Equine Herpesvirus

Definition: Equine herpesvirus is a common DNA virus that occurs in horse populations worldwide. The two most common strains are EHV-1, which causes abortion, respiratory disease and neurologic disease; and EHV-4, which usually causes respiratory disease only but can occasionally cause abortion.

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

EHV-1 myeloencephalitis results from widespread vascular injury after damage to the vascular endothelium of the blood-brain barrier. Neurologic signs result from vasculitis, thrombosis, and necrosis of neurologic tissue. Herpesvirus myeloencephalitis cases occur singly or as outbreaks affecting 20-50% of the population. They may or may not be associated with a previous ongoing EHV-1 respiratory disease outbreak.

Clinical signs
Fever: commonly precedes other clinical signs, but may be the only clinical sign and goes undetected.

Respiratory disease:
Nasal discharge

Neonatal foals infected in utero are usually abnormal from birth:
Respiratory distress
CNS signs
Death commonly occurs within 3 days.
Older foals: nasal discharge is most common sign of illness.

Usually no warning signs of impending abortion
Typically occurs late in pregnancy (8+months); occasionally as early as 4 month

Neurologic disease:
Incoordination of the hind (and occasionally fore) limbs
Urine retention/dribbling
Recumbency/ inability to rise
Neurologic signs may be preceded by fever and respiratory signs

After infection by any route, incubation period may be as short as 24 hours, is typically 4-6 days, but can be longer

EHV abortion can occur from two weeks to several months following infection with the virus

Aerosol transmission (most common route)
Inhalation of droplets from coughing and snorting
Mares who have aborted, or whose foals have died, transmit infection via the respiratory route

Shedding by the respiratory route typically lasts for 7-10 days, but can persist much longer.
Therefore a 28-day isolation period is recommended after diagnosis

Indirect transmission
Virus can be viable for several weeks in the environment once it has been shed by the horse.

Inanimate objects (grooming tools, water buckets, etc.) are a significant factor in transmission

Aborted fetuses, fetal membranes and/or fluids are significant sources of infection

Infected foals are highly contagious and can transmit infection to other horses via the respiratory route

Shedding time of organism past resolution of clinical signs
Likely for up to a week, possibly for 21 days or more

Recovered horses typically develop latent infections and are capable of shedding virus (with or without clinical disease) particularly at times of stress for the remainder of their lives.

Booster vaccination of healthy animals may have some value, and is not known to lead to complications.

Modified-live vaccines generate two-types of immunity – humoral (just antibody) and cell-mediated (lymphocytes). Killed vaccines generate antibody only, which might not offer enough protection when the horse is faced with a strong challenge.

Modified-live vaccines tend to create a stronger reaction in the vaccinated animal, so some individuals might have a greater risk of vaccine reactions (not disease) when modified-live vaccines are used

Duration of measurable immunity in challenge studies tends to be longer with vaccines that create cell-mediated immunity (modified-live and vectored)

Source: “The Horse”, February 2007, pg. 18.


New animals entering a facility:
Isolate from existing population for 28 days
Consider having vaccination requirements before entering your facility
Update vaccinations at the recipient facility if a new/suspect horse is due to arrive

Animals exposed within a facility:
Separate from non-exposed animals
Isolate from non-exposed animals for 28 days

Source: www.aaep.org; guidelines for infectious diseases