that has been inactivated or killed using either heat or chemicals.  

The modified live vaccine is administered intranasally and offers quicker protection. There is no scientific basis to indicate that the modified live vaccine will cause disease. 

The killed vaccine is given intramuscularly. Vaccine usage in light of the recent outbreaks of neurological EHV-1 is currently being reevaluated. Consult your veterinarian for recommendations.

To prevent an outbreak, horses arriving on a farm from other locations should be isolated for three to four weeks before being introduced into the resident horse population. 

Reduce management-related stressors that may increase the possibility of stress-induced reactivation of latent EHV-1 in carrier horses. 

Keep horses separated by physiological state or group, especially with regards to pregnant mares, which should be kept away from weanlings, yearlings and performance horses that frequently travel.

Facility Considerations

The neurologic form of EHV, called equine herpes myeloencephalopathy (EHM), is a reportable disease in Florida.

In the case of an outbreak, infected horses should be isolated from other horses. 

The stable should be quarantined for at least two weeks after signs of clinical disease in the last case resolve. 

All horses exposed to the affected horse should have their temperatures taken twice daily. If an exposed horse develops a fever, that horse should be isolated immediately.

All stable equipment should be disinfected. 

People handling the infected horses should be sure to wash their hands after handling each horse, dip their shoes in a disinfecting footbath, and change clothes before working with healthy horses. 

Some sources suggest that bedding be removed and disposed of in a sanitary manner. 

Barn stalls, aisles and other surfaces should be cleaned and disinfected as well. 

Although this virus can last for several weeks in the environment, it is readily killed by most common disinfectants; phenol-based disinfectants are commonly used.

Suspected or confirmed cases of EHM must be reported to the State Veterinarian’s Office. To report EHM or other reportable diseases, call (850) 410-0900 from 8 a.m. to 5 p.m., Monday through Friday. For after-hours reporting, call 1-800-342-5869 or email RAD@FDACS.gov.

Resources

• Basic Equine Biosecurity [ 162.8 kB ]
• EHV PCR Sample Collection [ 210.5 kB ]
• Equine Herpesvirus Myeloencephalopathy Guidance [ 760.1 kB ]
• USDA Equine Brochure [ 3.8 MB ]