The American Horse Council and the American Association of Equine Practitioners are working with the U.S. Department of Agriculture on the reporting of horses infected with equine herpesvirus after an outbreak that is believed to have started in late April.
As of June 2, 84 confirmed cases of equine herpesvirus (EHV-1) or its neurologic form, equine herpesvirus myeloencephalopathy (EHM), had been reported in 10 states—Arizona, California, Colorado, Idaho, New Mexico, Nevada, Oklahoma, Oregon, Utah and Washington.
The USDA reported that of the 84 cases:
• 58 involved horses that attended the National Cutting Horse Association’s Western Regional Championship, held April 29-May 8 in Ogden, Utah.
• 12 horses were dead or had been euthanized.
• Two newly identified premises had suspected or confirmed cases.
The collaboration between state and federal veterinarians and private practitioners is significant given the lack of federal regulations requiring reporting of EHM cases.
To contain the outbreak, show organizers canceled competitions, the Bureau of Land Management postponed a Mustang adoption event in Tulare County, California, and horses were not shipped to remote adoption centers.
EHM affects the brain and spinal cord, often leading to weakness, ataxia and bladder paralysis. Infected horses may shed the virus for as little as one day but more commonly for seven days and occasionally up to 14 days.
The first fever disappears by the fourth or fifth day. The second wave of fever starts around days five, six or seven and can continue until Day 10. The second wave coincides with the virus’ appearance in the blood.
“This is why we advise practitioners to take blood samples in addition to nasal swabs,” said Paul Lunn, BVSc, MS, Ph.D., head of the Department of Clinical Sciences at Colorado State University’s College of Veterinary Medicine.
“During that period of viremia, the virus can infect the cells that line the blood vessels of the central nervous system, especially the spinal cord. This damages the blood vessels, triggering clotting or hemorrhaging, and causing little ministrokes. The more severe cases have a much poorer prognosis, but it’s not impossible to treat those horses.”
Dr. Lunn said it isn’t known why the strain is producing such a high frequency of disease, adding that genetic variations within the virus make it more prone to severe neurological disease.
“We have been working on growing the virus in the lab, and we believe we have it isolated,” he said. “There will be a lot of research done on it. Hopefully we will be able to identify some characteristics that might help explain what happened.”
The quarantine of horses that may have been exposed to EHV-1/EHV is key to mitigating further outbreaks, experts said. A minimum two-week quarantine is recommended for horses returning from an event.
Owners of facilities that house an infected horse should quarantine the entire property for three to four weeks after the first appearance of a fever.
EHV-1 can be transferred through an aerosol, such as a sneeze, so separating horses at least 100 yards apart likely will prevent disease transmission, experts said. Because humans can transfer the virus, strict biosecurity guidelines should be practiced during farm visits. Clients also should be advised to clean any equipment used on an infected animal.
EHV vaccines are available, but none claim protection against the neurologic form. Dr. Lunn said some of the more potent vaccines may decrease shedding of the virus.
“However, once your horse gets infected we don’t know if any of these vaccines currently prevent the neurological disease from developing,” he says. “In a time like this I would consider using a vaccination to booster horses that are already vaccinated in the hope you could prevent the virus from getting a toehold on your facility,” he said.
Julie Wilson, DVM, Dipl. ACVIM, a former member of the AAEP’s Infections Disease Committee, said veterinarians disagree over whether there is any advantage to using a modified live vaccine.
“It has been shown in at least one study to reduce the shedding and reduce the clinical signs,” she said. “I recommend [the modified live vaccine], and I’d give it to my own horses at risk in a public facility. The downside is that veterinarians on occasion have seen some pretty significant vaccine reactions.
“The veterinarian should be aware of a higher risk of reaction with any modified live vaccine and should have that conversation with the horse owner.”
Dr. Wilson noted differences of opinion with boosters as well. Horses in areas not at an elevated risk should be boosted every six months, she said, but in areas of high exposure, veterinarians have chosen to boost every three months, which is extra-label.
HEALx Booster Concentrate Available
Harrisons’ Pet Products of Lake Worth, Fla., is offering free Booster Concentrate to veterinarians willing to try it on horses infected with equine herpesvirus (EHV-1).
No clinical studies involving the product have been done on EHV-1 horses, but the company said Booster Concentrate has been shown in laboratories to inactivate enveloped viruses and fungi.
The product is a highly purified monolaurin nutritional lipid.
Veterinarians who want to use Booster Concentrate and report the results may contact Dana O’Donoghue.
States such as Colorado and Wyoming imposed travel restrictions, so clients who want to transport their horses across state lines should first contact state health officials to ask about enhanced requirements. Washington’s state veterinarian, Leonard Eldridge, recommended June 3 that any horse that had not tested positive, exhibited symptoms or been exposed to a confirmed positive horse be cleared for travel.
“I continue to recommend that horses that are confirmed positive for [EHV-1/EHM] or were exposed to a positive case be isolated for 28 days after all symptoms have cleared up,” Dr. Eldridge said. “While the animal may appear to have recovered, it can still transmit this highly contagious disease to another horse.”
Practitioners who suspect or confirm a case of EHV-1/EHM should notify state animal health officials.
For more information on the outbreak, visit AAEP.org.