Researchers at the University of California–Davis have developed a new way to diagnose a sporadic lung infection associated with equine herpesvirus-5.
Equine multinodular pulmonary fibrosis (EMPF) causes damage to the alveoli, the sacs within the lungs that facilitate oxygen exchange, leaving scarring that takes the form of fibrous nodules.
Outward signs of EMPF vary from exercise intolerance to coughing, fever and weight loss, making it difficult to distinguish from other respiratory conditions. The disease, however, is progressive and breathing becomes increasingly difficult. Supportive care can help horses with EMPF, but most are eventually euthanatized.
One distinct feature of EMPF is its association with equine herpesvirus type 5 (EHV-5), which is wide-spread in horses but had not previously been implicated as a cause of illness. Earlier research established a link between EHV-5 and EMPF, determining that the lung condition is an inflammatory response to the viral infection that only some horses, for unknown reasons, develop.
“We all struggle to understand the why and where [of EMPF],” says Nicola Pusterla, DVM. “EHV-5 is highly adapted to its host. That said, it generally stays at the level of the upper airways. We are not sure what predisposes the colonization of the lower airways in some horses and the development of EMPF. The host immune response, however, is definitively involved in this process.”
The current “gold standard” for diagnosing EMPF is a lung biopsy to look for signs of fibrosis, combined with a positive polymerase0 chain reaction (PCR) test for EHV-5 within the tissue samples. Lung biopsies, however, can be difficult to obtain from a horse showing severe respiratory signs and may lead to serious complications.
Looking for an easier alternative, the Davis researchers sought to determine how effective the testing of blood, nasal secretions and lung fluid could be in identifying EHV-5-associated EMPF.
The researchers first used clinical findings, radiographic imaging and analysis of lung tissues and fluid to divide 70 horses into four groups: those with no respiratory illnesses, those with inflammatory airway disease, those with non-EMPF interstitial lung diseases and those with EMPF. PCR was then used to test each horse’s blood, nasal secretions and lung fluid for EHV-5.
The data showed that the presence of EHV-5 in lung fluid was strongly associated with EMPF. “Although possible, it is very rare to find a horse that tests positive for EHV-5 in lung fluid that does not have EMPF,” says Pusterla.
EHV-5-positive blood and nasal secretion tests were also associated with EMPF, but with lower sensitivity (meaning the tests could miss some cases) and specificity (meaning the tests could produce false positives). Considered together, however, positive blood and nasal secretion tests were more reliable in detecting horses with EMPF than each was on its own.
These findings, says Pusterla, show that less invasive measures can be used instead of lung biopsies in making a diagnosis of EMPF. “In a case suspected on the basis of clinical and radiographic abnormalities, blood and nasal secretions positive for EHV-5 can support that diagnosis,” he says. Confirmation of the diagnosis can be made via PCR testing of lung fluid or a lung biopsy.
“Assessment of quantitative polymerase chain reaction for equine herpesvirus-5 in blood, nasal secretions and bronchoalveolar lavage fluid for the laboratory diagnosis of equine multinodular pulmonary fibrosis,” Equine Veterinary Journal, published online January 2016
This article first appeared in EQUUS issue #467, August 2016.