The Horse

NOV 2018

The Horse:Your Guide To Equine Health Care provides monthly equine health care information to horse owners, breeders, veterinarians, barn/farm managers, trainer/riding instructors, and others involved in the hands-on care of the horse.

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24 November 2018 The Horse | "We surveyed hundreds of horse owners in Queensland, Australia, and only 3.3% of those owners felt that their horse had a medical concern involving their eyes. We subsequently conducted complete ocular examinations in 339 of the 974 horses we obtained completed surveys for and found that almost 88% actually had abnormal ocular findings," says lead author Fernando Malalana- Martinez, DVM, GPCert(EqP), Dipl. ECEIM, FHEA, MRCVS, senior lecturer in equine internal medicine at the Univer- sity of Liverpool's Institute of Veterinary Science, in the U.K. "An estimated 1 to 2% of the American equine population currently suffers uni- lateral (in one eye) or bilateral (in both) blindness, equivalent to approximately 95,000-190,000 horses. This is a substan- tial number of horses, making vision loss an important issue in equine operations," Dwyer adds. Let's take a closer look at potential causes of ocular abnormalities and vision loss in horses. We'll also describe behav- iors and signs you can watch for that suggest visual impairment. Finally, we'll review management strategies for helping horses deal with deteriorating eyesight and blindness. However, we haven't included details regarding treatment of specific ocular conditions because they lie beyond this article's scope. Dwyer encourages owners to maintain an open mind as they read this article. "It's very important for owners to recog- nize that many horses without any vision at all can be successfully managed and potentially even continue to compete ath- letically," she says. "Deteriorating vision is not synonymous with a death sentence." Leading Ocular Abnormalities Trauma Considering the large size of the horse's eyes relative to his head and the proximity of those eyes to the ground, where dust and debris, vegetation, and other horses' tails and feet tend to aggre- gate, it's no surprise that trauma remains a leading cause of equine ocular issues. "Trauma causing injury to the surface of the eye, called the cornea, is usually readily observable," says Dwyer. "A red, painful, swollen eye that the patient holds closed with an obvious defect or even embedded foreign body makes diagnosis relatively straightforward." Indeed, data collected in the above- cited survey by Malalana-Martinez and colleagues showed that "owners more readily identify corneal lesions (than other types of ocular abnormalities), which often occur in cases of trauma." Malalana-Martinez did additionally note, however, that "very few owners reported an ocular traumatic injury as a specific entity—0.3% of 974 horses." Trauma cases must be addressed im- mediately because secondary infections— both bacterial and fungal—can develop rapidly, potentially leading to more advanced and serious disease, including melting corneal ulcers. "Some horses lose vision if ulcerative keratitis (fungal infection of the cornea) advances to infection within the globe (eyeball) or the corneal disease becomes so severe that enucleation (eye removal) is required," says Dwyer. "Fungal keratitis that is severe frequently results in vision loss, even if the keratitis is eventually controlled. "A simple injury can quickly manifest into an even larger, more complex, ex- pensive, and potentially eye-threatening condition," she adds. "Never wait to have any horse with any eye problem exam- ined by a veterinarian." Cataracts and retinal atrophy "In addition to trauma, leading causes of ocular abnormalities noted in our study included cataracts and age-related retinal atrophy," says Malalana-Martinez. Specifically, 34.3% of the horses had cataracts and 31.8% had senile or age-related retinal atrophy, which is degeneration of the membrane lining the back of the eye that essentially transmits information from the eye to the brain. An additional 10.1% of examined horses also had so-called "bullet-hole" lesions in their retinal tissues, which are pin-sized defects that might or might not impair vision. "Cataracts are areas of focal or diffuse cloudiness within the lens of the eye," says Dwyer. "As with other species, cata- racts can affect vision, and both mature and hypermature (beyond full develop- ment) cataracts can be blinding." "In terms of the retina, I think probably senile atrophy is more significant to these horses than bullet-hole lesions, as these have to be quite extensive to significantly affect a horse's vision," says Malalana- Martinez. Uveitis The most common cause of blindness in horses is equine recurrent The eye conditions that most commonly cause vision loss in horses are (left to right) cataracts, equine recurrent uveitis, and glaucoma. PHOTOS COURTESY DR. FERNANDO MALALANA-MARTINEZ An estimated 1-2% of the U.S. horse population suffers some form of blindness DEALING WITH DETERIORATING VISION

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