For the Health of It
Equine Herpesvirus Associated Ocular Disease
Reprinted with permission from the April 2016 edition of Equine Disease Quarterly.
Herpesviruses are widely disseminated in nature and renowned
in equine medicine for inducing
respiratory, reproductive and
neurological diseases. The most
important biological characteristic
of these viruses is their ability to
remain latent. Reactivation of latent
virus can occur and result in virus
replication, shedding, and disease.
While the precise mechanisms of
reactivation are unclear, it is believed to be triggered by stressors
such us exercise, pregnancy, changes in management, or concurrent
disease. The equine herpesviruses
(EHV) that have been implicated in
ocular disease, to date, are EHV-2
and EHV-1.
Equine herpesvirus-2 has been
proposed as the putative and
primary cause of viral keratitis in
the horse. Nevertheless, the virus
has been inconsistently found in
corneal and conjunctival scrapes
of affected animals and can be
frequently found in asymptomatic
horses. In addition, co-infection
with EHV-5 is common, not only
in ocular disease but also in respiratory cases, making it difficult to
attribute causation of disease to a
particular virus. Evidence for EHV2 induced ocular disease is based
on several reports of successful
virus isolation from outbreaks and
single cases of keratoconjunctivitis,
conjunctivitis, or corneal edema in
foals; however, despite such reports
its role as a pathogen is still debatable. In contrast to other viral
etiologies, herpetic keratitis is not
12 • Walking On
usually accompanied by systemic disease. Clinically, horses have
sporadic, multifocal, subepithelial,
punctate opacities that stain variably with fluorescein but commonly with Rose Bengal. Superficial
vascularization is associated with
the opacities, and horses exhibit
persistent to moderate ocular pain
that can lead to reflex uveitis. Continuation of the epithelial lesions
into the corneal stroma is possible
as a consequence of viral antigens
extending from epithelial lesions
or from viral replication in stromal
cells. Evidence of virus in corneal
and/or conjunctival scrapes may be
challenging as viral inclusion bodies, within injured epithelium, are
rarely seen at the time of sampling.
PCR techniques are more reliable
in detecting viral DNA; however,
the presence of herpetic DNA is
not diagnostic of itself. Corneal
lesions in the horse seem to resolve
quickly after antiviral treatment
or heal spontaneously, which is
in contrast to human and feline
induced herpesviral ocular disease.
Treatment with topical antivirals
is often recommended if there is
a strong suspicion of herpesvirus
involvement and subsequent PCR
confirmation. Topical ganciclovir
gel in combination with topical corticosteroids has been recommended
as the treatment of choice.
Chorioretinal lesions have been
described following experimental
EHV-1 infection in several studies.
Focal and multifocal lesions, either
unilateral or bilateral and located
in the non-tapetal fundus as donut-shaped depigmented regions
with pigmented centers were observed several weeks after infection.
This type of lesion is most likely
caused by ischemic injury of the
chorioretina with death of the overlying retinal pigment epithelium.
These findings do not seem to impact vision, but are a main concern
during pre-purchase examination,
particularly if previous history is
unknown.
Although there is sufficient evidence to support the role of EHV-1
in chorioretinitis, the implication of
any member of the equine herpesvirus family in corneal disease
remains controversial.
- Sonia Gonzalez-Medina, Ldo
Vet,
CertAVP(EM),MRCVS
44-2032148025 (Ext. 4025)
Royal Veterinary College
London, United Kingdom