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Equine Herpes Virus: Types, Treatment and Prevention PDF Print E-mail
Written by Tonya Randall   

Most horse owners are familiar with the common respiratory disease known as Rhinopneumonitis, or simply “Rhino,” and vaccinate for it along with influenza. However, few people realize that by vaccinating for Rhino, they’re actually vaccinating against the respiratory form of the Equine Herpes Virus (EHV).


Running HorsesIn recent days, two cases of Equine Herpesvirus (Rhinopneumonitis) were confirmed in horses that competed at The Four Sixes/National Cutting Horse Association Western Nationals, a show held in late-April and early-May at Ogden, Utah. One Colorado cutting horse was euthanized because of complications of Equine Herpesvirus and another one remains in quarantine, according to the Colorado Department of Agriculture. Ogden is being investigated as a “point of interest for the infection.’ The disease is highly contagious.

At the Kern County cutting in California on May 13, a horse died of the disease and another was rushed to University of California, Davis, according to one cutting horse person present at the event in a report done by Quarter Horse News. The show was immediately cancelled. Also, the Pacific Coast Cutting Horse Association announced the cancellation of its Tejon Ranch Cutting, which was scheduled for May 19-22. On Saturday, May 14, a horse died in Idaho. Also, the Breeders Invitational Derby and Classic/Challenge was postponed, though it was reported that no horses at that event exhibited symptoms related to the disease.

Back in 2006, (EHV) outbreaks forced the quarantines of racetracks, training facilities and private farms in Kentucky, Maryland and Wisconsin. Tragically, some horses with the respiratory form of EHV developed the deadly neurological form. Several horses were euthanized.

Benjamin J. Darien, DVM, MS, experienced the outbreak firsthand when a horse admitted to the University of Wisconsin-Madison (UWM) was diagnosed with the neurological form of EHV-1, one of five types of EHV, and subsequently euthanized. Darien, a diplomat of the American College of Veterinary Internal Medicine and an associate professor of internal medicine UWM, oversaw the treatment of the remaining horses at the private farm that was the site of the outbreak.

“EHV is mostly commonly associated with respiratory disease and abortion in mares,” Darien said. “All horses carry some form of EHV, and all it takes for it to reemerge is stress. There is a latent infection located in a nerve, just like herpes simplex in people. It stays latent until periods of stress, such as under performance conditions for horses, then the horse has a recrudescence, a recurrence, of the disease, like a human getting a cold sore.”

Darien along with other veterinarians and virologists are continually studying the dynamics of the herpes simplex in horses, especially EHV-1, the type responsible for producing the neurological form.

 

Types of EHV

EHV has five forms. They’re distinguished by their genetic make-up, or DNA. EHV-2 and EHV-5 are present in the body, but haven’t been linked to any disease. EHV-3 is a sexually transmitted form of the virus. EHV-1 and EHV-4 are the most common forms of the virus, and pose most of the problems for competitive barrel racers.

Both EHV-1 and -4 can cause respiratory disease, commonly referred to as Rhinopneumonitis, Darien explains. “Rhino” means nose, “pneum” means lungs, and “itis” means inflammation. So they can affect the respiratory system anywhere from the nose to the lungs.

EHV-4 is most commonly associated with respiratory disease, especially in younger horses. But, Darien notes, you can see the respiratory form in foals, weanlings and adult horses.

The respiratory forms of EHV-1 and -4 have the same clinical signs, or symptoms. The most common clinical sign, says Darien, is an acute febrile episode, so 104 to 105 degree temperature. The normal range for an adult horse is 98.5 to 101 degrees and slightly higher in younger horses.

The fever is generally accompanied by a snotty nose, coughing, lack of appetite and general depression.

EHV-1 is also responsible for reproductive problems such as abortion, still birth and weak foals. Abortions generally occur after seven months gestation and usually the mare shows no signs of illness at the time.

In fact, EHV-1 accounts for 10 to 15 percent of all diagnosed abortions. The number may be much higher since few mare owners have post-mortems on their lost foals.

Although not as common, EHV-4 can also cause abortion, Darien says.

EHV-1 also has a neurological form, a myeloencephalitis, or inflammation of the brain and spinal cord. It’s this neurological form that was responsible for deaths of horses in Maryland, Kentucky and Wisconsin.

The neurological form produces symptoms very similar to that of other neurological diseases such as West Nile Virus (WNV) and Equine Protozoal Myeloencephalitis (EPM). These symptoms include lack of coordination and the inability to perform normal bodily functions.

“At first the horse is weak in the backend, but is bright and alert. It can eat and drink, but it can’t urinate and can’t pass manure. It’s often dog sitting,” Darien describes. As the disease progress, the horse may lose the ability to stand completely.

 

Treatment

Rhinopneumonitis doesn’t respond to antibiotics because it’s caused by a virus, not bacteria. Generally, the symptoms are treated with anti-inflammatory drugs such as Banamine®, DMSO® and some corticosteroids, as well as intravenous fluids to maintain hydration.

With the neurological form, measures such as bladder catheterization and manual manure removal from the rectum may be required. Body slings may be used if the horse is uncoordinated or unable to stand.

In some instances of both the respiratory and neurological form, the anti-viral drug, Acyclovir® may be administered.

With his Wisconsin cases, Darien used a combination of Acyclovir®, Tahitian Noni International’s Equine Essentials® and the occasional use of non-steroidals, namely Banamine®, to help maintain the health of the remaining diseased and exposed horses at the infected farm.

 

Vaccination controversy

Vaccines are available to protect against the respiratory forms of EHV-1 and -4. None exists to protect against the neurological form.

It’s believed that vaccination may reduce the severity of symptoms of the respiratory disease. By lessening those symptoms, vaccination may reduce the risk of neurological form development.

However, a controversy does exist in regards to the frequency and timing of vaccination. Retrospective studies following previous outbreaks have linked frequent vaccination to an increased risk for the neurological form of EHV-1, Darien says.

The problem lies in the fact the neurological disease also spreads to the immune system.

“If you vaccinate in the face of an outbreak (when it’s known to be in your area), you are increasing the antibodies in the blood. In essence you can actually make the horse more susceptible to the neurological form because the antibodies are there and can now develop a bigger, more robust reaction to the virus (due to the horse’s effected immune response),” he said.

His own experience concurs. Immediately following the initial outbreak of EHV-1 in six horses and the death of the two neurological case, the remaining horses in the barn were vaccinated by the farm’s veterinarian. The four remaining horses with the disease were not vaccinated. Darien experienced more problems in the vaccinated group as opposed to the non-vaccinated group. Luckily all were maintainable with the Acyclovir®, Equine Essentials® and the occasional use of Banamine®, and none of the horses developed the neurological form.

Darien advises a common sense approach to vaccinating for EHV.

“My recommendation is that you would vaccinate for equine herpes no more frequently than is indicated for your performance and it’s exposure during the performance season. If your horse is exposed to a large number of susceptible horses and you have a good vaccination level you will have less clinical signs and less clinical disease of the respiratory form.”

For example, you don’t need to vaccinate when you horse isn’t exposed to outside horses such as during winter turnout. However, if you plan on hauling to bigger events starting in May, you would want to start vaccinating for EHV with your spring shots in April, says Darien.

Protection from the vaccine lasts two to three months. The vaccine is effective seven to 10 days after the horse receives the initial injection and first booster (two shots). So, vaccination should be repeated as necessary during the competition season.

For broodmares, vaccination should occur during gestation to prevent abortion.

“The recommended schedule against the reproductive form was traditionally five, seven and nine months,” Darien said. “What most people actually do is three, five, seven and nine.”

 

Boosting the immune system?

“There is nothing preventing a horse from getting EHV,” Darien said. “What you are trying to do is lessen the severity of the disease.”

Vaccination is the most effective method, but good general nutrition can also help a horse fight off infection.

Boosting the immune system naturally is why Darien put all the horses at the quarantined farm on Equine Essentials®. “I put every horse in the barn on Equine Essentials, because it’s an immune-enhancing and stress-reliving-type supplement,” he said.

The supplementation of the amino acid lysine has as been recommended by some practitioners.

“We put our last horse on lysine when it was shedding virus,” Darien said. “There is nothing in the equine literature to show that it is at all helpful. I doubt that it does any harm. I don’t think there is any prophylactic benefit of it.”

 

General prevention tips

Infection by EHV is airborne. Particle of the virus can spread as far as 35 feet. It’s also possible to spread by contact with contaminated objects, such as stalls, buckets, shared bits, and even the rider’s hands.

Along with a judicious vaccination schedule, other preventive measures include isolation and disinfection.

Isolating new horses, sick horses or those just returning from an event helps prevent the spread of disease. New horses or those returning from competition should be isolated for at least seven days. Horses returning from shows where illness was present should be isolated for 21 days. Sick horses are quarantined for 30 days or longer.

“It’s just like your kids. You don’t want them sharing the same glass of juice when one of them is sick,” quipped Darien.

Luckily the virus is effectively killed by disinfection of all contaminated areas, stalls and equipment, including boots and shoes. One part bleach to 10 parts water generally does the trick.

 

Points to remember

While no true preventative to EHV exists, steps can be taken to lessen the symptoms of outbreaks and reduce the spread of the disease.

Darien reminds, “Good nutrition to enhance strong immunity, vaccinate only at the times when your horses are going to be exposed to a lot of at risk horses – those under stress and possibly shedding the virus, and know that vaccination in the face of an outbreak is very controversial.”

 

According to w May 13 press release issued by the Colorado Department of Agriculture, two cases of Equine Herpesvirus (Rhinopneumonitis) were confirmed in horses that competed at The Four Sixes/National Cutting Horse Association Western Nationals, a show held in late-April and early-May at Ogden, Utah. One Colorado cutting horse was euthanized because of complications of Equine Herpesvirus and another one remains in quarantine, according to the Colorado Department of Agriculture. Ogden is being investigated as a point of interest for the infection.  The disease is highly contagious.

According to a report released by Quarter Horse News, at the Kern County cutting in California on May 13, a horse died of the disease and another was rushed to University of California, Davis. The show was immediately cancelled. Also, the Pacific Coast Cutting Horse Association announced the cancellation of its Tejon Ranch Cutting, which was scheduled for May 19-22. On Saturday, May 14, a horse died in Idaho. Also, the Breeders Invitational Derby and Classic/Challenge was postponed, though it was reported that no horses at that event exhibited symptoms related to the disease.

To read CSU's press release, click here and for AAEP's fact sheet on the disease, click here.

For more on EHV-1, see: http://www.aaep.org/ehv.htm