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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26 November 2018 The Horse | uveitis (ERU), also referred to as moon blindness, a condition most owners have at least heard of, particularly in the Appa- loosa world. Uveitis itself is defined sim- ply as inflammation of the uvea, which comprises several tissues inside the eye, including the iris. Classic signs suggestive of uveitis include a red, painful, cloudy eye accompanied by miosis, a profound constriction of the pupil. Unfortunately, even with aggressive treatment, about half of all horses with uveitis eventually suffer severe vision loss. "Reasons for loss of vision associated with ERU vary," says Dwyer. "Some horses suffer detached retinas, others go blind from maturing cataracts, others lose vision when the eye is so damaged that it just scars in on itself and becomes what we call phthisical." Glaucoma Defined as a "multifactorial neurodegenerative ocular disease," glaucoma typically appears as eyes that are "big and blue," either over the whole corneal surface or a portion of it. This condition is relatively uncommon in horses and was not identified in any horses in Malalana-Martinez's study. "When it does occur, glaucoma fre- quently occurs secondary to uveitis and requires aggressive multimodal therapy to address the underlying changes in the eye," Dwyer says. Other Less common causes of vision impairment and loss stem from a wide variety of injuries/diseases, including: ■ Cranial trauma, especially training accidents in which a horse flips over, causing skull trauma that affects the visual pathways. ■ Neoplasia (abnormal growths) involving the globe, retrobulbar region of the orbit behind the eye, or brain region near the optic nerves. Neoplastic conditions can either arise primarily in the orbit or can start in the paranasal sinuses and spread or expand into the orbit. ■ Congenital defects in which foals are born with rudimentary, nonfunctional eyes. "There is actually a genetic condition called 'multiple congenital ocular anomaly' that has been linked to the silver dapple coat color," says Dwyer. "Like many genetic linked diseases in horses, it presents with a spectrum of severity, ranging from minor variants to blinding sequelae. I have seen this syndrome in horses of this coat color, as well as Miniature Horses and some other breeds that are both silver dapple and other coat colors." Dwyer also describes having seen West Nile virus cause acute blindness in a few horses within 48 hours of onset of clinical signs. What Impaired Vision Looks Like Based on the available data, it seems horses are masters of disguise, frequently hiding deteriorating eyesight behind their lovely lashes. Owners shouldn't feel bad about not recognizing their horses' diminished vision; veterinarians attest that even the most experienced horseperson can be fooled about what a horse can or cannot see. "Ridden horses with major ophthalmic abnormalities, including extensive and bilateral lens opacification (a "fogginess" to the lenses) commonly show no behavioral evidence of visual compromise," said Andrew G. Matthews, BVM&S, PhD., Dipl. ECEIM, honorable Member ACVO, FRCVS, an equine ophthalmologist from Scotland, during a presentation at a recent AAEP Focus on Ophthalmology session. "On the contrary, however, horses with minor abnormalities such as central focal lens opacities or vitreal 'floaters' can exhibit behavior suggestive of visual dysfunction." This can include headshaking, shying, and/or exaggerated startle responses. "Other horses with gradually declining vision, which occurs more commonly than a sudden loss of vision, can begin to show signs of hesitation or uncertainty in certain situations, herd behavior may change, and they can actually be observed bumping into objects or obstacles," adds Dwyer. In his presentation Matthews described the following abnormalities and how they can potentially cause visual dysfunction: ■ Retinal issues presumably impact visual acuity (this is the 20/20 type of vision reported in humans, indicative DEALING WITH DETERIORATING VISION SHELLEY PAULSON When assessing a horse's vision, veterinarians might perform a dazzle reex test to see how the animal reacts to illumination of his eye.

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