Equine herpes virus is a common virus that occurs in horse populations worldwide. It has been established that up to 70% of adult horses may have the latent virus in their system. While there are several strains, the two most common are EHV -1 which causes abortions, respiratory disease and neurologic disease, and EHV-4 which usually causes respiratory disease only but, in rare cases, causes abortion.

The most common signs of infection with either form of the virus are fever, depression and loss of appetite. Cough and clear to whitish nasal discharge are commonly seen as well.

Respiratory disease caused by EHV is most common in weaned foals and yearlings, often in autumn and winter. Older horses are more likely than young ones to transmit the virus without showing signs of infection (inapparent shedding). Although EHV-1 can cause outbreaks of abortions, EHV-4 has only been associated with single occurrences and is not considered a risk for contagious abortions.

EHV-1 myeloencephalitis (i.e. nervous system disease) is being seen more commonly and results from inflammation of, and damage to, the blood vessels of the central nervous system. EHV-1 nervous system disease does not always occur when a horse has EHV-1 infection. Depending upon the strain of the virus and possible host factors (horse’s age, antibody status, degree of stress, etc) nervous system disease may be seen in 10% to 50% of horses infected with EHV-1. Nervous system disease almost never occurs with EHV-4.

Fever – This is common in all infected animals.

Respiratory – Coughing and nasal discharge are common symptoms.

Abortion – It is usually an abrupt abortion without any signs of vaginal discharge, etc. Typically this occurs in late gestation (8+ months), although earlier abortions can also be seen.

Neonatal Form – This is seen when the fetus is infected in-utero late in gestation. Foals are born weak, and may demonstrate jaundice. Respiratory distress is commonly seen. This form is almost always fatal to the neonate.

Neurologic – This results in incoordination of the hind limbs primarily, although front limbs can be involved. Urine dribbling and rectal fecal impactions may be seen. In severe cases the horse may be unable to stand. Although this may be fatal in its most severe form, the majority of cases respond to supportive care.

Following infection, the incubation period is typically seven to ten days, although earlier signs of illness may be seen. ***Fever usually precedes clinical disease by one to three days. Abortion may occur two weeks to several months after infection.

The most common form of transmission is by the aerosol route with the virus carried on droplets, or direct contact. Indirect transmission can occur when the virus contaminates hands, feed and water buckets, etc. Aborted fetuses and associated placental membranes are significant sources of infection.

Culture – Culture of the virus is taken from blood, nasal secretions or cerebrospinal fluid.

Polymerase Chain Reaction (PCR) test – This looks for viral DNA. Test cannot differentiate between live and dead virus.

Serology – This tests for the presence of antibodies to the virus, and it also gives a measurement of the amount of antibodies present (titer). Antibodies are produced by the horse in response to the presence of the virus in the body. Typically, paired samples are taken five to ten days apart and the values are compared. If the titer is increasing, that signals active infection. Paired samples are recommended because most horses have some level of antibody to the virus in their bloodstream whether they are infected or not arising from either previous natural exposure that has cleared or from vaccination.

Horses shed the virus in nasal secretions. This shedding is most likely to occur during the period of fever, but can continue for anywhere from 14 to 21 days after fever has resolved. Shedding of the infective virus can be detected by use of the PCR test taken from a naso-pharyngeal swab. A large percentage of horses that are recovered from the virus may develop latent infections meaning that the virus stays hidden in the body and can be shed again during periods of stress.

There is some disagreement about the ability of the virus to live in the environment. Some older literature suggests that it may survive for two to four weeks; however consultation with current leading EHV researchers indicates that on a cleaned, disinfected and dry surface, the virus can live no longer than a few days. In a moist environment protected from sunlight, survival may be up to one week! The virus requires a live cell and/or intact viral envelope to remain viable – these conditions cannot be supported on a clean, dry surface.

Minimizing the horses’ exposure risk and keeping stress levels low are key factors in preventing clinical disease. Maintaining animals in a clean, dry, well ventilated environment will reduce viral spread. Monitor exposed animals by checking rectal temperatures daily to twice daily. A rise in temperature (fever) indicates a likely onset of clinical disease in 1 to 3 days and the horse is likely shedding virus at the point of developing the fever.

Vaccination – Because most adult horses have been previously exposed to EHV and have a latent infection, vaccination may be of little benefit and in some cases could be harmful as it appears as if horses that are more highly vaccinated may be more at risk for the neurologic form and it does often appear to be more severe in these animals. Killed vaccines have little to no effectiveness in preventing the neurologic form. Some researchers believe the Modified Live Virus (MLV) vaccine may have more effectiveness in minimizing shedding of the virus and has been less often associated with the severe neurologic form. Pregnant mares should continue to be vaccinated in the 5^th, 7^th and 9^th months of pregnancy using a killed product.

Isolation in the Face of an Outbreak – Maintain infected or exposed animals in absolute quarantine. No direct contact with other horses should be permitted. Feed buckets, waterers and grooming equipment should not be shared between horses. The infected horse should ideally be treated by one dedicated individual who does not have contact with other horses. If this is not possible, the infected horse should be attended to after all others have been taken care of, then the persons clothing laundered. Use of barrier protection in the form of water impermeable gowns and latex gloves is ideal. Hands should be washed frequently and between contact with every horse. Frequent use of hand disinfection (i.e. 62% ethyl alcohol) is advisable and the label should be checked to make sure that it kills viruses. Manure and bedding from infected horses should be disposed of in an area that cannot be contacted by other horses. Wheels and tires of muck carts, tractors and other equipment used in the barn should be cleaned, ideally with a high pressure sprayer and then sprayed with a 1:10 dilution of bleach before leaving the area. Feed and water buckets should be disinfected by immersion in a solution of 1:10 bleach.

Facilities – Exposed areas (including stalls, barn aisles, and any place someone in contact with an infected horse has walked or touched) should be cleaned with a commercial disinfectant that will kill viruses or 1:10 bleach solution. Detergents will also kill herpes virus. Disinfection should occur only after a thorough cleaning and removal of all visible dirt and organic matter. Horse trailers and vans should be cleaned by removal of all bedding, feed, hay and manure, followed by vigorous scrubbing with hot water and detergent. A dilution of 1:10 bleach can then be applied using a standard garden sprayer and allowed to dry – do not hose off.

This disease and its manifestations (i.e. the neurologic form) are currently being studied at a variety of locations, most notably by Dr. Klaus Osterrieder of Cornell University. Current investigations include determining which horses might be at risk for developing the neurologic form and more importantly how to prevent this form of Equine Herpes Virus. Updates will be provided as they become available.

Groton City Animal Hospital
794 Cortland Rd. (Rte. 222)
Groton, NY 13073
(607)898-4482