Walking On Volume 4, Issue 11, November 2017 | Page 10
For the Health of It
Equine Herpesvirus Associated Ocular Disease
Reprinted with permission from the Equine Disease Quarterly, Volume 25, Number 2
Herpesviruses are widely dis-
seminated 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 be-
lieved to be triggered by stressors
such as exercise, pregnancy, chang-
es 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 pri-
mary cause of viral keratitis in the
horse. Nevertheless, the virus has
been inconsistently found in cor-
neal and conjunctival scrapes of af-
fected animals and can be frequent-
ly 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 EHV-2 induced
ocular disease is based on several
reports of successful virus isolation
from outbreaks and single cases
of kerato- conjunctivitis, conjunc-
tivitis, 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 usually
accompanied by systemic disease.
10 • Walking On
Clinically, horses have sporadic,
multifocal, subepithelial, punctate
opacities that stain variably with
uorescein but commonly with Rose
Bengal. Superficial vascularization
is associated with the opacities, and
horses exhibit persistent to mod-
erate 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 repli-
cation 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 antivi-
ral 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 sub-
sequent 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 stud-
ies. Focal and multifocal lesions,
either unilateral or bilateral and
located in the non-tapetal fundus as
donut-shaped depigmented re-
gions with pigmented centers were
observed several weeks after in-
fection. This type of lesion is most
likely caused by ischemic injury of
the chorioretina with death of the
overlying retinal pigment epitheli-
um. These findings do not seem to
impact vision, but are a main con-
cern during pre-purchase examina-
tion, particularly if previous history
is unknown.
Although there is sufficient evi-
dence to support the role of EHV-1
in chorioretinitis, the implication of
any member of the equine her-
pesvirus family in corneal disease
remains controversial.
CONTACT:
Sonia Gonzalez-Medina
Ldo Vet, CertAVP(EM), MRCVS 44-
2032148025 (Ext 4025) Royal Veterinary
College London, United Kingdom