There are many breeds that are prone to excessive tear production just as some breeds are more prone to dry eye conditions and lack of tear production. Either condition can be a direct result of injury, genetic conditions or infections to the eyes that are inhibiting or stimulating the tear gland functioning.
In cases where excessive tearing is noted there are usually both genetic and health factors at work. In breeds such as the Maltese, Miniature Poodle, Yorkshire Terrier, Shar Pei, Pomeranian and American Cocker Spaniel excessive tear formation, also known as epiphora, is very common. In white and light colored dogs the result may be a reddish stain down the muzzle from the eyes, typically known as red yeast stain. In darker colored the dogs the tear stain may be much less noticeable, but there may be a slightly yeasty or dank smell about the head of the dog. These yeast infections become problematic as they can lead to further complications of the eye and increasing likelihood of bacterial infections. The constant tearing keeps the area moist, making an ideal growth and breeding area for both bacteria and yeast.
In breeds such as the American Cocker Spaniel there is actually an inherited defect in the tear removal system from the eye that causes the tearing. In normally functioning eyes the tears are produced from glands in the upper eyelid and the third or nictitating eyelid. These tears are produced when the eye becomes dry or irritated as a natural lubricant and moisturizer for the cornea or outer layer of the eye. The tears wash over the eye, providing nutrients and moisture, then drain into a small sac, the lacrimal sac, that is just below the eye under the skin. This sac empties into the nasal passage via a small tube or shunt. In some breeds the tube is close to the nostrils but in others it is farther back in the nasal passage, depending on the shape of the dog's head and muzzle. If the tube, known as the nasolacrimal duct, becomes plugged or stopped up at either end, tears can no long be naturally drained from the eye. The tears will, in fact, only be pushed out of the eye when the dog blinks, leading to the excessive tearing on the face.
In some breeds the tear ducts become inflamed or damaged due to other problems with the eyes such as entropion, ectropion or corneal ulcerations. This damage to the tear ducts can lead to excessive tear production or complete lack of tear production, depending on the type of long term damage that is caused by the primary condition.
Any type of infection or debris in the eyes will stimulate tear product as the body works to try to wash away the cause of the irritation. Some breeds are prone having hair that grows inwards, toward the eyes rather than outwards, away from the eyes. In these breeds the constant irritation of the hair brushing against the cornea or even against the lashes can trigger excessive tear production.
Excessive tearing should always be checked by the vet to determine if it is a structural problem of the eye or if it is due to an infection. In the case of infections antibacterial eye drops are often all that is needed for treatment. Clogged ducts can be treated with medications to dissolve the blockages and monitoring can be put in place to prevent the problem from reoccurring.