Equine Herpesvirus 1: Pathophysiology and Commonly Asked Questions

The following information was posted on The Ohio State University Veterinary Teaching Hospital's web site Feb. 26. The author, Catherine W. Kohn, VMD, Dipl. ACVIM, is professor of equine medicine and surgery at OSU.

Typical Clinical Signs of EHV-1 Infection
Upper respiratory infection is the most common manifestation of EHV-1 infection. Commonly, young horses (weanlings, yearlings, and 2 year olds) have depression, poor appetite, nasal discharge and cough. If young horses are housed or pastured together, many horses in the herd may experience disease from which they recover uneventfully. Disease may be mild or unapparent in older horses.
EHV-1 infection in pregnant mares may result in abortion. Appropriate vaccination has greatly reduced the incidence of outbreaks of virus abortion on breeding farms.

Less typical Clinical Signs of EHV-1 Infection
Neurological signs occur infrequently as a result of EHV-1 infection; however, outbreaks of neurologic disease associated with fever, nasal discharge and cough have been reported both in the USA and elsewhere (see below). Neurologic signs may include incoordination that can progress to an inability to stand. Horses may be unable to urinate or may dribble small volumes of urine. Horses may have difficulty producing manure. Sometimes the neurologic signs are accompanied by cellulitis (inflammation or swelling of the limbs) and petechiae (small hemorrhages on the gums)
Pathogenesis of EHV-1 Infection
The primary reason that EHV-1 causes disease is that it causes vasculitis (inflammation of the endothelial cells that line blood vessels). Vasculitis of the brain and spinal cord are the most common abnormalities seen under the microscope in tissues from horses that die of the neurological form of this disease. Vasculitis elsewhere in the body is less commonly encountered in horses with neurologic EHV-1 infection. Signs of vasculitis are edema (swelling) of distal limbs, sheath, or ventral midline.
In studies of experimental intranasal EHV-1 infections, viremia (virus in the blood) develops in 6 to 8 days. We presume that a similar incubation period occurs in field cases of EHV-1 infection. Infected horses shed the virus in respiratory secretions (snotty nose, sneezing) during the viremia. This is termed the infectious period. The infectious period for EHV-1is usually about 7 days (range 6-10 days). Horses displaying neurological signs usually are no longer viremic (do not have the virus in their bloodstream), and they usually do not shed the virus while they are showing signs of nervous system disease.

The infection is likely life-long with the dormant virus residing in the trigeminal ganglion of the brain. Reoccurrence of viremia and shedding of the virus in nasal secretions of horses with dormant infections has been produced in a research setting (Slater 1994) by administering corticosteroids.(Edington 1985). The cycle of persistent latent (dormant) infection and intermittent shedding is thought to keep herpes viruses circulating in the equine population. Intermittent shedding by carrier horses is thought to induce sporadic outbreaks of respiratory and/or neurological disease in susceptible populations. When carrier horses shed the virus, reoccurrence of neurological or respiratory signs in the carrier horse is not reported.
Transmission and Spread of EHV-1 Infection
Transmission from horse to horse is usually by close contact aerosol. The virus is short-lived in the environment in the absence of organic material (dirt, bedding, manure, etc.). The virus is very susceptible to disinfectants in clean environments (in the absence of organic material).
Diagnosis of EHV-1 Infection
There are four tests that can be used to help diagnose EHV-1 infection. One or all of these tests may be employed during an outbreak of suspected EHV-1-induced disease.

  1. The virus can be isolated from pharyngeal or nasal secretions. Isolation of the virus from respiratory secretions strongly suggests that the virus is the cause of the disease.
  2. PCR can be used to look for the virus antigen in pharyngeal or nasal secretions. A positive test indicates presence of the virus.
  3. PCR can be used to look for virus antigen in the blood stream. The test looks for the virus in the buffy coat (packed white blood cells). The presence of viral antigens in the blood indicates that the horse is currently or has recently been viremic. Negative PCR tests do not rule out EHV-1 as the cause of the disease.
  4. Serology is used to look for antibody to EHV-1. The horse produces antibodies in response to virus infection, and this process takes time (7-14 days, depending on whether the horse has been previously exposed to the virus. Previously exposed horses produce antibodies more rapidly than unexposed horses.) Serum antibody titers are therefore often low during the acute phase of the disease, but increase dramatically as the horse responds to the infection. When assessing serum antibody concentrations, it is therefore important to have two blood samples drawn 7 to 14 days apart. A 4-fold increase in antibody titer indicates recent virus infection.

Questions Asked Commonly About EHV-1
What is the risk of infection if I take my horse to a show or if horses from my barn travel to a show?
Herpesvirus infection is likely life-long in horses. Latently infected horses may intermittently shed the virus at any time and potentially infect other horses. For this reason, there is always a risk of community-acquired disease. The aggressive EHV-1 virus that caused the outbreak of fever and neurological disease at the University of Findlay is likely circulating in the general horse population, but is probably less common than other strains of EHV-1. Outbreaks of EHV-1 neurologic disease are uncommon, but do occur regularly. Risk of exposure at a show is low if horses at the show are healthy.
How dangerous are horses that have been at the University of Findlay?
Horses at the University of Findlay are still isolated as of February 26, 2003 and no movement of horses out of or into the facility is allowed.
In general, we recommend isolating horses for 21 days after potential exposure to the virus. This interval represents approximately 3 times the infective period of the virus. It is unlikely that a horse will be actively shedding the virus 21 days after exposure.
If my horse has been exposed to this EHV-1 virus, when is it safe to assume that the horse will not develop signs of disease?
If your horse has not developed fever, cough, snotty nose or neurological signs within 21 days of potential exposure, it is unlikely the horse will do so. For potentially exposed horses that have not been isolated, the risk of developing disease or transmission of the virus decreases as the time after the exposure increases.
What is the incubation period for EHV-1?
After experimental intranasal infection, horses become viremic in 6– 8 days.
What is the likelihood that a recovered horse will infect other horses in its home community?
We can find no reports of outbreaks of EHV-1 neurological disease after re-introduction of a recovered horse to the herd at home. It is true that virus shed by latent carrier horses is likely the source of infection in herd outbreaks of EHV-1 disease. The factors that induce shedding of virus by carriers, and the risk factors for infection in susceptible populations of horses are poorly understood at this time.
Reoccurrence of viremia has been demonstrated experimentally after administration of corticosteroids; however, we can find no reports of reoccurrence of clinical neurological or respiratory disease in recovered horses that are shedding the virus.
What is the treatment for EHV-1 neurologic disease?

  1. The antiviral drug Acyclovir has been used empirically, but efficacy is unknown. The drug is expensive
  2. Symptomatic treatment: anti-inflammatory drugs (Butazolidin, Banamine, DMSO, Corticosteroids); nursing care, sling if needed.

How often is treatment for the neurologic disease successful?

  1. In most outbreaks, horses that remain standing have a good prognosis for life. Full recovery may take weeks, months, or years.
  2. Horses that display profound and rapidly progressive neurologic deficits have a much poorer prognosis, although some may live with intensive care. 

How is the disease transmitted?

  1. Usually by close contact aerosol. Experimentally, transmission has occurred over a distance of 35 feet. Respiratory secretions are usually infected. Common airspace enhances transmission. Fetal tissues, placenta and uterine fluids from mares that abort are infective.
  2. Fomite transmission (the virus can be transmitted via organic material on clothes, shoes, or tack for example). Contaminated water or feed may also spread the virus.
  3. The source of the virus is usually horses that are unapparent carriers. 

What is the best disinfection protocol?

  1. Clean away all organic material.
  2. Disinfect surfaces with a solution of 1 part bleach (sodium hypochlorite) and 10 parts water. Wear gloves to dispense.
  3. Wait for surfaces to dry.

Can transmission occur in a horse trailer?
Yes. Horse to horse transmission may occur because of the close proximity of passengers. Thorough cleaning and disinfection of a contaminated trailer should be effective in removing virus.
Are there different strains of the EHV-1?
Yes. (Note from TheHorse.com: to read more about equine herpesviruses, click here.)
Do we know anything specifically about the virus that is responsible for the University of Findlay outbreak?

  1. The virus has been isolated from the buffy coat or pharyngeal secretions of at least 8 horses from the University of Findlay.
  2. Currently, the virus genome (DNA) is being sequenced to see if this virus is similar to viruses isolated from horses in other outbreaks of EHV-1 neurologic disease. This information will not change the treatment of any current case of this disease or change the recommendations for reducing the risk of the spread of the virus. This information could help lead to the development of a new vaccine for this disease
  3. The EHV-1 in the University of Findlay outbreak appears to be very aggressive and virulent. 

Should I vaccinate my horse to help prevent the neurological form of EHV-1 infection?

  1. The question of appropriate vaccination strategies to reduce the likelihood of neurological disease in an EHV-1 outbreak is complex. Vaccination may reduce the severity of the viremia experienced by infected horses, and may therefore reduce the amount of virus shed by infected horses, hence reducing the risk of all manifestations of EHV-1 infection. There is no vaccine currently available that claims to protect against the neurological form EHV-1 infection.
  2. Investigators of recent outbreaks of EHV-1 disease have noted an association between frequent vaccination for EHV-1 and development of neurological signs. Although “frequent” has not been clearly defined, vaccination every 60 days is not recommended; vaccination every 4 to 6 months is suggested. The relationships among vaccination, infecting herpes virus, and other risk factors in the selection for a high percentage of horses with neurological disease in an outbreak are poorly understood.
  3. Consult your veterinarian to determine what vaccines have been given to your horse. If the horse has been vaccinated within the last 6 months, we do not recommend revaccination. If the horse has not been vaccinated for herpes in the last 6 months, we recommend using a univalent vaccine containing killed EHV-1. Do not vaccinate horses in an outbreak of EHV-1 disease.
  4. Follow your veterinarian’s recommendation for vaccination of pregnant mares to prevent virus abortion. The neurological form of EHV-1 is infrequently encountered in appropriately vaccinated brood mares. 

What precautions can I take at my barn?

  1. Isolate new arrivals, sick horses, and horses returning from a show, Isolate horses for at least 7 days if you have a high index of suspicion of exposure. If you know that horses at the show had confirmed EHV-1 infection, isolate your horses for 21 days.
  2. Disinfect all areas of the barn in which a suspect horse has been housed or worked. Use bleach (1 part bleach to 10 parts water.
  3. Encourage barn personnel and riders to wear leather or rubber boots/shoes that can be disinfected. Use a disinfectant tub at the entrance to the barn and ask all visitors to step in it before entering and leaving. This includes blacksmiths and veterinarians. Change bleach in the tub daily. Wash hands before handling horses. Do not share water buckets, feed tubs or stalls among horses.
  4. Segregate horses into the smallest possible groups. Large groups of horses sharing a common airspace may all be infected by 1 or a few horses shedding the virus.
  5. Take rectal temperatures daily. Isolate any horse with a fever (adult horses, >101F). Consult your veterinarian immediately. Samples for EHV-1 testing can be sent by your veterinarian to the Ohio Department of Agriculture Laboratory, 8995 East Main St, Reynoldsburg, OH 43068; 614-728-62220. Your veterinarian should contact the ODA laboratory directly before sending samples.
  6. Vaccinate horses every 4 to 6 months against EHV-1. Broodmares should be vaccinated according to the schedule recommended by your veterinarian to prevent virus abortion.

To read about OSU Veterinary Hospital's Management of Horses from the University of Findlay EHV-1 Outbreak, click here.

About the Author

Catherine Kohn, VMD

Catherine Kohn, VMD, currently at The Ohio State University College of Veterinary Medicine, serves as United States Equestrian Team veterinarian and was the FEI veterinary delegate at the U.S. Olympics in 1996 and at Rolex Kentucky in 1998.

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