The outbreak of equine herpesvirus infection that was confirmed on 8th January caused particular concern because cases were identified first at an equestrian centre and then at a large veterinary hospital, so there was a high risk of disease spread to other premises.
Equine herpesvirus (EHV) is a very common virus that does not affect other animal species or humans. Many horses will carry the virus without developing disease. Some strains of the virus are more likely to cause disease because they damage blood vessels, which results in disease of the affected tissues and the occurrence of respiratory disease, abortion, death of newborn foals or spinal cord damage. Different strains seem to affect different areas of the body. The strains of equine herpesvirus that affect the spinal cord and nervous system are thankfully the least common — as this form of the disease is severe, highly contagious and frequently fatal. In the UK there are typically only a couple of outbreaks of neurological EHV (also known as equine herpesvirus myeloencephalopathy or EHM) each year.
The strains of virus that cause EHM may be highly contagious between horses that are in close contact, but the virus does not spread over large distances. Effective biosecurity and isolation of affected horses therefore stops the spread of the virus. At both of the affected premises in this outbreak, appropriate precautions have been taken, movements have been limited and horses that had already moved from the affected premises have been isolated. The threat to the wider equine population is therefore likely to be low, and if no new cases develop in the coming week then we can be confident that the outbreak has been contained.
Where there is a potential risk of EHM owners should take horses' temperatures twice daily and monitor for signs of general malaise or inappetence, as well as for more specific signs of EHM such as dragging or incoordination of the hindlimbs and loss of bladder control.
Vaccination can be performed to reduce the risk of EHV-related abortion and respiratory disease. It is not known whether vaccination is helpful in preventing the neurological form of the disease and there is some evidence that vaccinating prior to infection with the neurological form may actually increase the severity of disease.
In populations of horses that are at higher risk of infectious disease (competition horses, horses on yards where there are frequent horse movements, yards in the region of a recent case of EHM) then vaccination against EHV is prudent; however, once cases have been identified on a property or once there is a suspicion that an affected horse may have entered a property then it is too late for vaccination to be beneficial and vaccination may even be harmful. Horse owners are encouraged to speak to their own veterinary surgeon regarding vaccination in their specific circumstances.
Following on from the equine influenza outbreak in 2019, this outbreak serves as another reminder of the susceptibility of horses to infectious disease and the importance of routine, regular vaccination in protecting both individual horses and the equine population as a whole. It takes two vaccinations, injected a few weeks apart, and then a further few weeks for the immune system to be stimulated sufficiently to protect against infection. Vaccination is therefore best used as a means of protecting horses in the long-term, not as a knee-jerk response to an imminent threat of disease.
More information on equine herpesviruses is available in a roundtable discussion document that was published in this journal during 2019 (Ivens et al, 2019). In response to this UK outbreak of neurological herpesvirus, the roundtable has been made free to access online at https://www.magonlinelibrary.com/ doi/full/10.12968/ukve.2019.3.S2.1 — do visit the website and read the roundtable, either online or in PDF format.