Since it was first identified in 2007, deadly equine multinodular pulmonary fibrosis (EMPF) has been reported in numerous horses across North America and Europe. While still considered a rare disease, EMPF appears to be related to a very common one—equine herpesvirus (EHV)—and early treatment appears to be the main hope for survival.

Specifically, it’s the “gamma” herpesviruses that seem to be involved in EMPF development, explained Bianca Schwarz, PhD, DVM, Dipl. ECEIM, head of the Internal Medicine Service in the Equine Clinic of Altforweiler, Germany, and a former researcher at the Equine Clinic of the University of Vienna in Austria. There are two known gamma herpesviruses in horses: EHV-2 and EHV-5. These EHV forms often go unnoticed in horses, as the infection can be subclinical (i.e., it doesn’t cause outward clinical signs). But if signs do occur, they are usually related to the respiratory tract, causing nasal discharge or breathing difficulties, and in more severe cases fever and lethargy. There is no vaccine for EHV-2 or -5.

Here’s how it all ties together: In all documented EMPF cases, the affected horses have also tested positive for EHV-5, Schwarz said. And about one-third of those horses tested positive for EHV-2, as well. Schwarz says that, even so, EMPF itself cannot be considered a “viral” disease: “At the moment we think that it is a wrong immunoreaction of certain horses to a viral pathogen,” she said.

Looking at five of the seven EPMF cases seen in two European referral clinics between late 2008 and mid-2011, Schwarz and colleagues noted two distinct forms of EMPF