Researchers have recently identified a new equine respiratory ailment that presents clinical signs similar to heaves, but that is progressive and has a poor prognosis for affected horses. The researchers have postulated that the disease, dubbed equine multinodular pulmonary fibrosis, might be linked to equine herpesvirus-5, a common virus that has been considered to be nonpathogenic (not causing disease). A peer-reviewed paper on the pathology of the disease (structural changes to the organs of the horse) will be published in an upcoming issue of Veterinary Pathology.

Equine multinodular pulmonary fibrosis is characterized by coalescing nodules of fibrosis (scarring) in the alveoli of the lungs. Clinical signs include a low-grade fever, weight loss, and progressive respiratory disease that is often mistaken for heaves or bacterial pneumonia, but that doesn’t respond to therapy.

Pam Wilkins, DVM, MS, PhD, Dipl. ACVIM, Dipl. ACVECC, associate professor of medicine at the University of Pennsylvania, and chief of Emergency/Critical Care/Anesthesia at the university’s New Bolton Center, said she has seen 10-15 cases and is working on another paper, which will focus on the clinical presentation of the disease.

“It is a clinically distinguishable syndrome,” Wilkins said. “The chest X rays will show sort of a diffuse bronchial interstitial pattern, but laid on top of that are these nodules that you can see. When radiologists look at it, their big rule-outs are going to be fungal pneumonia and neoplasia (cancer).”

Corticosteroids are currently the treatment of choice for equine multinodular pulmonary fibrosis, but prognosis remains poor (and is complicated by the amount of time that generally passes before the disease is identified and these horses begin treatment), and it can take several weeks of treatment before any positive progress is made.

“We’re trying to work out what clinical indicators might give us a good idea of prognosis, but there are so few cases that have been evaluated clinically, I can’t really say,” Wilkins said.

She emphasized the need for practitioners to be certain of a diagnosis of equine multinodular pulmonary fibrosis before beginning treatment, as corticosteroids will make fungal pneumonia worse, should that be the actual cause of the clinical signs. She recommended performing bronchoalveolar lavage (“washing” of the deep lung with fluid and collecting the fluid for analysis), as well as ultrasound-guided biopsy of the lung masses to confirm X ray findings and look for fungal organisms before proceeding with treatment.

Kurt Williams, DVM, PhD, Dipl. ACVP, assistant professor in the department of Pathobiology and Diagnostic Investigation at Michigan State University’s College of Veterinary Medicine, said that prior to the current study, there were limited descriptions of fibrotic lung diseases in the horse.

He has now seen around 40 cases of the disease, 24 of which are discussed in the upcoming Veterinary Pathology paper. Williams identified the disease and made the connection with EHV-5 after noting inclusion bodies (structures that replicating viruses leave in cells) in samples of affected tissue. In collaboration with Steve Bolin, DVM, PhD, and Roger Maes, DVM, PhD, at Michigan State University’s Diagnostic Center for Population and Animal Health, he performed polymerase chain reaction testing on the samples and found the affected horses were positive for a gammaherpesvirus (which could be EHV-2 or EHV-5). The scientists narrowed it down to EHV-5 through subsequent testing. The exact nature of the relationship between the fibrotic disease and EHV-5 is not yet known.

Along with the 24 cases detailed in the study, researchers also looked at 23 horses of a similar age in a control group, many of which had chronic lung diseases of a different nature. They did not find EHV-5 in any of these animals.

“It’ll be interesting to really investigate whether this is garden-variety EHV-5, or some new variant,” Williams said. “Has there been some sort of shift in the virus that’s now making it pathogenic, or is it a very closely related, yet different gammaherpesvirus? We really need to pin that down.”

According to Williams, there is limited information in the scientific literature on the prevalence of EHV-5 infection in horses, with most studies identifying the virus in nasal swabs or peripheral blood leukocytes using PCR. A recent study has identified the virus in 64% of nasal samples from young racehorses. Researchers on the study by Williams et al. did not find EHV-5 in the lungs of control horses.

Practitioners wanting more information about equine multinodular pulmonary fibrosis can contact Williams via e-mail at williamsk@dcpah.msu.edu or Wilkins at pwilkins@vet.upenn.edu.  

“I think the really interesting thing about this disease is once you’ve seen one, you’ll recognize it earlier the next time,” Wilkins said. “It is out there