Equine herpesvirus‐1: Dealing practically but effectively with an ever present threat

Alpha herpesviruses are ubiquitous pathogens affecting many mammals, including equids (equine herpesvirus [EHV]-1, EHV-3 and EHV-4). Infections with EHV-1 and EHV-4 are among the most common causes of equine respiratory disease worldwide and estimates of prevalence show that most mature horses are infected with EHV-1, EHV-4 or both during their lifetimes [1]. The establishment of lifelong latency in a large proportion of infected animals ensures the survival of herpes viruses in horse populations and is of major importance for EHV-1 and its ability to cause several clinical syndromes (clinical features of note outlined in Table 1). Initial EHV-1 infection occurs through the respiratory route by either inhalation of aerosolised infectious virus or close contact with an infectious individual or fomites. The virus replicates in the respiratory mucosa and spreads quickly to adjacent tissues establishing a cell-associated viraemia between 4 and 10 days after infection. During this phase the virus is transported to sites of secondary infection, particularly the vascular endothelium of the central nervous system and/or endometrium, where the infection may trigger neurological signs and/or abortion/neonatal foal death. All 3 disease syndromes are widely recognised after EHV-1 infection, and chorioretinitis has also been recently described [2]. Although EHV-4 infection is widely recognised as a cause of respiratory disease and occasionally as a cause of individual abortions, it has not been shown so far to be a direct cause of neurological disease.