By Peter Fenton BVM&, MRCVS
Uveitis, also sometimes known as moon blindness, is a painful condition of the eye and can be caused by a multitude of different factors. An initial episode of acute inflammation can lead to the development of equine recurrent uveitis (ERU), an autoimmune disease that can be challenging to treat. Uveitis should be treated as an emergency and veterinary assistance should be sought as soon as possible.
Uveitis is inflammation of some structures within the eye, these include the iris, ciliary body and the choroid. The inflammation associated with ERU can lead to retinal disease, which if severe enough can lead to blindness in the affected eye. Uveitis can be defined as anterior or posterior depending on which of these structures are affected. An initial episode of uveitis can be caused by a number of different factors and treatment will depend on what the primary trigger is. This initial episode can be followed by periods of quiescent and active disease therefore developing the syndrome known as equine recurrent uveitis. ERU is a complex disease and is now believed to be multi-factorial. This means that there are a number of risk factors for the disease and they way in which these interact increases a horse’s chance of developing ERU. Certain breeds of horses high a higher risk of developing ERU. It has been noted that in Appaloosas there is a much higher prevalence of the disease. In addition to this those Appaloosas that have a lighter coat seem to be at a greater risk of developing the disease. However even in other breeds ERU is the leading cause of blindness in horses, meaning that medical treatment should be sought as soon as possible.
Although there are a number of different causes of uveitis there is one infectious agent that warrants further discussion. Leptospira is a form of bacteria that has been linked to the development of uveitis in some horses. This is suggested to explain the differences in prevalence of ERU between different geographical areas, as they will have varying levels of leptospira in the environment. For example studies from certain areas of Germany have seen 50% of cases of ERU that are associated with leptospira. The mechanisms by which leptospira cause uveitis is not fully understood but it is hypothesised that they are somehow interfering in the immune system of the eye. The bacteria can also cause other clinical signs such as abortion and there are currently no vaccinations available against the disease.
Horses with acute uveitis show clinical signs associated with ocular pain, these can include ocular discharge, closure of the eyelids, corneal oedema (a hazy blue appearance to the eye) and swelling of the area around the eye. A veterinary examination at this stage may reveal a constricted pupil (miosis), protein in the anterior chamber of the eye (aqueous flare) and a decrease in intra-oscular pressure. Other changes related to the chronic condition can also be noted, this includes changes in the iris and cataract formation. After this examination is done your veterinarian may then stain the eye with a green/yellow dye to check the cornea for damage and use an ophthalmoscope to check deeper structures in the eye.
After a diagnosis of uveitis is made treatment will commence, aiming at reducing ocular pain and inflammation in the hope of preserving vision. If there is an underlying cause for the episode this should be identified and treated accordingly. Both topical and systemic inflammatories will then be used, often in the form of eye drops and oral medication. It is also important to dilate the pupil to prevent permanent changes to the eye. This is achieved through the administration of topical atropine, which can lead to decreased gut mobility known as ileus. Therefore horses being treated with this drug should be monitored for signs of gastrointestinal discomfort. Depending on the severity of the disease some of these topical treatments may need to be administered every 2-4 hours. This can cause a few problems as horses do not tend to tolerate this very well for a prolonged period of time and it is very important that they are being medicated appropriately. If a horse becomes difficult or dangerous to medicate the veterinarian may place a drain into the horse’s eyelid to make treatment easier. In some severe cases it may be appropriate to have the hose hospitalised to ensure they are able to receive their topical treatment.
There are a couple of surgical procedures that are being used in some places to attempt to control uveitis long term. The first of these is the placement of a cyclosporin implant (immunosuppressant) with the eye. This aims to reduce the immune response and chance of developing uveitis, therefore offering a long term treatment option for those horses suffering from ERU. Another procedure being performed is known as a vitrectomy and is particular popular in places such as Germany. This involves the removal of the vitreous and replacement with a suitable alternative. The theory being that this is also removing organisms such as leptospira and inflammatory cells that are leading to chronic inflammation within the eye. Both of these surgical techniques are relatively modern and further research will have to be undertaken to determine their effectiveness.
There are a number of recognisable equine medical emergencies, for example colic, however ocular disorders such as Uveitis should also be placed in this category. As it is hard to determine the significance of clinical signs associated with the eye without proper veterinary training, it is always advisable to seek a professional opinion. The prognosis of horses with uveitis is highly variable and depends on the underlying cause. Horses that suffer from ERU carry a fairly guarded prognosis due to the possible development of cataracts, lens luxation and blindness. It is always important to seek early intervention and carefully monitor those horses with the disease to ensure not permanent ocular damage occurs.