Types of Equine Herpesvirus
There are many types of Equine Herpesvirus (EHV) but the ones that affect the domestic horse are EHV-1, -2, -3, -4 and -5. EHV-2 and EHV-5 are found in most horses and rarely cause disease, although they have been associated with conjunctivitis and respiratory disease. It has also been suggested that they might predispose a horse to other infections. EHV-3 can cause a venereal disease but again this is quite rare and does not cause death or need treatment. EHV-1 and EHV-4 however can cause more serious problems.
Equine herpesvirus-1 (also known as equine rhinopneumonitis virus) can cause neurological disease, respiratory disease, abortion and death of newborn foals. Neurological symptoms are varied but can include hind limb weakness and loss of coordination (ataxia), which can progress to problems in the horse getting up (recumbency) and paralysis. EHV-1 is also the cause of a recently identified syndrome, peracute vasculitis, which is fatal to adult horses.
Equine herpesvirus-4 mainly causes respiratory disease, and only rarely has been reported to cause neurological disease or abortion. The associated respiratory disease can be severe but is non-fatal.
EHV-1 and EHV-4, are extremely widespread and although most horses will have the virus many of them will not show signs of illness most of the time. However, periodically there are outbreaks of disease. This is because the virus, as with other herpesviruses, establishes latency where only the viral DNA resides in the cells of the host animal. If the virus reactivates, it goes on to multiply and possibly cause clinical illness in the horse. It is at this stage that the virus is spread through aerosolised droplets, which can infect other horses through inhalation or ingestion. Reactivation of latent virus can occur when the horse is stressed, (for example during illness, transportation or weaning), however the actual mechanism of reactivation is unknown.
There are no vaccines currently available that provide complete protection against equine herpesvirus. The vaccines that are available give some protection against respiratory disease and can help to reduce the spread of infective virus. However, vaccination should be viewed as assistance to disease control alongside good equine management. To help reduce the incidence of EHV outbreaks, any sick animals should be quarantined from healthy horses and strict hygiene precautions established. Similarly horses at risk from reactivating virus (for example weanlings and horses going into training or showing) should be isolated. This is particularly important, as infectious horses often do not show signs of disease. A rise in temperature can be a warning that a horse is developing illness so taking regular mesurements of at risk horses is advisable.
Diagnosis of EHV can only be confirmed in the laboratory.
EHV-1 and -4 are DNA viruses, with the DNA genome enclosed by an icosohedral capsid that is surrounded by a tegument layer and finally an outer membrane, also known as the virion envelope.
Both the EHV-1 and EHV-4 genomes have been sequenced. They have double-stranded linear DNA genomes and share extensive sequence homology. The EHV-1 genome is 150 kb in size and encodes 76 genes (open reading frames), whereas the EHV-4 genome is slightly smaller (145 kb) but also has 76 genes. Some of these genes are 'essential', encoding proteins that are absolutely required for virus replication (eg. important structural components of the virus particle or enzymes required for replicating the viral DNA). Others encode 'accessory' proteins, which are not absolutely required for virus replication, but which contribute to virus survival within infected animals.
Within each type of EHV (for example EHV-1), there are many different strains circulating in the horse population. These strains can differ from each other by just a few amino acids, which can be identified by sequencing the DNA. These mutations can make the strain behave quite differently from other strains. For example, a single mutation within the EHV-1 gene, ORF 30 has been shown to be associated with neurological disease.
Diagnosis of EHV can only be carried out in the laboratory. The Animal Health Trust is a World Organisation of Animal Health (OIE) reference laboratory for EHV-1. For further information on EHV testing at the AHT please see our diagnostics section.
You can also view a copy of our downloadable EHV .pdf by clicking here.
Guidelines for minimising risk of spread of infectious disease
In the light of recent Equine HerpesVirus (EHV-1) outbreaks, the AHT has launched a set of guidelines for horse owners and event co-ordinators to help minimise the risk of any infectious disease spread.
You can download the guidelines here