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