Vet Vibes: The eyes have it

VET VIBES

The eyes have it

Eyes are precious and should be treated with care. DR DOUG ENGLISH goes into detail in this informative article.

It’s important to be very clear that if your horse has either injured their eye or has an ailment of some kind, calling you vet is always the best policy. That said, having a basic understanding of the eye and potential problems can help you gauge the severity of the situation. Ulcers: Ulceration typically involves the

cornea (the eye’s outer coating) and

is usually the result of a trauma from

bushes; sticks; displaced hoods, halters

and fly masks; and even clods thrown up The cornea is a very tough organ. Composed of three layers, it resembles an onion because there are layers within each primary anatomical area. Treatment for an uninfected cut to the cornea is relatively easy - it will heal quickly, even within a day. Horses have a third eyelid (the nictitating membrane) that protects and produces substances that heal any damage to the cornea.

Rinse and clean the eye with a warm saline solution. A warm compress is helpful because it encourages tears and calms inflammation and pain. It if you have a tube of eye ointment on hand, apply it after cleaning the eye (first ensuring the ointment doesn’t contain a cortisone like prednisolone, hydrocortisone, or triamcinolone, which will delay healing and could lead to rupture of the eye). However, if the cut becomes infected the situation can be serious and you should call your vet.

Vision: The pupil is the circular area of the lens that is exposed by the iris which dilates and contracts according to the ambient light. In a normal horse, both irises are the same size. A difference in size is caused by concussion and brain issues such as tumours and infections. If there is anisocoria (unequal pupils) then a veterinary examination is essential.

If the pupils are constricted (miosis) then there is a pain event that needs diagnosis. If gentle pressure on the globe of the eye through the eyelid elicits pain and you cannot see anything wrong with the eye then there is a problem (such as glaucoma) that needs to be identified. Dilation of pupils (mydriasis) is an indication of trouble and causes include:

ƒ Various plant toxins (e.g. bracken fern) which destroy thiamine and produce cortical blindness with dilated eyes.

ƒ Other plants causing blindness are those containing pyrrolizidine alkaloids such as the Rattlepods and

Senecio, a genus of flowering plants that includes ragworts and groundsels.

Other signs are poor muscle control, discomfort in bright light, aimless wandering, and head pressing.

ƒ Alkaloids in plants such as Datura and Nightshades cause mydriasis and impaired vision.

ƒ Ivermectin overdose produces mydriasis and younger animals are more sensitive. Other effects are poor muscle control, lethargy, muscle twitching, drooping lips, recumbency, and coma. Horses that recover do not have residual effects.

ƒ Trauma to the head by falling over and striking the poll or blunt trauma from running into a fixed object can cause permanent blindness with dilated unresponsive eyes as a result of the optic nerve being severed by moving fractures of bone. ƒ Optic neuritis from the migration of parasites, orbital infection, guttural pouch disease, and the accidental intra-arterial injection of phenylbutazone. ƒ PPID (Pituitary Pars Intermedia

Dysfunction) or Cushings disease has ocular manifestations with bulging fat pads above the eye sockets, and weeping eyes. ƒ Vitamin A deficiency is rare in well fed horses on good pastures. In drought conditions accompanied by poor dry grass, a vitamin A supplement should be given to avoid retinal atrophy; night blindness; tearing; abnormal thickening of the cornea; and if extreme, blindness with dilated unresponsive pupils.

Neoplasia: Squamous cell carcinoma of conjunctival tissue and the third eyelid are a common cancer. Surgical removal with wide margins is the gold standard response. Onchocerca: Caused by a parasite. The adult, which lives in the nuchal ligament running along the neck at the base of the mane, produces larva that migrate through the tissues and cause a dermatitis reaction when they ultimately die, as well as an eye lesion producing depigmentation in the upper eyeball.

Tetanus: signs are ‘flashing’ of third eyelids and retraction of the eyeball as the horse reacts to a menacing action (e.g. waving your arms around in front of them). A serious disease, the sooner antiserum is given the better the prognosis.

Botulism: from chewing bones. Signs are a retracted eyeball, droopy eyelids, mydriasis, staggering and paralysis.

ERU: Equine recurrent uveitis (moon blindness) is the most common blindness in horses. It is a severe autoimmune syndrome characterised by recurrent episodes of inflammation separated by periods of remission. The

If your horse has either injured their eye or has an ailment of some kind, calling you vet is always the best policy.

condition is complex, and very difficult to accurately diagnose and treat.

Leptospirosis: historically associated with ERU, leptospirosis is a family of water and sewer dwelling bacteria with 21 types commonly found in Australia. Rodents both introduced and native are active shedders. There is no recognised vaccine for horses but there is for dogs, cattle, and pigs. The organism is responsive to antibiotics like penicillin and doxycycline.

Vestibular disease: signs are staggering, head tilt and involuntary rhythmic eye movement.

Horner’s Syndrome: caused by nerve damage, the signs are droopy eyelids, sweating on the head and neck, mitosis, retracted eyeball, elevated third eyelid, increased tearing, no loss of vision. Causes are many and include trauma, cancers, and guttural pouch infections.

Viral Encephalitis: West Nile (Kunming) virus is spread by mosquitoes. This disease has been limited to horses in Australia. Symptoms include staggering, blindness, muscle tremors, altered mental state, depression, circling, and excessive physical sensitivity.

Managing visibility issues

The horse has a wide peripheral monocular (two dimensional) vision and narrow binocular (three dimensional) front vision. If one eye is blind then there is a wide area on that side that the horse cannot see. To avoid startling, talk to them if you approach them on their blind side.

Chronic irritation from dust and flies, conjunctivitis, and blocked tear ducts can cause weeping eyes. Make a saline wash in a spray bottle (9gms sea salt per litre of water) and, with the nozzle set to misting, spray into the irritated eye and into any wounds. Accurately measured, 0.9% saline is the same concentration as blood and does not sting, particularly if the water is at body temperature.

Fly masks: Prevention is always better than cure, and fly masks are a great option. But not all masks are created equal. As a minimum, look for a mask made from black 95% UV proof mesh

Anatomy of a horses eye (Diagram courtesy Lynne Sandmeyer, DVM).

(black allows better vision than grey, and UV rated mesh won’t melt in really hot weather). The mask should have a builtin visor so it can’t collapse down and touch the eyes.

Some masks offer 100% UV protection and give improved clearance around the face area so that no part of the mask touches the horse. If your horse has had eye surgery, look for a solidly constructed recovery visor with at least a 92% UV rating. And finally, avoid masks with lightweight mesh, which, when they get wet, can become heavy enough to touch the eyes and cause irritation or an injury.