New horse pulmonary disease has few treatment options

Equine multinodular pulmonary fibrosis, a newly discovered respiratory disease in horses, severely affects the lungs and greatly reduces their ability to exchange oxygen.

First described in 2007 in North America, this rare disease has since been found to affect horses in Europe, New Zealand and Australia. It is probably a disease that went unrecognized rather than one that is completely new. Even though it is rare, the disease is severe with limited treatment options and few survivors.

Horses affected by EMPF suffer from weight loss, mild fever and varying degrees of breathing difficulty, which can include rapid and laboured breathing and reduced exercise tolerance.

The average age for affected horses is 14.5 years old, and there does not seem to be a sex or breed predilection.

Normal lungs should be fluffy and pink, like a strawberry gelatin salad. Air enters the lungs with each breath, where it is dispersed into millions of thin-walled sacs called alveoli. Tiny blood vessels run directly beside these alveoli, allowing blood to pick up fresh oxygen and release carbon dioxide. Carbon dioxide is breathed out and the cycle begins again.

Horses with EMPF have deposits of extra, dense fibrous tissue, which thickens the alveolar walls and decreases oxygen and carbon dioxide exchange. The fibrosis also likely prevents the lungs from expanding and contracting normally.

This fibrosis consists of the same type of connective tissue that occurs in tendons. If the normal alveolar lining were like tissue paper, this fibrosis would be like laying down a brick wall. These areas eventually progress to form variably sized, dense lumps. These reduce the ability of the horse to breath properly and could be life-threatening.

Researchers have linked EMPF to an equine herpes virus, EHV-5, which is distantly related to the herpes viruses (EHV-1) associated with abortions, upper respiratory tract inflammation (rhino) and neurological disease.

Infection with EHV-5 is widespread in horse populations, but usually there are no signs of illness.

However, researchers think EMPF is an abnormal inflammatory response to the viral infection rather than direct damage from the virus itself.

It could also be that certain strains of EHV-5 are more likely to cause EMPF or that certain horses have a genetic predisposition to developing the disease after becoming infected.

Some horses infected with this virus also develop lymphoma, a form of white blood cell cancer.

Diagnosis of EMPF is challenging because respiratory signs are similar to other, more commonly diagnosed lung diseases, including pneumonia (shipping fever) and heaves. Ruling out these other causes of respiratory illness is the first step in diagnosing EMPF.

Diagnostic tests include listening to the lungs with a stethoscope, blood work and X-rays. Fluid samples collected from the airways can be tested for EHV-5 and signs of inflammation. As well, an ultrasound-guided lung biopsy to obtain tissue can be examined for microscopic evidence of fibrosis.

The presence of EHV-5 and confirmation of the microscopic fibrosis by a pathologist is the current standard for diagnosis. The diagnosis can also be made from an autopsy examination, as long as the veterinarian is aware of the disease and collects the appropriate samples for testing.

Treatment includes supportive care such as intravenous fluids, supplemental oxygen and anti-inflammatory steroids to reduce lung inflammation and fibrosis.

Some horses have been treated with antiviral medications, although there are probably too few reports to determine if the medication is effective, and they can be prohibitively expensive.

The discovery of this disease in horses may shed light on idiopathic pulmonary fibrosis, a similar condition that affects people. It has been speculated that the human disease is also associated with a chronic herpes virus infection.

Ongoing research into EMPF may improve diagnosis, treatment and prevention in horses, but it also might be an important model for understanding the similar disease in people.

Dr. Jamie Rothenburger is a veterinarian who practices pathology and a PhD student at the Ontario Veterinary College. Twitter: @DrJamieR_Vet

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