Equine Herpesvirus-1 : An Elusive Target
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By Neil Bryant
Infectious diseases are not uncommon in racehorses in training, breeding stock, and pleasure horses. Some of the more serious diseases can be financially devastating to the animal’s owners and to the equine industry on the whole. Viruses belonging to the herpesvirus family cause some of the most well characterized equine infectious diseases, and the most problematic of these is equine herpesvirus 1 (EHV-1; species Equid alphaherpesvirus 1). EHV-1 is ubiquitous in most horse populations in the world. It is responsible for major economic and welfare problems causing respiratory disease, neurological disease (mainly seen in adult horses), and abortion and neonatal foal death in pregnant mares.
This was most notably highlighted by the multiple abortion outbreak recorded in Hertfordshire, England, between February and April 2016 in fully vaccinated animals (http://www.aht.org.uk/cms-display/interim-report16-april2.html). Studies have determined that EHV-1 is a common cause of abortion. Occasional cases have also been linked to EHV-4 infection, but this is much rarer and doesn’t account for episodes of multiple abortion, as is seen occasionally with EHV-1.
The virus
EHV-1 was first isolated from an equine abortion in the U.S. in the 1930s. At the time of first isolation the vets weren’t sure what it was, but they knew it was infectious. Subsequent genetic analysis much later led to the classification of the virus in the genus Varicellovirus (family Herpesviridae), together with its close relatives equine herpesvirus 4 (EHV-4; species Equid alphaherpesvirus 4) and equine herpesvirus 8 (EHV-8; species Equid alphaherpesvirus 8). Interestingly it is grouped with, and is therefore genetically similar to, the human herpesvirus responsible for chickenpox, the Varicella Zoster virus. Initial infection of horses was thought to occur around weaning, when virus-neutralizing antibodies transferred to the foal from the mare’s colostrum had declined enough to make them susceptible to infection. However, virus has been isolated from foals as young as seven days old with high antibody levels but without any significant clinical signs. Immunity to re-infection after primary infection is relatively short-lived, lasting between three-six months, but it is rare for naturally infected mares to abort in consecutive pregnancies.