A reader asks: My dog’s eye is red and bloodshot. I know that she has seasonal allergies; is that all it is? How do I know whether it is something serious?

While itchy, red, and irritated eyes can definitely be a symptom of seasonal allergies in our dogs (as well as in ourselves!), the key that something more is going on is that only one eye is affected. This is a good reason to get it checked out by a vet.

Eyes are delicate — and very important — so don’t take chances here. It can be allergies, a cold, or a minor irritation, but it could be something much more serious.

In this case, the dog, Hannah, had scratched her cornea badly. The vet put in some drops that showed the scratch in vivid green. Hannah went home with eye drops. She was an amazingly cooperative patient (I’ve had otherwise sweet-tempered, you-can-do-anything-to-them dogs threaten to rip my arm off if I came near them with that eyedropper again). Medication seemed to help at first, but ultimately, Hannah needed surgery, but she’s recovering. It’s a good thing she’s nice about the eye drops, though, because she needs several types of drops and ointments during her long, uncomfortable recovery.

A scratched cornea is not the only thing that can cause a single red eye. The most common cause is an irritant — a piece of dirt, dust, or plant matter or a spray, such as a skunk’s spray or a household aerosol. But bleeding in the eye or clear discomfort (the dog is pawing at her eyes) can signal a serious health problem like glaucoma or a tumor. These are rare, but the only way to be sure is to get the eye examined.