By Vet Hamish Leslie
Uveitis is one of the most frequent causes of blindness in horses.
It is a painful, inflammatory condition of the middle layer of the eye and can cause significant discomfort.
There are many factors associated with flare-ups of uveitis. Here is a summary of how flies may play a part.
What Is Uveitis?
Uveitis is the inflammation of one or more parts of the uveal tract, the layer between the retina and the outer sclera, made up of the iris, ciliary body and choroid. It can be either acute or chronic. Acute uveitis causes sudden and very painful symptoms, while chronic uveitis persists and can cause permanent damage to the eye.
Uveitis can be either primary or secondary. Primary uveitis is where there is no identifiable underlying cause, or secondary, where it is triggered by trauma to the eye, infections or systemic diseases.
Equine recurrent uveitis (ERU) is a condition where the horse has suffered repeated episodes over time and can ultimately lead to blindness if left unmanaged.
Recognising the Signs of Uveitis
Early detection is vital for the best prognosis from uveitis. Clinical signs of uveitis include squinting or excessive blinking, eye discharge, colour change in the eye, constricted pupil and swelling around the eye. Horses may appear to have a dropped upper eyelid, resulting in the angle of the eyelashes pointing downward.
Chronic cases left untreated may also lead to cataracts (cloudy lens), retinal detachment, glaucoma (leading to damage to the optic nerve) and shrunken or misshapen eyes.
If you notice any of these signs, you should contact your vet for examination of the horse.
How Flies Are Involved
Stable flies, houseflies and horse flies are the most common flies which irritate horses. Not only do they cause annoyance, but they can also aggravate or even cause ocular disease. Firstly, irritation is a major factor. Flies group around the eyes to feed on tears and other ocular secretions. Their constant presence can irritate and introduce bacteria or foreign particles into the eye, which can therefore cause inflammation.
Flies can be vectors for infections when they encounter infected fluids and then move to another horse’s eye, transmitting pathogens that cause an immune response and leading to inflammation.
Finally, flies can cause exacerbation of pre-Existing Conditions. In horses with a history of uveitis, flies around the eyes can cause a relapse or worsen symptoms due to irritation, stress, and secondary infections.
Fly Control
Fly control is an essential part of managing uveitis-prone horses. Measures include using fly masks protect the eyes and offer some UV protection, environmental management reduces attraction to flies and breeding areas, repellents around the face but never directly in the eyes and using a screen or fan around barns and stables to disrupt the movement of flies.
Veterinary Management of Uveitis
In the unfortunate case of uveitis, a vet will likely prescribe the following medication. Topical corticosteroids are used in the eye to reduce inflammation. NSAIDs (nonsteroidal anti-inflammatory drugs) and mydriatics to dilate the pupil and reduce pain.
For recurrent or severe cases, further treatments such as cyclosporine implants or even enucleation (eye removal) may be considered to improve the horse’s welfare.
In conclusion, uveitis is a serious, potentially vision-threatening condition in horses, and flies are a common yet underestimated factor in its development. Through proactive fly control and prompt veterinary care, horse owners can significantly reduce the risk of flare-ups and help maintain their horse’s eye health and overall quality of life.