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 can affect adult horses of any breed. Horses with EMPF develop lung nodules, scarring, and inflammation, which are often fatal. Although rare, it's important for equine owners and enthusiasts to understand what experts have discovered about EMPF in horses.
EMPF: An Emerging Disease
Equine herpesvirus is a common virus among horses worldwide. According to some reports, all horses will have been exposed to at least one strain of EHV by the time they reach 2 years of age. The fifth herpesvirus strain, 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.
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 EMPF. This 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.
Horses with EMPF display symptoms similar to bacterial lung infections. In blood tests, horses with EMPF have low red blood cell counts (anemia), lymphocytes (lymphopenia), and oxygen levels (hypoxemia). This makes EMPF difficult for veterinarians to diagnose because these are the same symptoms associated with pneumonia. Some of the most common signs of infection include the following.
- Decreased appetite
- Weight loss
- Nasal discharge
- Crackling or wheezing when breathing
- Rapid heart rate (tachycardia)
- Rapid breathing (tachypnea)
The diagnostic method your veterinarian uses may depend on the stage of the disease and your horse's condition. In later stages of EMPF, it can be difficult to obtain samples due to risk of complication.
- X-rays: Your vet will probably want to do an X-ray first to help get a preliminary diagnosis. Radiographs are a non-invasive diagnostic tool. These images will allow the veterinary team to visualize the lungs and identify any possible nodules or areas of concern. Ultrasound can sometimes be helpful, as well.
- Lung biopsy: A lung biopsy is the most common way veterinarians definitively diagnose EMPF and it considered the "gold standard" diagnostic. To get a sample, your veterinarian may use an ultrasound to visualize the area, then insert a needle into the lung to extract cells. The vet team will then examine the tissue and perform a polymerase chain reaction (PCR) test to confirm EHV-5. However, veterinarians may struggle to perform the lung biopsy on horses in advanced stages of the disease if the horse is in severe respiratory distress or is unable to undergo sedation for the procedure. Those horses are more prone to severe complications from the procedure, such as puncturing vital vasculature.
- Bronchoalveolar lavage: Veterinarians may also use bronchoalveolar lavage or a tracheal wash to diagnose EMPF. In this procedure, veterinarians send a bronchoscope through a horse's throat to the lungs. Then the vet instills liquid (usually harmless saline) into the lungs where it will pick up cells. The fluid is them removed and sent out to the laboratory for evaluation.
- Less invasive methods: 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.
Unfortunately, treatment for EMPF is not always successful. Treatments only manage symptoms and do not stop the disease's progression. Common therapies include giving affected horses supplemental oxygen for hypoxemia, intravenous fluids for dehydration, and anti-inflammatory steroids to minimize lung inflammation.
Some antiviral medications, such as valacyclovir, may cure this disease, but these are expensive and it's still unclear if horses consistently respond to this treatment. Most horses with EMPF are humanely euthanized due to deterioration.
Preventing EMPF in Horses
Prevention is difficult because some horses carry the EHV-5 virus without clinical symptoms. Although vaccinations are available to protect horses from certain herpesvirus strains (EHV-1 and EHV-4), no vaccine exists for EHV-5 or EMPF.
Many Unknowns Surrounding EMPF Remain
The prognosis for horses diagnosed with EMPF is poor due to the number of variables associated with this disease, as well as lack of controlled studies. The exact relationship between EHV-5 and EMPF remains vague, though researchers in North America and Europe are working to get a better understanding of this disease.