our expertise: VET COLUMN
Equine Herpes
- What You Need
To Know
BY TOM HUTCHINS, DVM, DABVP
F
ive different herpes virus are commonly found
in domestic horses. Equine Herpes Virus (EHV)
infection is ubiquitous in the equine population and
most horses are infected in the first few months of life.
Following infection with an EHV, the virus is able
to essentially hide from the immune system in the
lymphoid or neurologic tissues; horses are then said
to have a “latent infection.” Once infected with EHV,
horses are thought to be permanently infected. A latent
viral infection can become reactivated via currently
unknown mechanisms often during times of stress.
Horses with re-activated latent infections are a source
of infection for other horses. EHV-1 was one of the
first equine herpes viruses to be described. EHV-1 is
termed an “alpha herpes virus” that can cause severe
pneumonia in newborn foals, mild respiratory disease
in young horses and abortion in pregnant mares.
The neurologic form of disease is a consequence
of damage to blood vessels and the adjacent soft
tissues. Depending on what part of the neurologic
system is affected (i.e., the spinal cord and brain)
clinical signs will vary. The neurologic form of EHV-
1 infection is generally referred to as Equine Herpes
Myeloencephalopathy (EHM) as both the brain and
spinal cord can be involved. This form of herpes virus
infection can cause clinical signs in any age, breed, or
gender of horse.
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TRANSMISSION: Horse to horse transmission of the
herpes viruses is significant when horses are kept in
close contact. Contaminated equipment (e.g., water
buckets, water hose handles, cleaning and grooming
equipment, etc.) can also be a source of infection and
people can transmit the virus on their hands or clothes.
An infected horse will excrete and aerosolize the virus
in respiratory secretions. All horses with clinical signs
are expected to be contagious, although horses not
showing any clinical signs can shed EHV. Neurologic
horses shed large quantities of EHV-1 and should be
securely isolated. The virus is estimated to be viable
for up to 7 days in the environment under normal
circumstances but may remain viable for a maximum
Dr. Tom Hutchins
of one month under perfect experimental conditions.
The virus is easily killed in the environment by most
disinfectants.
DIAGNOSIS: Once exposed and infected with EHV-1
the virus may be detected in blood and nasal swabs
for 21 and 14 days respectively in laboratory settings.
Virus shedding maybe more transient and difficult to
detect in the latter stages of the disease and in clinical
outbreaks. Nasal shedding may be prolonged (21+
days) in cases infected with certain neuropathic strains
of EHV-1. Horses can develop clinical signs as early as
1 day after exposure to the virus, although clinical signs
can be delayed up to 10 days after exposure. Typically
EHM develops 5 to 6 days after the primary infection.
Infection usually develops following exposure to a
horse shedding the virus but in a small percentage of
cases, infection occurs by reactivation of latent virus.
EHV-1 typically causes a biphasic fever peaking on
day 1 or 2 and again on day 6 or 7. With respiratory
infections there is often significant nasal and ocular
discharge, but not a lot of coughing. There may be
some persistent enlargement of submandibular lymph
nodes. With the neurologic form there are typically
minimal respiratory signs, with fever (rectal temperature
greater than 102°F) being the only warning. Neurologic
disease appears suddenly and is rapidly progressive
reaching its peak intensity within 24 to 48 hours.
In horses infected with the neurologic strain of
EHV-1, clinical signs may include: nasal discharge,