You should seek the advice of your veterinarian if your pet is ill as only he or she can correctly advise on the diagnosis and recommend the treatment that is most appropriate for your pet.

When you look into the eyes of an animal the black pupils in the middle of the eye are usually the same size and shape

The hole in the centre of the coloured iris is called the pupil. It usually appears black, but sometimes a bright coloured reflection from the retina at the back of the eye can be seen (yellow, green, blue or red are common colours), and occasionally a white discoloured lens (called a cataract) can be seen through the pupil - particularly in old animals, or animals with diabetes mellitus. 

Different species of animal have different shaped pupils - dogs are round, cats are oval and snakes are very slit shaped. The size of the pupil will vary depending upon how bright the light is in the environment. In very bright light the pupil will be very small, in dark light the pupils dilate and become very large to allow as much light as possible to enter they eye. When extremely dilated pupils that usually look oval in daylight may appear to be round.

Normal cats eyes in daylight 

A normal cats eye - dilated  pupil 

However, in all pets the pupils of both eyes should be the same size....because both pupils work together. This is complex to understand, but basically if a bright light is shone into the left eye, BOTH pupils will become small - so the response to light reflex is linked to both eyes. 

Because the muscles surrounding the pupils contract or relax together the shape of the pupils should also be the same in both eyes.

What should you do if your pet has odd-sized or odd-shaped pupils ? Ask your veterinarian to examine the eyes . There are several reasons why one pupil might be larger or smaller than the other, or odd-shaped - for example  if one eye has been injured or inflamed in which case adhesions can form stopping the muscle around the pupil from contracting properly, and also distorting the pupils shape. Also, damage to the nerve supply to the muscle may alter its ability to work properly.

Finally, shining a bright light into the eye is a useful test because a positive response means that the eye can "see" and isn't blind.

Updated October 2013