Equine multinodular pulmonary fibrosis (EMPF) is an interstitial lung disease caused by equine herpesvirus 5 (EHV-5). This disease was first identified in 2007 and affects adult horses of all breeds over age 4.
An Emerging Disease
Until 2007, EHV-5 had not been associated with any equine illnesses. All horses with EMPF have EHV-5, but not all horses with EHV-5 develop EMPH. EHV-5 is an airborne virus that can spread as far as 35 feet. Horses may also contract the virus by using contaminated buckets or feeders.
EMPF disease develops from an inflammatory response when EHV-5 moves into the lower airways. Although all breeds may develop this disease, Thoroughbreds are especially prone to it.
This disease causes scarring and fibrous nodules on the alveoli in the lungs. The alveoli are where the body exchanges oxygen for carbon dioxide, and damage to this tissue makes breathing difficult. Radiographs of affected horses show distinct patterns unique to the disease, so researchers believe this is an emerging disease rather than one that has been undetected.
EMPF is a progressive disease, and most horses who develop it die. The average age of infection is 14.5 years.
Horses with EMPF display symptoms similar to bacterial lung infections, including cough, fever, nasal discharge and lethargy. This disease also causes rapid heart rate (tachycardia) and rapid breathing (tachypnea). Some horses have crackling or wheezing when they breathe. Decreased appetite leads to weight loss. In blood tests, horses with EMPF have low red blood cell counts (anemia), lymphocytes (lymphopenia) and oxygen levels (hypoxemia). These symptoms make EMPH difficult for veterinarians to diagnose because these are the same symptoms associated with pneumonia.
Your vet will probably first want to do an x-ray to help give a preliminary diagnosis. However, lung biopsy is the most common way veterinarians definitively diagnose EMPF. The vet team will examine the tissue and perform a polymerase chain reaction (PCR) test to confirm EHV-5. For horses in advanced stages of the disease, veterinarians may struggle to perform the lung biopsy, and those horses are more prone to severe complications from the procedure.
Veterinarians may also use bronchoalveolar lavage to diagnose EMPF. In this procedure, veterinarians send a bronchoscope from horse's throat to the lungs. Then the vet squirts liquid into the lungs, and removes that liquid for evaluation.
In 2016, University of California-Davis researchers found that testing nasal secretions, blood and lung fluid for EHV-5 was an effective, less-invasive way to determine if a horse has EMPF. Although researchers note that a horse may test positive for EHV-5, but not have EMPF, they suggest that this situation is rare. Horses with EHV-5 in both their lung fluid and in their blood likely have EMPF.
Treatments include giving affected horses supplemental oxygen for hypoxemia, intravenous fluids for dehydration, and anti-inflammatory steroid injections for lung inflammation. These treatments only manage symptoms and do not stop the disease's progression. Some antiviral medications may cure this disease, but these are expensive and little research exists showing this is an effective treatment. Most horses with EMPF are euthanized for humane reasons.
Prevention is difficult because some horses carry the EHV-5 virus without clinical symptoms. No vaccine exists for EHV-5 or EMPF.
Prognosis for horses diagnosed with EMPF is poor due to the number of variables with this disease and lack of controlled studies. The exact relationship between EHV-5 and EMPF remains vague, and researchers in North America and Europe are working to understand this disease better.