What you see. The starting point for addressing any equine health related issue is your observation.


Pupils Seem Large or Dilated


The size of the pupil relates to the muscle of the iris, which is controlled by nerves that go from the brain into the back of the eye. Horses' pupils are dilated in response to low light conditions. Often, horses that are very excited or anxious also have dilated pupils. In most cases, when people notice changes in pupil size, it is not of much concern and relates to one of the factors above.

When assessing pupil size, it is important to compare one pupil to the other and to those of your other horses. Pay attention to whether the two pupils are being exposed to the same light. In rare cases, and usually in horses showing other signs of a problem, a dilated, unresponsive pupil in a horse can also be a sign of brain, nerve or eye injury.

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    • If you are convinced that there is a significant difference between the two pupils but the horse seems normal otherwise.
    • If the horse's appetite and attitude are normal and you see nothing else wrong.
    • If the results of the Whole Horse Exam (WHE) suggest the horse is otherwise normal.

your role


What To Do

Assess your horse's general health using the Whole Horse Exam (WHE), and compare the appearance of the two eyes. Consider the light in the area in which you are examining the horse. Is it similar on the right and left sides? Monitor appetite and attitude and contact your vet with your findings and concerns.

your vet's role

Your vet looks at signs like this in light of the general health of the horse, and the results of the rest of the neurologic and eye examination.
Questions Your Vet Might Ask:
  • How do the pupils compare to one another?
  • How does the finding compare to other horses?
  • Is the horse on any medication?
  • What are the light conditions that you are examining in?
  • Is the horse excited?
  • Do you notice anything else wrong with the horse's eye?
  • Is the eye watering or swollen?
  • What are the results of the Whole Horse Exam (WHE)?

Diagnoses Your Vet May Consider

The cause of the problem. These are conditions or ailments that are the cause of the observations you make.

Very Common
Less Common
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Treatments Your Vet May Recommend

A way to resolve the condition or diagnosis. Resolving the underlying cause or treating the signs of disease (symptomatic treatment)

Very Common
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Author: Doug Thal DVM Dipl. ABVP