How to deal with summer eye problems *H&H Plus*

Guard against sore, scratched or sun-damaged eyes this summer, with advice from Ria Chalder MRCVS

Summer can play havoc with a horse’s eyes, with flies, UV light, pollen and dry vegetation all posing potential risks.

Houseflies (the small, black pest termed Musca domestica) are attracted to the tears produced by a horse’s eyes and can often be seen swarming around the face. As they feed and breed on sewage, animal dung and other waste products, they can transfer bacteria on to the eyes and cause an infection called bacterial conjunctivitis.

Often referred to as “fly eye”, bacterial conjunctivitis can make the eyes weepy and red – often with a greeny-yellow discharge. The condition is not particularly painful. A mild bout can usually be managed by bathing the eyes twice daily with cotton wool pads soaked in sterile saline, or salty water that has been boiled and left to cool. The eyes should then be covered with a good-quality fly mask to prevent re-infection.

If this fails to improve symptoms within a day or two, call your vet. Any sign that your horse is squinting, or that one or both eyes seem painful, should be treated as a veterinary emergency.

Never buy over-the-counter eye drops for humans to treat your horse if you suspect that he has bacterial conjunctivitis. Using antibiotic drops without veterinary supervision increases the likelihood of resistant “superbugs” developing, which can make treatment of subsequent, more serious infections difficult. It also complicates matters if the infection fails to resolve and diagnostic tests are required.

A correctly fitting mask that completely covers the eyes, in combination with a strong insect repellent, will go a long way to preventing bacterial conjunctivitis. Homemade fly repellents are rarely effective. Instead, try one containing at least 2% permethrin, or a synthetic pyrethroid such as cypermethrin.

Light relief

In humans, long-term exposure to excessive amounts of UV light from the sun has been linked to damage to the retina, at the back of the eye, and the formation of cataracts. These abnormalities tend to develop after decades; since horses have a shorter lifespan, UV-induced damage inside the eye is less of a concern.

Even so, these invisible rays have a lot to answer for when it comes to equine eye problems.

UV light is partly responsible for a type of cancer called ocular squamous cell carcinoma (SCC). Tumours can develop in several locations on or around the eye, including the skin, the membrane that lines the inside of the eyelids (the conjunctiva), the third eyelid and the clear cornea at the front.

The tumours are usually pink, raised nodules with a rough appearance and tend to cause a thick, whitish-yellow eye discharge. Fortunately, this type of tumour does not tend to spread internally, but it can invade important structures in the area.

Horses with pink skin around their eyes are at greater risk of developing ocular SCC, as are certain breeds including Belgian Draughts and Haflingers. If your vet suspects that your horse has an ocular SCC, it may be necessary to take a sample of the tissue, known as a biopsy, which can be sent for analysis to confirm the diagnosis.

Treatment options depend on the location and/or size of the tumour. Removal of the tumour is recommended, in combination with an additional treatment such as chemotherapy (in the form of creams and injectables), radiation therapy, photodynamic therapy or freezing (cryotherapy).

Tumours can recur, however, after treatment. Researchers in America found that the only factor associated with a decreased risk of ocular SCC recurrence was the use of a fly mask with 90% or more UV light protection.

Although not proven, UV light also appears to trigger flare-ups of equine recurrent uveitis (ERU) – a condition that causes recurrent bouts of painful inflammation inside the eye and is the leading cause of blindness. While ERU is still poorly understood, owners of affected horses will testify that the sun and wind are major contributing factors to an episode of this devastating disease.

Again, it is advisable to use a fly mask that also blocks harmful UV rays. Sun visors are now available for use while the horse is being ridden, offering 100% UV protection.

Itch and scratch

Warmer, drier weather will also see an increase in the occurrence of corneal ulcers, the term for scratches on the surface of the eye. As trees and bushes become dry and spiky, a horse’s prominently positioned eyes are vulnerable to scratches as he grazes.

The problem is usually obvious, as the pain will cause your horse to squint and the eye to water. And the offending piece of twig or thorn may still be seen embedded in the surface of the eye.

A corneal ulcer requires immediate veterinary attention. Your vet will most likely stain the surface of the eye with a green dye called fluorescein to determine the size and depth of the ulcer. Prompt treatment, including pain relief and frequent application of antibiotic eye drops, is then essential to prevent sight-threatening infection.

Itchy eyes, also common during spring and summer, can be a sign of an allergic reaction such as allergic conjunctivitis. The eyes will tend to be puffy, the conjunctiva bright pink and the eyes slightly watery.

Treatment is similar to that of hay fever in humans: avoid the trigger. If your horse’s signs worsen when the pollen count is high, for example, try to keep him stabled during the day.

Although it may seem a relatively trivial disease, it is important to address allergic conjunctivitis with the help of your vet. A horse may try to rub his itchy eyes against sharp edges, potentially injuring himself. If they are so itchy that the horse is self-traumatising the eye area, your vet may administer strong anti-inflammatory medications to provide immediate relief.

While antihistamines are also commonly used to help relieve the clinical signs of allergies, their efficacy in horses is not well understood. They appear to work best as a preventative measure before the beginning of the allergy season, rather than once a horse is itchy. If he is affected this summer, it’s worth planning ahead next year.

Choosing and using protective fly masks

Fly fringes are relatively ineffective and do not provide any UV protection, so are best avoided. Instead, opt for a mesh fly mask that covers the eyes – this will not only keep bugs away, but will shield the eyes from sharp objects and foreign bodies while he grazes.

Invest in a quality mask with high UV protection properties, taking care to:

● Make sure it fits. Too loose and the mask will allow flies to crawl underneath it; too tight and it will chafe the skin. An ill-fitting mask may even rub the eyelashes or push them on to the surface of the eye, causing serious injury. A good design will feature several darts around the eye area, to ensure that
the mesh sits well clear. Additional features may include adjustable fastenings and soft padding at the brow and around the nose for comfort.

● Keep it clean. A dirty mask will shed dust into a horse’s eyes and
can make problems worse, so shake the mask out each time it is fitted and clean it regularly. A machine-washable version will make this easier.

● Check it frequently. Even if you are not riding your horse, take the mask off at least once a day to check his eyes and to ensure that he is not being rubbed.

Ref Horse & Hound; 11 June 2020

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