Experimental EHV-5 infection-histopathology-horse-E4-C1 Nodules of fibrosis horse

In the summer of 2021, Ohio horse owner Ginny Telego noticed her Miniature Horse mare named Dolly was struggling with respiratory issues and ongoing fevers. Asthma was the initial diagnosis, but a respiratory infection and pneumonia compounded the situation.

After working with her local veterinarian, Telego took Dolly to her first of what would be four trips to the Galbreath Equine Center at the Ohio State University (OSU) Veterinary Medical Center, in Columbus.

“Dolly was treated with antibiotics and anti-inflammatories for suspected equine asthma and bacterial pneumonia, but she continued to have recurrent bouts of fever and difficulty breathing,” said attending veterinarian Laura Dunbar Hostnik, DVM, MS, Dipl. ACVIM, an assistant professor at the Galbreath Center.

Dolly did not improve despite treatments and diligent care both at home and at OSU, so Hostnik ordered a radiograph of the mare’s caudoventral (underside and toward the rear of the horse) lung. The results did not look promising—the interstitial tissue pattern had remained unchanged by treatment for several months.

Dolly continued to experience respiratory distress and intermittent fevers, along with weight loss. In November 2021 she made her fourth trip to OSU, where Telego asked the vets to think outside the box to find a cause for Dolly’s worsening condition.

The next step was to draw and test fluid from a lung for equine herpesvirus-5 (EHV-5). This gamma-herpesvirus can be found within the lung nodules in horses with equine multinodular pulmonary fibrosis (EMPF).

Dolly’s polymerase chain reaction (PCR) test result came back positive.

At this point, identification of EHV-5 within lower airway samples or lung tissue in horses with signs and imaging abnormalities consistent with EMPF can aid in diagnosis. In addition to severe respiratory distress, horses might also exhibit a cough, nasal discharge, weight loss, and poor body condition.

It is essential for owners to understand that a positive nasal swab or blood sample in an otherwise normal horse or a horse without evidence of pulmonary nodules/fibrosis does not indicate the disease and is likely not a cause for alarm, Hostnik said.

Indeed, EMPF is a rare illness. In one retrospective study, “Prognostic indicators and long-term survival in 14 horses with equine multinodular pulmonary fibrosis,” researchers reviewed 14 cases diagnosed at the University of California, Davis, veterinary referral hospital over eight years. Unfortunately, the long-term prognosis for horses with EMPF, regardless of treatment, is poor. Corticosteroid treatment can improve short-term survival, but the prognosis remains poor.

Experimental EHV-5 infection-histopathology-horse-E4-C1 Nodules of fibrosis horse

Most horse owners think of EHV-1 when they hear about equine herpesvirus and corresponding respiratory disease, neurologic illness, abortion, and neonatal death. But the equine herpesvirus family includes five forms. EHV-1 is an alpha-herpesvirus that can also cause mild or latent (inapparent) infection and lead to upper respiratory disease, Hostnik explained. Conversely, EHV-5 is most likely to cause a latent infection and possibly mild disease/fever.

“Both are relatively common, and many horses have been exposed to these viruses, and both are likely to establish latency in immune cells in horses,” Hostnik said.

Ultimately, Dolly succumbed to the respiratory illness in mid-November 2021.

“While Dolly had a positive EHV-5 PCR, we could not confirm EMPF on post-mortem exam,” said Hostnik, as a necropsy wasn’t performed. “However, I still believe this is an important topic for people to be aware of. At this point, we don’t understand why some horses develop EMPF and why this virus is present in those nodules.”

“We did everything we could to find out what was causing her illness but, by the time it was uncovered, too much damage had been done,” Telego said.