Various types of herpes viruses infect humans and animals, causing a wide variety of diseases.  The name is derived from the Greek word herpein, which means “to creep,” related to the fact that once herpes infects a host, the virus can remain dormant indefinitely until something triggers reactivation.  There are several strains of equine herpes; the most commonly recognized is the strain that causes abortion.  It is this strain that we try to protect broodmares from when we vaccinate for Rhino during pregnancy. 

The other strains are also important.  One strain in particular, a highly contagious and lethal variety that causes neurological disease, has become more prevalent and gained more attention in recent years.  The focus of this article is a review of various strains of equine herpes virus (EHV) and the diseases they cause.

Strains of EHV

There are 9 strains of equine herpes virus.  The strains that affect horses are EHV 1-5 (EHV 6-8 affect donkeys and EHV-9 affects zebras).  Of these five strains, EHV-5 is poorly understood, so we’ll concentrate on the other four:

EHV-4

This strain infects the upper respiratory tract of horses, where it causes Rhinopneumonitis.  It more commonly results in disease in younger horses (especially from weaning to 3 years of age), but can be seen in horses of all ages.  Generally, the infection is limited to the upper respiratory tract.  After an incubation period of 2 to 8 days, infection results in fever, depression and nasal discharge.  The nasal discharge is initially watery and later can become thicker and cloudy.  Infection will weaken the horse’s normal defenses, opening the door to bacterial infections that can cause more serious disease, including pneumonia.  Infected horses shed the virus in nasal secretions.  These secretions are spread through direct nose to nose contact, aerosol droplets in the air, on clothing or tack carried by people, and discharges carried on the bodies of other animals (e.g. dogs, cats, rats, mice).  People and other animals are not infected with equine herpes virus but simply inadvertently carry infectious material on their bodies.  The virus can be shed weeks after infection.  Antibiotics do not affect viruses.  Treatment is limited to supportive measures such as providing a clean well-ventilated stall, rest, medication for fever, and treatment of secondary infections.

EHV-3

This strain causes a sexually transmitted skin disease called coital exanthema.  Although most commonly spread by coitus, insects and contaminated grooming supplies can potentially spread the virus.  After an incubation period of 4 to 7 days, the skin breaks out in painful, blister-like lesions or vesicles.  Numerous such lesions appear on the penis and prepuce of the stallion and the mucosa of the vulva and perineal skin of the mare.  Eventually the lesions break open and heal over a period of 10 days or so.  The healed blisters will have permanent loss of pigment, leaving a white spot for each lesion.  Affected individuals should be rested from any sexual activity for a minimum of three weeks and the lesions treated with antiseptics to prevent secondary bacterial infections.

EHV-2

Also called Equine cytomegalovirus, this strain can infect the respiratory tract of horses.  It also can infect tissues of the eye such as the conjunctiva, causing conjunctivitis, and the cornea, causing inflammation of the cornea (the medical term is keratitis). 

EHV-1

This complicated, devious strain of equine herpes virus is capable of causing three separate disease syndromes in horses: respiratory disease, abortion and neurological disease.

Respiratory disease: EHV-1 causes respiratory disease similar to that caused by EVH-4.  Horses are infected by inhaling the virus.  It more commonly causes respiratory disease in younger horses, but can affect horses of any age.  After an incubation period of 2 to 8 days, susceptible horses develop symptoms of Rhinopneumonitis, including fever, depression and a watery nasal discharge.  The fever can be high and is generally biphasic, meaning there is an initial fever that returns to normal only to return again in a few days.  While the sick horse’s defenses are compromised, secondary bacterial infections can occur.  The virus can spread by the same routes described above for EHV-4.  Most horses are infected with the virus by the time they are yearlings.  The virus survives in the individual in a dormant or latent state.  Stress such as hauling, other illnesses, pregnancy, change of location, and change of stable mates can trigger the virus to replicate.  These individuals may show no clinical signs of disease but they will shed the virus and are therefore called silent shedders, infecting susceptible horses or possibly triggering other horses with latent infections to reactivate.

Abortion: this is the problem that the public seems most aware of.  Mares can harbor latent infections of EHV-1 that are reactivated, or can develop respiratory disease from a new infection.  Abortion can occur within a couple of weeks or can occur months later.  Typically, mares abort without any indication of a problem.  The virus actually infects the placenta and invades the fetus, causing damage to the blood vessels.  The aborted fetus and placenta contain high levels of virus and are extremely contagious.  Infected foals that survive gestation typically die soon after delivery.  Abortions generally occur between the 7^th and the 11^th months of pregnancy.  Usually the abortions are limited to isolated cases but occasionally large percentages of broodmares will be involved in abortion storms.  Intensive vaccination against EHV-1 using one of the only two approved products (Pneumobort-K and Prodigy) at the beginning of the 5^th, 7^th, and 9^th months of pregnancy will help prevent abortion. The immunity generated from the vaccination is short-lived, hence the need to vaccinate every 2 months during this period of greatest vulnerability.

Neurological form:  This is an extremely serious disease.  Although this disease syndrome is much less common it is highly contagious and potentially lethal, and the number of outbreaks has increased in recent years.  In the five-year period between 2001 and 2005 there were 32 outbreaks in the United States and the UK.  In the previous six five-year periods, there were between 1 and 6 outbreaks.  Recent research has demonstrated that 30 of the 32 outbreaks between 2001 and 2005 were caused by a mutant variant of EHV-1.  This mutant variant has tremendous replicative capacity, producing five times the number of virus particles in the bloodstream as does typical EHV-1.  Retrospective studies of outbreaks in Wyoming, Ohio, Oregon, New Hampshire and Maryland involved 403 at-risk horses, 133 cases with neurological disease (between 22% and 50% at the various outbreaks) and 53 that had to be euthanized or died (between 20% and 50%). 

The virus damages the lining of blood vessels and has a special affinity for the central nervous system.  This leads to impaired blood flow and subsequent damage to the central nervous system.  The result is the sudden appearance of weakness and incoordination, particularly in the hindquarters.  Some horses will loose the ability to stand from a sitting position.  Paralysis of the bladder can lead to dribbling of urine.  Generally, the symptoms get as bad as they are going to get within 72 hours.  Horses that are down for more than 24 hours have a poor prognosis for survival.  Recovery time in some cases takes only a week or so.  Others may take over a year and still others have permanent neurologic disability.

Currently, there is no vaccine available that has a label claim against the neurological form of EHV-1.  Prevention centers on good, solid management practices that reduce stress: reduced crowding, good nutrition, effective parasite control, gradual weaning, protection from harsh weather, etc.  Isolation of new arrivals is recommenced for at least three weeks with daily monitoring and recording of temperatures.

Summary

There are many strains of equine herpes virus with various levels of virulence that infect horses.  Equine herpes viruses are ubiquitous and found worldwide.  Although uncommon, the neurological form of disease caused by EHV-1 is extremely contagious, deadly, and outbreaks have been disturbingly more common in recent years.  Recognizing these threats, management practices can be implemented to reduce the spread of the health risks posed by these and other contagious diseases.