Equine Herpes Virus (EHV) causes horse owners and breeders a large amount of concern due to its potentially devastating effects. 

The virus is ever-present in the horse population worldwide and cases of Herpes virus infection are seen sporadically across Australia. It can cause mild to life-threatening disease affecting the respiratory and neurological systems, as well as being responsible for abortions in all ages and breeds of horses and donkeys.

The equine herpes virus has five different strains; the main strains of concern are Equine Herpes Virus 1 (EHV-1) and Equine Herpes Virus 4 (EHV-4).

How does the disease spread?

The vast majority of horses will be exposed to the virus during their first year of life and will develop low-grade respiratory disease which generally, in uncomplicated cases, will resolve in 1-2 weeks. These horses will go on to develop short-term immunity to the virus.

Infection is spread by the inhalation of infected droplets or eating contaminated material. Infected horses shed the virus in nasal secretions and abortion products, such as the placenta, aborted foetus and uterine discharges. The virus can remain in the environment for up to two weeks.

The immune system, however, often fails to eliminate the virus from the horse’s body completely and, as a result, the horse will then remain latently and asymptomatically infected with Herpes virus for life (these horses are called ‘silent carriers’). These inapparent infections can be re-activated later in life during periods of stress, such as periods of poor nutrition, heavy parasite infestations, and lengthy periods of transport or stressful herd situations.

When the virus is re-activated, the horse may not show any signs of illness and still shed the virus, therefore, he becomes infectious to other horses. This explains why outbreaks of Herpes virus can occur in a closed herd situation, without the need for introduction of new animals.

Clinical signs of Equine Herpes Virus

EHV-1 is the most serious of the strains of Herpes virus as it has the ability to target the respiratory, reproductive and nervous systems in the horse, and can affect either individuals or large groups of horses.

Respiratory disease

EHV-1 is a regular cause of ‘the common cold’ in young horses. They normally present with varying degrees of fever, nasal discharge, inappetence and depression. The majority of these infections are mild, but can result in large numbers of horses, especially weanlings up to 3-year-olds, becoming infected. EHV-2, EHV-4 and EHV-5 can all also cause respiratory disease, which is generally mild in severity.

Abortion 

EHV-1, and occasionally EHV-4, can cause abortion, which usually occurs in the last four months of gestation, but can occur as early as four months gestation. The mare will normally abort a foal, often still within the placenta 10 days to 12 weeks after being infected with the virus. Often the mare will not show any signs of being ill before aborting the foal. Some mares if infected in late pregnancy may deliver a live foal. However, these foals are often born with severe disease and often die despite intensive veterinary treatment. These abortion products and foals are also a major source of infection to other foals and horses on the same property.

Neurological disease/Myeloencephalopathy 

Neurological signs associated with EHV-1 are uncommon, but can be life threatening – horses showing neurological symptoms will often be housed on a property which has had an outbreak of respiratory illness or abortions within the last few weeks.

Symptoms often appear suddenly, approximately 6-10 days post-infection. Mild cases become weak and slightly wobbly in the hind legs, and can dribble urine. More severe cases show weakness and paralysis of the hind limbs, which often results in the horse sitting like a dog and being unable to stand. Even though the neurological symptoms appear suddenly, they tend to stabilise in the first 24-48 hours. Most horses with the neurological form of EHV-1 do not have a fever.

Diagnosis of Equine Herpes Virus

Herpes is a notifiable disease in Australia. If you suspect your horse is suffering from the abortion or neurological effects of the disease, you need to contact your veterinarian who will then contact Biosecurity Queensland. EHV is specific to equines and it is not transmissible to humans.

Testing for Herpes virus is difficult due to the presence of antibodies in most horses’ blood from their first exposure to the virus as young animals. 

Paired blood samples which are taken 7-21 days apart and swabs from the nasopharynx can be tested for the virus, but both tests can result in false negatives once the horse is showing symptoms. It is worthwhile testing horses that have been in contact with the horses showing symptoms as they may test positive prior to becoming ill.

Laboratory testing of placenta and aborted foals can identify the Herpes virus as cause of the abortion. Samples can be taken by your veterinarian or the whole foetus can be submitted for testing.

Treatment 

All horses that are suspected of suffering from Equine Herpes infection should be isolated immediately from any other horses on the property as the virus is highly contagious.

Horses with mild respiratory signs will recover uneventfully or may require a short period of antibiotics if a secondary bacterial infection has occurred.

Mares which abort due to Herpes infection do not require treatment and the virus only remains in the genital tract for a few days. The infection should not affect the mare’s future reproductive potential, and she can be bred successfully shortly after the abortion and go on to foal normally.

Horses with the neurological form of Herpes can be extremely difficult to treat especially if they are unable to stand. There is no specific treatment for Herpes virus Myeloencephalopathy (EHM), treatment is aimed towards fluid and nutritional care, and supportive nursing.

Very often these horses will still have a good appetite, but may require intravenous fluids and sometime laxatives to prevent impaction colic. Urinary incontinence may also need to be managed by evacuating the bladder or catheterising the bladder until the horse is able to urinate voluntarily.

Horses that remain standing have a good prognosis for a full recovery and improvement generally occurs within a few days. However, complete recovery can take up to a year. Horses that remain unable to rise for longer than 24 hours and who are unable to stand with the assistance of a sling have a much poorer prognosis, due to potential secondary complications.

Equine Herpes Virus (EHV) Prevention 

Prevention of Equine Herpes virus can be managed by taking several steps:

1. Isolate:

Prompt isolation of suspect cases and the horses that have been in contact with the sick horse will help to decrease the risk of further infection and disease. These in-contact horses should be isolated for a minimum of one month and pregnant mares should be isolated until they have foaled.

2. Vaccinate:

Whilst vaccination does not protect against the neurological form of EHV-1, it can reduce the severity of clinical signs. Vaccination of pregnant mares is recommended at 5, 7 and 9 months of pregnancy, and will help to reduce the risk of abortion. Vaccination of horses that are required to travel extensively for competition or breeding is also recommended. Infected horses should not be vaccinated.

3. Quarantine:

New additions to the property should be isolated and quarantined for a 21 day period prior to coming into contact with any horses on the property.

4. Separate pregnant mares:

Pregnant mares should be kept separate from other horses on the property, especially weanlings and yearlings.

5. Minimise stress:

Good management practices, such as regular worming, good nutrition and minimising lengthy periods of transport will help to reduce the incidence of the virus.

Whilst Equine Herpes virus is something that horse owners and breeders need to be aware of, simple management practices along with good biosecurity and vaccination can significantly reduce the risk of your horse developing any form of the disease.

About the Author: Emily Mabbott graduated from the University of Edinburgh in Scotland. After graduation, she moved to the much warmer climate of Asia and worked at the Hong Kong Jockey Club for four years. After taking some time off to travel, she joined the team at WestVETS Animal Hospitals in May 2010. Emily enjoys all aspects of equine veterinary work and has completed further training in equine dentistry. She also enjoys the challenges of the more intensive patients in the equine hospital and equine anaesthesia. In her spare time, Emily can usually be found racing dragon boats and making the most of Queensland’s amazing beaches.