Pulmonary Fibrosis in Horses: Chronic Lung Scarring and Prognosis

Pulmonary fibrosis is a chronic lung condition where normal, stretchy lung tissue is replaced by scar tissue. In horses, that scarring makes the lungs stiffer and less able to exchange oxygen. Over time, affected horses may breathe faster, tire more easily, and struggle with even light activity.

In equine medicine, the best-known fibrosing lung disease is equine multinodular pulmonary fibrosis (EMPF). EMPF is an interstitial lung disease that has been strongly associated with equine herpesvirus-5 (EHV-5). Not every horse with EHV-5 develops fibrosis, and not every horse with chronic breathing problems has EMPF, so your vet usually needs imaging and additional testing to sort out the cause.

This condition is different from common equine asthma. Horses with pulmonary fibrosis often have more obvious breathing effort at rest, more pronounced weight loss, and imaging changes that suggest deeper lung damage rather than primarily airway inflammation. Because the disease can progress gradually or present after signs have been missed for weeks, early veterinary evaluation matters.

For pet parents, the most important takeaway is that pulmonary fibrosis is usually a serious chronic disease, not a short-term cough. Some horses can be managed for a period of time, but long-term outlook depends on how much lung tissue is already scarred and how well the horse responds to treatment and environmental support.

In horses, pulmonary fibrosis most often refers to equine multinodular pulmonary fibrosis (EMPF), which has a strong association with EHV-5, a gammaherpesvirus found in horses worldwide. Researchers have shown that EHV-5 is linked to this disease, but the exact reason one horse develops severe lung scarring while another does not is still being studied. It may involve a combination of viral activity, the horse's immune response, and ongoing inflammation inside the lung tissue.

Fibrosis itself is the end result of repeated injury and abnormal healing. Instead of returning to normal after inflammation, the lung lays down excess fibrous tissue. That scar tissue reduces elasticity and interferes with oxygen exchange. In practical terms, the horse has to work harder to breathe.

Your vet will also consider other causes of chronic respiratory signs before labeling a horse with pulmonary fibrosis. Severe equine asthma, chronic bacterial pneumonia, pleuropneumonia, fungal disease, neoplasia, and other interstitial lung diseases can overlap clinically. That is why diagnosis usually involves more than one test.

There is no evidence that routine vaccination prevents EMPF specifically. Good biosecurity and respiratory disease management may help reduce infectious pressure in a herd, but they cannot fully eliminate the risk of EHV-5 exposure because these viruses are common in the horse population.

Diagnosis starts with a full history and physical exam. Your vet will pay close attention to resting respiratory rate and effort, body condition, temperature, and lung sounds. Bloodwork may show inflammation, and low oxygen levels can sometimes be documented with arterial blood gas testing in more advanced cases.

Imaging is usually the next step. Thoracic ultrasound can identify pleural irregularities and peripheral lung changes, while thoracic radiographs may show a diffuse nodular or interstitial pattern that raises concern for fibrosis. Because several lung diseases can create similar signs, imaging helps narrow the list but does not always confirm the diagnosis by itself.

Airway sampling such as a tracheal wash or bronchoalveolar lavage may be recommended to look for inflammation, infection, or other clues. Testing for EHV-5 by PCR on respiratory samples or lung tissue may support the diagnosis, especially when combined with imaging findings and clinical signs. In some horses, your vet may recommend an ultrasound-guided lung biopsy, which can provide the most definitive confirmation and may help with prognosis.

Because these horses can be fragile, your vet will balance diagnostic value against safety, stress, and cost range. A horse with severe breathing effort may need stabilization before advanced testing. That stepwise approach is a core part of spectrum of care planning.

There is no guaranteed way to prevent pulmonary fibrosis in horses, especially EMPF. Because EHV-5 is common in the horse population, prevention is more about reducing respiratory stress and catching disease early than eliminating exposure completely.

Good barn ventilation, low-dust forage and bedding, and prompt veterinary attention for chronic cough, weight loss, or reduced stamina are practical steps. Avoid pushing a horse that is breathing harder than normal, and work with your vet to investigate persistent respiratory signs instead of assuming they are routine fitness issues or stable dust.

Biosecurity still matters. Isolate horses with fever or active respiratory disease when possible, avoid sharing buckets and equipment, and follow your vet's guidance during outbreaks of infectious respiratory illness. While these steps are not specific protection against EMPF, they support overall respiratory health and may reduce infectious burden in the herd.

For horses already diagnosed with chronic lung disease, prevention shifts toward limiting flare-ups. That may include environmental management, careful exercise decisions, regular rechecks, and a clear plan with your vet for monitoring respiratory rate, effort, appetite, and body condition over time.