A cataract is an opacity of the lens inside the eye. Not all cataracts lead to significant visual loss, but if they are severe, blindness can result. As cataracts progress, the eye can become cloudy blue or grey, leading to reduced vision and, in some advanced cases, inflammation and other problems such as glaucoma.

There are many different causes of cataracts. Some are inherited (genetic) in certain breeds, but sometimes they can be secondary to other conditions such as diabetes, intra-ocular inflammation (uveitis), trauma, retinal disease, or old age. Often, the cause of cataracts is not identified (idiopathic).