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Equine
Herpesvirus
By Nancy S. Loving, DVM
Horses usually are infected with herpesvirus at an early age, with estimates that 80-
90% of the horse population has encountered herpesvirus exposure before the age of
2. Herpes viral presence persists for the life of a horse following the initial infection. It
is common for horses to be infected with both EHV-1 and EHV-4. However, EHV-4 is
not associated with viremia, so there are rarely non-respiratory disease manifestations
following EHV-4 infection.
Equine herpesvirus has been able to adapt within the horse host and thereby undergo
a period of latency during which time an affected horse shows no clinical signs of
infection, yet still can actively shed virus.
EHV-1 is highly infectious and can be transmitted through fomites, aerosols, an aborted
fetus or placental parts, or through direct horse-to-horse contact. Broodmares might
provide continuous horizontal exposure from dam to foal, particularly if the virus
underwent viral recrudescence from stress associated with pregnancy and foaling.
In horses, EHV-1 has the ability to infect many cell types: endothelial cells of inner
organs, respiratory epithelial cells, and mononuclear cells in lymphoid organs and
peripheral blood. Latent virus can "hide out" in lymphocytes and/or sensory nerve cell
bodies within the trigeminal ganglia. Reactivation of virus—particularly during periods
of stress—enables it to spread to susceptible horses through the respiratory tract. Not
all "infected" horses show signs of illness; some are simply silent viral shedders.
As the competition season
begins for many riders in the
United States, it is particularly
important to implement
increased biosecurity measures
against infectious disease.
One of the more concerning
outbreaks to control is
that of equine herpesvirus
myeloencephalopathy (EHM, or
neurologic herpesvirus) caused
by equine herpesvirus type 1
(EHV-1).
This article was first published in EquiManagement magazine.
Harnessing Practice Health
