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