EHV-1 discovered in Montana |

EHV-1 discovered in Montana

Heather Smith Thomas
for Tri-State Livestock News
An affected horse may need to be put in a sling and held in an upright position during recovery until it is once again able to support its own weight. Courtesy photo Dr. Steve Reed

There are several types of equine herpes virus. Some are more damaging than others. EHV-1 can cause respiratory illness, abortion in pregnant mares, and on occasion a more severe (and sometimes fatal) illness involving the nervous system.

Historically the neurologic form of this disease has been rare, but in recent years there has the number of outbreaks has been on the rise, both in North America and other parts of the world. During the past decade there have been outbreaks in the U.S. at various race tracks, and in horses attending other events or shows where horses congregate from various areas.

Earlier this year a few cases cropped up in various states. During the past month two cases were identified in Montana. Dr. Tahnee Szymanski, Assistant State Veterinarian, Montana Department of Livestock, says the horse that tested positive in Flathead County demonstrated neurological signs.

“This horse had attended an event in Ravalli County (south of Missoula) in early March. The veterinarian submitted samples on that animal that came back positive for the non-neuropathogenic strain of EHV-1. Another term for that non-neuropathogenic strain is the ‘wild’ strain. This strain is capable of causing neurologic disease but doesn’t seem to produce neurologic signs as often as the neuropathogenic strain,” Szymanski explained.

“Part of the increase in cases may be the fact that there is increased awareness among horse owners and veterinarians, as well as increased incidence, and horses are being tested – if EHV-1 is suspected as a possibility.”
Dr. Tahnee Szymanski, Assistant State Veterinarian, Montana Department of Livestock,

“That animal in Flathead County is still under quarantine (as of the first of April) but appears to be making a full recovery. We are now out of the risk period for other horses who were also at that show. It has been more than three weeks since that animal was exposed to any other horses,” she said.

It’s hard to know exactly how this horse was exposed, but based upon the amount of time elapsed since it attended the event in Ravalli County (put on by a local horse club), and onset of clinical signs, this would have been within the optimum incubation period. “We think the horse was likely exposed at that event,” Szymanski said.

Even though no other animals that attended that event appear to have become ill, there might have been a horse or horses there that were shedding the virus. “This virus is always present in equine populations. Some horses can be carriers without signs of disease. Likely an animal that attended that event was shedding and this is where the horse picked up the disease,” she explained.

Fortunately this case was an isolated incident and the risk period is now past for this particular horse to transmit it to others. Thanks to the quarantine there are probably no other horses exposed from this one.

“The second case was a horse that arrived in Montana from a show in California, coming to a facility in Gallatin County. This horse developed respiratory illness and had no neurologic signs. The owner and veterinarian didn’t do any testing until a second group of animals was returning from a California show and they got a report that these horses had been at the same show in southern California where there had been an EHV-1 positive horse,” Szymanski said.

There was a chance for exposure, so the owners of the mare that was sick decided to test her. She had been at that same show and returned early. “That mare tested positive so she is still under quarantine. A second trailer load of horses returning from California is also under quarantine, as is the facility where the mare came home to. She was in a general population of horses at home, where it’s hard to say which animals were exposed to her and which ones were not. These horses are all quarantined and still being monitored,” explained Szymanski.

“We started our countdown for the quarantine from the time we got the positive test on the mare. The reality is that she had been home, at this facility, for about 15 days by the time the owners and veterinarian got the diagnosis. From the first time that those other animals might have been exposed to her until now, it’s been more than 20 days, and we’ve seen no other illnesses. So I think we are probably out of the risk period, but that facility is still under quarantine until we are sure the risk has passed.”

These were two unrelated cases. There have been cases and small outbreaks already in other parts of the country, including Illinois, Florida, Tennessee, California, Washington and Utah. A gelding in Santa Barbara County in California showed neurologic signs on March 20, and tested positive on March 25. A horse in San Diego County also tested positive after attending the same event – in Thermal, CA – and both have been quarantined. A three-year-old Thoroughbred filly at Santa Anita Park in California was euthanized March 5, after developing neurologic signs and testing positive to EHV-1.

Four positive cases, from two separate facilities, were found in Washington. Utah has been home to nine cases since late February, four of which were euthanized because of their deteriorating condition. The Utah State Veterinarian confirmed seven cases (in seven different locations) in early March in Cache County. Two were humanely disposed of and the rest are being treated. The quarantine of those seven facilities was lifted in late March after the other horses at those locations passed 28 days without becoming sick.

“Part of the increase in cases may be the fact that there is increased awareness among horse owners and veterinarians, as well as increased incidence, and horses are being tested – if EHV-1 is suspected as a possibility,” Szymanski said.

“At this point in time, we have two locations in Montana under quarantine, but we feel the risk to Montana horses from these two incidents is very low. People are taking the necessary precautions. If horsemen continue paying attention to simple things they can do for biosecurity, then it is fairly safe for to travel with their animals,” she said.

Watch your horse for signs

Average incubation period is four to seven days after exposure, but some horses don’t show signs of illness for 14 days. The California Department of Food and Agriculture recommends that any horses possibly exposed to EHV-1 should be isolated and have their temperatures checked twice daily. Horses with temperatures over 103 degrees should not travel or mingle with other horses.

“EHV-1 can cause respiratory or neurologic disease. The respiratory illness will be similar to other diseases that cause upper respiratory signs. You might see heavy nasal discharge and perhaps a cough associated with it. There is often a fever, early in the disease,” Szymanski said.

“Regarding neurologic signs, these tend to start in the hind end. Sometimes the horse may just seem a little lame or ‘off’ in the hind legs, but you can generally see hind end instability and incoordination. In mares we sometimes see urine dribbling because they lose control of the sphincter in the bladder,” she explained.

Then signs may progress to more obvious problems and inability to stand. The horse may try to get up, but the hind limbs don’t work, and the horse is sitting on the hindquarters. “We call this a dog-sitting position,” she said. The horse may eventually be unable to rise at all, and may die or have to be euthanized.


“If you are traveling with your horse, there are some risks you can’t avoid. But some of the things you can do include limiting close exposure to other horses. Try not to share water buckets, tack, grooming tools, etc. Wash your trailer thoroughly between trips. Avoid common tying areas,” said Szymanski.

“When returning home with your horses, if possible leave them separate from your other horses in a different facility or a separate pen on your property so they are not immediately commingling with the animals at home,” she suggested.

To make sure the returning horse(s) were not exposed and coming down with a disease, they should be kept separate for 14 to 21 days before you put them back with the other horses. Then if the horse develops clinical signs, consult with your veterinarian.

Szymanski does not recommend testing animals unless there is reason to suspect EHV-1, because this virus is present in the population. “Healthy animals may test positive, and this is difficult to interpret. So you definitely want to consult with your veterinarian and have some basis for doing that test,” she said.

“There are several reasons this disease is more concerning than many of the other equine diseases. One is that we don’t know why some horses develop the neurological form and others only have respiratory signs – and still others that are exposed develop no clinical signs at all. Also, there is no preventative vaccine; the vaccines currently only protect against the respiratory form. They don’t give any real protection against the neurologic form. Finally, there is no treatment – only supportive therapy,” she explained.

The vaccine given to mares to prevent abortion only protects against EHV-4, which is a different strain of the virus.


Being a virus, this pathogen does not respond to antibiotics. “Sometimes, if a horse has the respiratory form and gets a secondary bacterial infection, you may need to use antibiotics to treat the secondary infection,” Szymanski said.

“With the neurologic form, the most important thing is good supportive care, with IV fluids, etc. while the horse is unable to eat and drink. You also want to keep the horse on its feet.” If the horse goes down, prognosis for recovery is poor. You need your veterinarian to guide you through this situation and try to keep the horse’s body functioning. It is often beneficial to have the horse at a veterinary facility.

“When horses get to the point they are unable to stand, some are euthanized. Others might be taken to veterinary facilities where they can be put in a sling and held in an upright position during the course of their recovery – until they are able to support their weight on their own,” she said.