Bit & Bridle

Equine Pulmonary Fibrosis (EMPF)

Other Names

  • Equine Multinodular Pulmonary Fibrosis (EMPF)
  • EHV-5-associated pulmonary fibrosis
  • Equine progressive pulmonary fibrosis

What is Equine Pulmonary Fibrosis (EMPF)?

Equine Multinodular Pulmonary Fibrosis (EMPF) is a progressive, typically fatal fibrosing lung disease characterised by the formation of scar tissue (fibrosis) throughout the pulmonary parenchyma — the functional gas-exchanging tissue of the lungs. As the disease progresses, increasing areas of lung tissue are replaced by fibrotic nodules and interstitial scarring, leading to severely impaired gas exchange, respiratory distress, and ultimately death.

EMPF is associated with infection by equine herpesvirus-5 (EHV-5), a gamma-herpesvirus that is closely related to Epstein-Barr virus (EBV) in humans — the virus responsible for infectious mononucleosis and which is also associated with some forms of human pulmonary fibrosis. While EHV-5 is very commonly detected in horses with EMPF, the exact mechanism by which it triggers fibrosis remains incompletely understood, and not all horses infected with EHV-5 develop EMPF. The virus appears to trigger an aberrant immune and fibrotic response in some individuals.

EMPF is a relatively recently characterised condition in equine medicine, first described in detail in the early 2000s. It affects horses of various breeds and ages, though middle-aged to older horses (typically 6–20 years) appear most commonly affected. There is no apparent sex or breed predisposition, and it has been reported across Europe and North America.

Causes

The leading proposed cause of EMPF is EHV-5 infection triggering a fibrotic response in the lungs. EHV-5 is a ubiquitous gamma-herpesvirus present in a very high proportion of adult horses — most horses carry the virus as a lifelong, latent infection acquired early in life. What triggers the transition from clinically silent carrier status to active disease in some horses is not fully understood, but potential contributing factors include:

  • Viral reactivation: Stress, immunosuppression, concurrent illness, or corticosteroid treatment may reactivate latent EHV-5, leading to active viral replication in lung tissue.
  • Abnormal host immune response: Individual variation in immune response to EHV-5 may determine whether the infection is controlled or whether it triggers a fibrotic cascade.
  • Secondary viral or bacterial infections: Co-infections may play a role in promoting lung injury and fibrotic response.

It is important to note that EHV-5, unlike EHV-1 and EHV-4, is not associated with respiratory epidemics, abortion storms, or neurological disease. The fibrotic lung disease it causes appears to be an individual, non-contagious response rather than a transmissible outbreak disease.

Symptoms and Signs

  • Progressive respiratory distress: Increasing difficulty breathing, with laboured respiration, flared nostrils, and an exaggerated abdominal effort on expiration — similar to heaves (equine asthma), but typically more progressive and less responsive to bronchodilators.
  • Weight loss: Often dramatic and rapid as the disease progresses. Affected horses may lose condition despite adequate or increased feeding.
  • Fever: Moderate intermittent pyrexia (elevated temperature) is common, reflecting ongoing inflammation and viral activity.
  • Depression and inappetence: Affected horses appear dull, disinterested in feed, and generally unwell.
  • Cough: A chronic, progressive cough, often productive, is frequently reported.
  • Exercise intolerance: Rapidly progressive inability to perform work, often leading owners to investigate as the horse’s fitness appears to collapse without explanation.
  • Abnormal lung sounds: Auscultation of the chest reveals diffuse crackles, wheezes, and increased bronchial sounds consistent with interstitial lung disease.

Diagnosis

  • Clinical examination: The combination of progressive respiratory distress, fever, weight loss, and poor response to standard respiratory treatments in a middle-aged horse is highly suggestive of EMPF.
  • Thoracic radiography: A characteristic pattern of multiple nodular infiltrates distributed throughout the lung fields is highly characteristic of EMPF. Chest radiographs are an essential early diagnostic step.
  • Thoracic ultrasonography: Multiple hyperechoic (bright) nodular lesions at and beneath the pleural surface are a consistent finding. Ultrasonography is particularly valuable as a quick, non-invasive bedside assessment.
  • Bronchoalveolar lavage (BAL): Cytological examination of lavage fluid typically reveals a mixed inflammatory pattern with macrophages, neutrophils, and increased protein. PCR testing of BAL fluid for EHV-5 is a key diagnostic test — a high viral load strongly supports the diagnosis.
  • Lung biopsy: Ultrasound-guided percutaneous lung biopsy can provide a definitive histopathological diagnosis showing fibrosis and lymphoplasmacytic infiltration. This confirms the diagnosis but carries some procedural risk.
  • Blood tests: Haematology typically shows anaemia, elevated fibrinogen, and leucocytosis. Serum protein electrophoresis may show elevated globulins consistent with chronic infection.

Treatment

There is no reliably effective treatment for EMPF, and the prognosis is generally poor. Management aims to slow progression and improve quality of life:

  • Corticosteroids: Systemic corticosteroids (dexamethasone or prednisolone) are the most commonly used treatment. They can reduce pulmonary inflammation and improve short-term respiratory function and quality of life. Some horses show a meaningful, though usually temporary, response. Long-term corticosteroid use carries risks including laminitis, and must be used judiciously.
  • Antiviral therapy: Ganciclovir (valacyclovir) and other antiviral agents have been used in attempts to reduce viral replication. Some clinical improvement has been reported in individual cases, though the evidence base is limited. These drugs are expensive and require long treatment periods.
  • Antifungal therapy: Itraconazole has antifibrotic properties in addition to antifungal activity and has been included in some treatment protocols for EMPF, though clinical evidence is limited.
  • Bronchodilators: May provide modest symptom relief in horses with a significant bronchospastic component.
  • Nutritional support: High-quality, calorie-dense feeds help combat weight loss. Feed at ground level to reduce respiratory effort.
  • Dust-free environment: Minimising airborne irritants (dust, mould spores) with soaked hay, dust-free bedding, and good ventilation reduces additional respiratory burden.

Many horses with EMPF deteriorate progressively despite treatment, and euthanasia on welfare grounds is frequently required within months of diagnosis.

Prevention

Effective prevention is difficult because EHV-5 is ubiquitous and most horses are already exposed early in life. There is currently no vaccine against EHV-5. General measures that support respiratory health and immune function include:

  • Maintain excellent stable ventilation — poor air quality increases the burden of all respiratory pathogens.
  • Minimise dust, mould, and ammonia in the stable environment.
  • Reduce unnecessary stress and immunosuppressive events where possible.
  • Avoid unnecessary or prolonged corticosteroid use, which can reactivate latent herpesvirus infections.

Prognosis

The prognosis for EMPF is poor. Median survival times from diagnosis in published case series range from weeks to a few months. Some horses show a transient response to corticosteroid therapy that provides improved quality of life for several months, but the disease is almost universally progressive and ultimately fatal. Complete resolution has been reported in very few individual cases. Early diagnosis allows for informed decision-making and ensures affected horses are not subjected to unnecessary distress.

When to Call the Vet

Any horse showing progressive weight loss, increasing breathlessness, and a persistent fever that does not respond to standard treatments should be investigated by a veterinarian promptly. Do not assume that a horse with apparent respiratory signs is simply suffering from a respiratory infection or heaves without a proper diagnostic workup — EMPF requires different management and carries a very different prognosis. For related reading, see our entry on Equine Rhinopneumonitis for other herpesvirus conditions, and equine respiratory anatomy. The British Equine Veterinary Association can refer you to an equine respiratory specialist.

Frequently Asked Questions

Is EMPF contagious between horses?

The fibrotic lung disease itself is not considered a contagious outbreak disease. While EHV-5 is present in most horses and can be shed, EMPF appears to reflect an individual susceptibility rather than a transmissible epidemic condition. Horses in the same yard with a confirmed EMPF case do not routinely develop the disease. Strict isolation measures (as would be applied for EHV-1) are not generally considered necessary, though good biosecurity practice is always sensible.

Can EMPF be confused with heaves (equine asthma)?

Yes, and this is one of the challenges in diagnosing EMPF. Both conditions cause progressive respiratory distress, coughing, and exercise intolerance. Key differentiators include the presence of fever (common in EMPF, unusual in heaves), rapid weight loss (more prominent in EMPF), a poor response to bronchodilators and environmental management in EMPF, and the characteristic multinodular radiographic and ultrasonographic pattern. Any horse diagnosed with “heaves” that does not respond as expected to management changes should be re-evaluated for EMPF.

How is EMPF different from regular EHV infection?

EHV-1 and EHV-4 (the viruses responsible for equine rhinopneumonitis, abortion, and neurological disease) are entirely different herpesviruses from EHV-5. EHV-5 is a gamma-herpesvirus with a very different clinical profile — it does not cause the respiratory epidemics, abortions, or neurological disease associated with EHV-1. EMPF is the primary clinical manifestation of EHV-5 disease, and it is a chronic, fibrotic condition rather than an acute infectious one.

Is there any hope for horses diagnosed with EMPF?

Survival times vary and a small number of horses have shown longer-term improvement with treatment, but recovery is rare. The most important thing is to obtain a confirmed diagnosis quickly, so that appropriate treatment can begin and so that the owner can make informed welfare decisions. Some horses gain several months of acceptable quality of life with corticosteroid management; others deteriorate rapidly. Open communication with your veterinarian about realistic expectations is essential from the outset.