Moon blindness in horses has nothing to do with the moon, but historically many people though that the occurrence or reoccurrence of the disease become worse during different phases of the moon, hence the name.
The correct name for moon blindness is equine recurrent uveitis or ERU and is still not fully understood by vets and horse owners. There are several different factors or precursors that can bring on the first episode of moon blindness, often it's relatively mild in nature and may not be problematic for the horse at all. Many times the first episode seems to correct itself, lulling the owner into a false sense of security about the horse's eyes. Every subsequent bout of the disease causes greater damage to the eye and the long term prognosis for horses with ERU is total blindness at some point in time. Since the episodes of moon blindness may occur frequently, within one or two weeks of each other or many only happen once a year or once every several years each horse will have a different progression of the condition.
Moon blindness is characterized by a pain in the eyes and a hypersensitivity to any type of light. The horses may suddenly shy or not want to have anyone working around the eye or touching the head. The eyes often tear excessively and the sclera or whites of the eyes become red and inflamed looking. The horses will have eye spasms that cause the eyes to blink randomly. The pupils and iris will appear to have a cloudy film and a thick, puss-like discharge will be noted from the eye within three days. Depending on the duration of the episode and how much damage was done to the eye, cataracts and pigmentation changes will occur that will eventually decrease vision over time. Glaucoma and limited pupil movement will become more pronounced as the horse ages. Each episode of moon blindness further damages the eye and contributes to the scarring and secondary problems.
ERU is linked to a bacterial known as Leptospira that is commonly present in horses exhibiting moon blindness. Since Leptospira can be transmitted from the urine of other infected animal species it is critical to keep horses in top condition to allow their immune system to fight off the bacterial infection. Moon blindness can also be triggered by a trauma to the eye, sensitivity to parasites such as strongyles or onchocera cervicalis or exposure to equine herpes virus or influenza. The exact cause of moon blindness is the horse's bodies reaction to these triggering factors that causes an immune-mediated process in the eye.
While there is not a cure for moon blindness immediately using anti-inflammatory drops in the eye will prevent further damage to the structure of the eye. In addition the drops will contain medications that cause the pupil to dilate, so the horse should be kept in a dark stable or even have the head covered with a dark hood to help prevent both pain and the sensitivity problems. Treating the trigger for moon blindness is the best possible management of the condition. Tests for Leptospira, Onchocera cervicalis and strongyles should be done immediately and treatment started with the necessary medications.