Paralysis - A Killer in our Midst
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
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
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
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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