Horse Side Vet Guide ®

Equine Health Resource

Pupils Seem Large or Dilated

Code Green - Contact Your Vet to Obtain Useful Advice & Resources

Code Green - Contact Your Vet to Obtain Useful Advice & Resources

  • 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.

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.


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 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.

Author: Doug Thal DVM Dipl. ABVP


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