Equine Herpes Virus

We all know how contagious strangles is, and there’s a growing movement, quite rightly, about reporting cases in the local community to try to reduce the spread of the disease. Hopefully the stigma associated with strangles is dissipating, but of course there is a new disease doing the rounds to get hot under the collar about. 

Equine Herpes Virus is not a new disease, but it is becoming more prevalent. There are several strains of EHV that affect domestic horses – EHV-1,-2,-3,-4 and -5. 

EHV-1 causes respiratory disease in young horses, abortion in pregnant mares, death of newborn foals, and paralysis in horses of all ages and types. 

EHV-2 also causes respiratory disease, but is linked to conjunctivitis and swollen submaxillary and parotid lymph nodes.

EHV-3 is a venereal disease that causes pox-like lesions on the penis of stallions and the vulva of mares. It is quite rare, rarely causes death or need treatment.

EHV-4 usually only causes low-grade respiratory disease but also occasionally causes abortion.

EHV-5 is a virus that is currently associated with unusual sporadic cases of debilitating lung scarring in adult horses.

EHV-2 and EHV-5 are found in most horses, but rarely cause disease. It’s been suggested that a horse carrying one of these strains of EHV predisposes them to other illnesses.

However, EHV-1 and EHV-4, which are both DNA viruses, cause more serious problems.

The neurological symptoms of EHV-1 are varied but include hind limb weakness and loss of coordination, which can progress to recumbency and paralysis. EHV-1 is also the cause of a recently identified syndrome, peracute vasculitis, which is fatal to adult horses.

EHV-4 rarely causes abortion or neurological disease, but the associated respiratory disease can be severe, although it is not fatal.

Unfortunately, once horses have been infected with a strain of EHV they carry the virus silently, a bit like humans with cold sores, reactivating at intervals throughout their life. This means that 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, like there is in the UK at the moment. This is because if the virus reactivates it goes on to multiply and possibly cause clinical illness in the horse. At this point 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.

EHV abortion can occur from two weeks to several months following infection with the virus, reflecting either recent infection or re-activation in a carrier horse. Abortion usually occurs in late pregnancy (from eight months onwards) but can happen as early as four months. Respiratory disease caused by EHV is most common in weaned foals and yearlings, often in autumn and winter. However, older horses can succumb and are more likely than younger ones to transmit the virus without showing clinical signs of infection. It is the continual cycling of EHV respiratory disease in young horses and the periodic reactivation of latent EHV in older horses that maintains the risk of EHV abortion in pregnant mares and EHV neurological disease in horses of all types and ages.

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, and often studs require mares to be vaccinated against EHV. However, vaccination should be viewed as assistance to disease control alongside good equine management. 

To reduce the incidence of EHV outbreaks, sick animals should be quarantined from healthy horses and strict hygiene precautions established. Yards with ill horses should go onto lockdown and reduce the spread of the disease to other yards. Weanlings and young horses are more at risk from the reactivating virus should be isolated, especially as carried horses often don’t have any symptoms.

The first symptom of EHV is a rise in temperature, and then respiratory symptoms of nasal discharge and coughing appear. If it is the neurological strain, then signs of stiffness will appear. However, diagnosis of EHV can only be confirmed in the laboratory. 

Within each type of EHV there are many different strains circulating in the horse population, in much the same way there are numerous strains of the flu virus, and the reason no one vaccine is all encompassing. These strains can differ from each other by just a few amino acids, but the mutations can make the strain behave quite differently from other strains. This also makes it harder to diagnose EHV and this yards can be slow to put bio security measures into place, thus allowing the virus to spread.

In my research about EHV, I found this useful article about how to prevent the spread of the disease – Have a read here.

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