Equine herpes virus
Equine herpes is a widespread and deadly condition in horses caused by the equine herpes virus 1 (EHV-1). It bears many similarities to human herpes viruses, including those that cause chicken pox, cold sores and genital herpes. The virus’s ability to lay dormant for years in infected animals has stymied attempts to prevent its spread.
The first symptoms are often a watery nose, loss of appetite, fever and swelling. The virus targets the respiratory, reproductive and central nervous systems, and gradually spreads throughout the body. This can lead to incontinence, an inability to stand and stillbirth in pregnant mares.
The watery nose, much like humans sneezing, is a potent way of spreading the virus. With many horses feeding from the same source or sharing bridles, the virus can transfer quickly. Stables are quarantined when there is a suspected case in order to contain the virus, but often the worst outbreaks are in the aftermath of horse shows. At the 2011 Ogden horse show in Utah, USA a single case of herpes spread to numerous horses that then went back to their home state and spread the disease to new stables, . This led to quarantines in affected stable and numerous other events being cancelled.
The virus is so widespread that most horses will become infected, usually early on in life. This bears a striking resemble to the herpes viruses that cause chicken pox and cold sores in humans. With the virus this prevalent, restricting the movements of horses and regular testing will achieve little.
Equine herpes viruses can also spread beyond horses and even other equines. In 2010, two polar bears at a German zoo caught equine herpes that had spread somehow from zebras. While the infections may have been latent in the zebras it caused encephalitis in the bears, which died shortly after becoming infected.
Research is currently going into developing antiviral drugs to prevent the virus from spreading through the body. While this would do little to prevent the transmission of the disease between horses, it could have the power to prevent it from progressing beyond a respiratory disease.
Acyclovir, a standard herpes virus drug that might be used to treat cold sores in humans, cannot reach high enough doses in the bloodstream in horses for it to be effective.
There are currently no vaccines that completely prevent infection of EHV-1, although there are several that offer limited protection against aspects of the disease. For example, Pneumabort K and Prodigy are inactivated vaccines that are marketed as protection against abortions in pregnant mares. Although the effectiveness of vaccines only tends to last a few months, some can reduce the chances of the virus being spread between horses and indirectly protect them.
Another approach for targeting EHV-1 is using small interfering RNA (siRNA) . This works by binding to the messenger RNA that instructs the cell to manufacture a certain protein. By intercepting this messenger RNA, the protein is not made and the virus cannot function normally. In 2009, researchers published work using mice showing that the small stretches of interfering RNA could be applied topically and repress viral replication and symptoms . In this case the proteins targeted were glycoprotein B and Ori helicase, which are necessary for cell entry and genome replication respectively. Because of the nature of this treatment, it is very difficult for the virus to develop resistance. This approach has since been tested in horses where it showed a significant decrease in neurological complications of EHV-1 infections, although appeared to have little impact on the amount of virus in the body .
- Fulton A et al (2009) Effective Treatment of Respiratory Alphaherpesvirus Infection Using RNA Interference PLoS ONE 4(1):e4118 doi:10.1371/journal.pone.0004118
- Brosnahan MM et al (2010) The effect of siRNA treatment on experimental equine herpesvirus type 1 (EHV-1) infection in horses Virus Research 147(2):176–181 doi:10.1016/j.virusres.2009.10.017