On November 2007, an article from Michigan State University, printed in Veterinary Pathology, reported on an unusual lung disease that was found in 23 euthanized horses. The disease was characterized by scarring and nodule formation.
There were inflammatory cells (neutrophils and macrophages) and some cells also had viral inclusion bodies. PCR (polymerase chain reaction) testing for viral DNA identified Equine Herpes Virus-5 in over 79% of the diseased lungs and only 8.7% of lungs from a control group with no lung disease. The authors proposed a name of equine multinodular pulmonary fibrosis, EMPF, for this new disease.
The finding of a high percentage of these horses having EHV-5 virus in their lungs makes it suspicious as a cause but doesn’t prove it was the cause. It may have been pure coincidence or something about weakened immune defenses that kept them from clearing the virus well.
However, an article from Iowa State, published in the Journal of the American Veterinary Medical Association, describes an identical disease in five horses that were brought there for treatment. EHV-5 was also isolated from the lungs of all of these horses.
This corroborating evidence increases the likelihood EHV-5 is either the cause or a co-factor in this unusual lung disease, although we still will not know for sure until an experimental infection with the isolated virus has been shown to cause the disease.
The EHV-5 virus is generally considered not to cause disease. There is only one prior report of EHV-5 isolation in the United States, from white cells of a healthy horse in California, while studies in other parts of the world have found it in the United Kingdom, New Zealand and Australia, but no connection with any respiratory disease had been made before. There is no vaccine available for EHV-5.
Bottom Line: The symptoms of EMPF resemble heaves/chronic obstructive lung disease. They include cough, rapid breathing, difficulty breathing, elevated heart rate and weight loss. Some of the five horses reported on by Iowa State also had fever and a nasal discharge. Fever is not a part of the picture with heaves.
This disease can be distinguished from other viral infections by the fact that it is chronic and doesn’t go away.
The five horses in the Iowa State University report were treated in the hospital, including with corticosteroids and an antiviral drug, Acyclovir. Three of the horses did not respond and were euthanized. Two horses recovered from the illness.
It’s important to note that:
• We don’t know for sure yet if this disease is infectious and caused by EHV-5, but it’s always very wise not to let your horse come in contact with sick horses.
• Horses with symptoms of viral respiratory disease that do not show quick improvement with appropriate treatment should be evaluated for EMPF.
• Horses with symptoms suggestive of heaves that do not respond well to the usual treatments should always be evaluated for EMPF.
• Scarring and nodules in the lung tissue can be diagnosed by chest X-ray or ultrasound. Fluid lavage of the lung by endoscopy can retrieve samples for cell type identification and PRC testing for EVH-5 virus.