Herpesvirus Paralysis - A Killer in our Midst

The Disease

Paralytic equine herpesvirus (EHV) is a neurological disease. Horses of all ages and breeds can be affected and it may occur in a single horse or in outbreaks. Outbreaks can be devastating, involving the death of several animals. The clinical signs vary in severity from mild hindlimb incoordination to severe paralysis and recumbency. Onset of neurological disease is usually acute (over 24 hours), and is sometimes preceded by a raised temperature, general malaise and/or respiratory signs one to two weeks earlier. Paralysis of the tail and bladder with faecal and/or urinary incontinence may occur. Animals that do not become recumbent have a good chance of full recovery, although this may take several weeks. The prognosis is poor for patients who have been unable to stand for 24 hours; these will usually die or euthanasia will be necessary.

What causes the disease?

The disease is caused by a common virus in the horse, called equine herpesvirus-1 (EHV-1). EHV-1 is associated with three disease syndromes: respiratory disease, abortion and neurological disease. It is not known why the virus sometimes causes respiratory symptoms only and causes neurological disease and/or abortion on other occasions. During an outbreak of paralytic EHV, abortions and respiratory symptoms may also occur. In outbreaks of the disease, not all infected animals will show clinical signs.

How does the disease spread?

An infected horse will excrete virus in bodily secretions, particularly nasal discharge, even if it is not showing any obvious clinical signs of disease. Infected animals usually shed the virus for up to two weeks after infection. The disease is transmitted between horses mainly via direct nose to nose contact, but indirect transmission via handlers also occurs, as well as limited spread of infection via the air. Because transmission is mainly via direct contact, appropriate isolation and management measures can limit the extent of an outbreak substantially.

After an infection, the virus can remain latent in the body and the animal is said to be a "carrier" of the virus. It is believed that up to 75% of the British horse population are carriers of herpesviruses. Carriers are not infectious, unless the virus is reactivated. This can happen under circumstances of stress (transport, competition, pregnancy etc) or when the animal's immune system is compromised. The horse will undergo the infection again, with or without showing clinical signs, and shed the virus. These animals may be the source of an outbreak of disease.

How does the disease occur?

The virus enters the horse via the nose and may cause signs of respiratory disease when it infects the airways and lungs. For nervous symptoms to occur, the virus must invade the blood stream and cause damage to the tiny blood vessels which supply blood to the central nervous system.


Although a presumptive diagnosis can be made on history and clinical signs, confirmation of EHV paralysis must be made through laboratory investigations. Specific tests on blood samples, taken at the onset of nervous disease, are very useful in the diagnosis, providing the horse is not on a vaccination programme. Diagnosis in live animals can be confirmed by virus isolation from nasopharyngeal swabs and blood samples. If an animal dies, diagnosis can be made on post mortem examination through characteristic histopathology and isolation of virus from tissues.


As this is a viral infection, treatment is directed towards relief of symptoms and nursing care. Anti-inflammatory therapy is probably indicated. Horses who are too weak to stand should be kept on a thick straw bed and turned regularly or put into a sling. Catheterisation of the bladder may be necessary. If a horse is severely affected, and does not respond to treatment, euthanasia should be considered on humane grounds.

Immunity and Vaccination

Natural immunity after a herpesvirus infection is not long lasting and the horse will be susceptible to re-infection after some months. With subsequent infections, however, clinical signs will be milder or even absent. Vaccination against EHV-1 and EHV-4 is possible and advisable. Current vaccines are licensed for prevention of respiratory disease and abortion only, and do not claim to protect against neurological disease. However, general use of the vaccine will raise the level of protection in a population, thus reducing severity of clinical signs and shedding of the virus. Vaccination in the fact of an outbreak is not advisable, as this may trigger clinical symptoms.


The following recommendations are aimed at controlling an outbreak of paralytic EHV:

When a horse is affected with acute onset neurological disease, especially affecting the hindlimbs, steps should be taken to determine whether EHV is involved.
Samples to be submitted to the laboratory for diagnosis of paralytic EHV include a nasopharyngeal swab, a clotted blood samples and a large volume (30ml) of heparinised blood.
Before the diagnosis is confirmed, the affected animal(s) should be kept in isolation and all movement on and off the premises must cease. Horses must be kept together in their existing or preferably smaller groups and these groups segregated as far as possible.
When EHV-1 infection has been confirmed, groups of horses should be monitored clinically, serologically and virologically for evidence of active infection. Movement restrictions within the premises should be maintained until active infection ceases. On studs, recently foaled mares and pregnant mares should be divided into small groups. Animals should only be allowed to leave the premises when there is no evidence of active infection in their group. On arrival at the new premises, animals should be kept in isolation and screened for evidence of infection, which may be triggered by the stress of transport.


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