Equine Herpesvirus-1 Outbreak in Oregon

In the last month, 16 of the 19 horses at Brookhill Stables in Goble, Ore., and two horses from a nearby private farm, have showed respiratory and/or neurologic signs consistent with equine herpesvirus type 1 (EHV-1), although not all have been tested for the disease. To date, three older victims have been euthanized--one from Brookhill Stables and the two horses from the private farm, which visited Brookhill for 4-H lessons in early July.

The Oregon State University's Veterinary Diagnostic Laboratory has confirmed the presence of EHV-1 in two horses from which samples were taken at the boarding facility, and more samples from those horses and "suspect" horses at nearby operations are pending at the Gluck Equine Research Center at the University of Kentucky (the Gluck Center houses the reference laboratory for herpesvirus in the United States). All horses at Brookhill have shown EHV-1 antibody levels consistent with exposure or vaccination at least (less than 1:256), and active infection at worst (1:256 and higher).

In a press release Don Mattson, DVM, PhD, a veterinary virologist at the OSU laboratory, said, "These samples showed extremely high antibody titers to EHV-1 (much higher than we have ever seen). The fact that the antibody titer was very high (suggesting generalized infection with much virus present), older horses were affected, and horses at the stable were alleged to be vaccinated against EHV-1 every three months, suggests that we may be dealing with a similar form of the virus (to the Ohio State University outbreaks early this year; see article #4214). This virus is very infectious and is spread by droplet aerosol and mechanically by handling exudates from animals that are just coming down with the disease."

Disease Progression
The first horse in the area to show neurologic symptoms was a 24-year-old mare from a nearby private barn who visited Brookhill Stables for 4-H lessons in early July, said treating veterinarian Katy Ostervold, DVM, of Valley Vet Clinic in Rainier, Ore. "On Aug. 10, the mare began showing signs that we initially attributed to old age: Progressive ataxia over a 24-hour period to recumbency, no tail tone, and an inability to defecate or urinate, although she was eating and drinking normally with a normal heart rate and an elevated respiratory rate. We put her down later that night. This case was not laboratory confirmed as EHV-1, but looking back I'd bet money that's what it was."

About a week later (Aug. 17), the family's other horse, another elderly mare, had much the same progression of signs and was also put down quickly (the next day) by another veterinarian, she said. The mare was reportedly recumbent with no hind end movement by the time she was euthanized, and necropsy examination showed lesions considered to be coincidental and an EHV-1 antibody titer of 1:32, indicative of exposure but not necessarily active infection.

Approximately a week after that (about Aug. 25), an older horse with prior chronic neurologic signs was found recumbent in a paddock at Brookhill, and had clearly been down for several hours. The horse was very ataxic (incoordinated) behind and squirting urine, and was put down.

Ostervold, who owns a horse boarded at Brookhill Stables, treated several of the cases that cropped up there during the next few weeks. The next one was a yearling filly a few days later which spiked a fever between 103º and 107°F for the next six days along with respiratory instead of neurologic symptoms.

"I took samples from her in the acute and convalescent stages, and her EHV-1 antibody titer went from a negative value to 1:256+ in seven days," Ostervold stated. "That's a definitive diagnosis."

Another horse spiked a fever about Sept. 5, she said, then two or three new horses every day showed temperature elevation (by this time, each horse was having his temperature recorded every day). By Sept. 9, all but two horses had shown respiratory or neurologic signs, fever, and a rash, at some point (only the yearling filly showed respiratory signs). These included ataxia and paresis (slight or incomplete paralysis), usually of the hindquarters; walking in tight circles; poor tail tone; and dribbling urine.

One horse moved from the facility on August 24 before any signs of the outbreak began, Kirby said, and was reportedly showing similar symptoms at her new home near Scappoose, Ore. However, she said the infection was reported to be contained there.

Kit Kirby, owner of Brookhill Stables, said, "Most of them are almost healed. Maybe three or four are still symptomatic, six if you looked hard, and two still might not come out of it totally. One is in a sling but improving slightly."

The Rumor Mill
Rumors have been running rampant regarding this outbreak, ranging from the Oregon governor banning horse transport in Oregon to further problems in Lake Oswego area horse facilities. These are untrue. The Oregon Department of Agriculture is suggesting that owners avoid travel until the outbreak is resolved, but the only travel restriction is the self-imposed quarantine at Brookhill Stables.

Control and Prevention

The equine herpesviruses (there are eight types and five affect horses) are known for their agility in evading a host's immune system. The EHV-1 organism can cause three different forms of disease, including rhinopneumonitis (a respiratory disease often affecting young horses), abortions in pregnant mares, and neurologic disease. Horses can survive the neurologic form of EHV-1 with supportive care, but if a horse becomes recumbent (stays down), it is difficult to nurse the horse back to health. The virus doesn't live long in the environment, and it can be killed with bleach.

"We'll pressure wash everything soon," said Kirby. "We've also been using bleach pans for everyone to walk through when they go into and out of barn. No visitors, no visiting horses, and those with horses at home are recommended to shower and change clothes after leaving here and before visiting their own."

Aside from disinfection and isolation, control of the respiratory form of EHV-1 is certainly aided by vaccination. "Vaccination will generally reduce the severity of the respiratory form of this infection," stated a release from the Oregon Department of Agriculture. "Vaccination has not been shown to offer protection against the neurologic form of the disease. There is also a vaccine available which helps in prevention of abortion, which is another possible consequence of infection with this organism."

The Brookhill horses were vaccinated with the flu-rhino combination every three months, with the latest given in late July or early August, said Kirby.