Gammaherpesviruses: Bystanders or the Real Culprits?

Gammaherpesviruses (equid herpesvirus 2 and EHV-5) occur throughout the world, which makes it difficult to determine their pathogenic importance. The problem is deciding whether they are bystanders or are truly causing disease.

"Gammaherpesvirus infection is predominately subclinical in adults," said Guillaume Fortier, DVM, MS, PhD of the Frank Duncombe Laboratory in France.

However, these viruses have been implicated in outbreaks of serious respiratory disease, especially in foals and young horses. They infect the upper airway of the foal and potentially suppress its immune system, leaving the animal vulnerable to secondary infections, such as bacterial pneumonia. In addition, gammaherpes can cause eye inflammation (conjunctivitis) in young foals.

A recent report has also implicated EHV-2 in airway inflammation of sport horses.

"Alpha herpes [EHV-1 and EHV-4] are responsible for the most severe herpes manifestations (myeloencephalopathy, respiratory signs, and abortion) while gammaherpes are more or less weaker, except in foals," said Fortier.

"The biggest challenge is protecting horses during breeding and against fast-spreading infections in training yards," he said, explaining that the viruses are easily transmitted from mare to foal, from foal to other foals, and then from foals to other mares.

Clinical signs vary from mild respiratory indications such as conjunctivitis and nasal discharge to severe outbreaks involving foals and adults with general symptoms, such as fever and lethargy. Because no vaccine exists against EHV-2 and EHV-5, early recognition is important. If owners suspect any herpesvirus, they should quickly get their veterinarian to examine the horse, and sick animals should be separated from the rest of the herd, Fortier advised.

In an editorial about Fortier’s review of gammaviruses, Stephanie A. Brault, DVM, PhD said that simply detecting these viruses by testing is not enough to tell whether they are causing illness because they are ubiquitous. She called for more research about these lesser-known herpesviruses. The study, “Equine gammaherpesviruses: Pathogenesis, epidemiology and diagnosis,” was published online in the September 2009 Veterinary Journal. The editorial, “Equid gammaherpesviruses: persistent bystanders or true pathogens?” was published online in the April 2010 Veterinary Journal. Both are in press for the print edition.

The abstract is available on PubMed.

About the Author

Marie Rosenthal, MS

Stay on top of the most recent Horse Health news with FREE weekly newsletters from Learn More