Corneal Ulcers, and Why Flat-Faced Dogs Get Them
A squinting, watering eye can be a corneal ulcer. Why flat-faced breeds are 11 times as likely to get one, and why a painful eye needs a vet now.
Written by thecanineledger
If your dog is squinting, holding an eye shut, pawing at it, or the eye is watering, red or cloudy: see a vet now. You can’t tell an ulcer from glaucoma or an injury at home, and some of these can cost the eye. Don’t put old eye drops in it while you wait.
In a study of more than 100,000 dogs at English primary-care practices, flat-faced breeds were “11 times as likely” as crossbreds to have a corneal ulcer (O’Neill et al., Canine Genetics and Epidemiology 2017). More than 1 in 20 Pugs in the study had one.
While a sore eye can look minor, an ulcer is an open wound on the clear front of the eye, and it hurts. This page explains what an ulcer is, why flat faces raise the risk, and why a painful eye needs a vet straight away.
1. What a corneal ulcer is
The Merck Veterinary Manual’s owner page explains that “The cornea is the clear dome on the front surface of the eye.” An ulcer is a sore where its surface layer has been lost (Merck, Disorders of the Cornea in Dogs).
The causes. “In dogs, most ulcers are caused by injury, such as nail scratches, foreign objects in the eye, or chemicals that enter the eye.” Infection usually comes second: “bacterial infection often occurs after the ulcer is already present.”
Why it matters. Ulcers can be shallow or deep. Merck warns that with a deep ulcer “the ulcer can create a full-thickness hole in the cornea.” The 2017 study lists the risks to the dog as “pain, reflex uveitis, perforation and even loss of the eye”. Uveitis is inflammation inside the eye.
2. The signs
Merck describes pain “shown by watering eyes and spasmodic winking”. In practice, owners may see:
- squinting, or holding the eye shut
- watering or discharge
- pawing or rubbing at the face
- a red eye, or a cloudy or bluish patch on the surface
A warning sign of infection. Merck says “A dense, white material at the edge of the ulcer indicates the presence of white blood cells and bacterial involvement.”
How the vet finds it. “To detect small ulcers, a veterinarian may put drops of a specialized dye into the eye.” In the 2017 study, the dye test was used in 92.8% of cases.
These signs overlap with glaucoma, an eye emergency. That is why the advice at the top of this page is the same as on our glaucoma page.
Our note: We say see a vet now because owners can’t tell an ulcer from glaucoma at home. Merck says only that eye problems should be addressed “promptly”.
→ Glaucoma: The Red, Painful Eye That Can’t Wait · → Emergency Signs in Dogs: When to Go Now, Not Tomorrow
3. Why flat-faced dogs get them
The 2017 study found 834 cases among 104,233 dogs, a prevalence of 0.80%. Compared with crossbreds:
- Brachycephalic (flat-faced) types had about 11 times the odds
- Spaniel types had about 3 times the odds
- Purebred dogs had about twice the odds
The breeds most affected. The Pug (5.42% of the breed), Boxer (4.98%), Shih Tzu (3.45%), Cavalier King Charles Spaniel (2.49%) and Bulldog (2.41%).
Why the face matters. The authors point to “a large palpebral aperture and a shallow socket”, meaning a wide eyelid opening and a shallow eye socket. Together these may play a role “by promoting greater exposure of the less sensitive cornea”. A prominent eye that the lids don’t fully cover is exposed and easier to scratch.
Eyelid problems add to it. A 2025 study from the same UK programme found that corneal ulceration was one of the commonest problems in dogs with entropion, where the eyelid rolls inward and rubs the eye (O’Neill et al., PLoS One 2025).
The authors’ conclusion. They suggest “that breeding focus on periocular conformation in predisposed breeds should be considered in order to reduce corneal disease.”
Our note: The odds ratios are from the 2017 study’s multivariable results (11.18, 3.13, 2.23), rounded. “Wide eyelid opening” is our wording for “large palpebral aperture”.
→ Choosing a Breed or a Mix: The Health Data Breeders Don’t Lead With
4. Treatment and outlook
Shallow ulcers. Merck: “Therapy for shallow ulcers usually consists of topical antibiotic(s) and correction of any causes, such as removal of a small splinter from the eye. The ulcers usually heal within a week.”
Slow-healing ulcers. “Syndromes of very slow-healing and recurrent shallow ulcers occur in dogs, especially in older animals.” For these, Merck says treatment “often includes removal of the dead, damaged, or infected tissue of the ulcer by a veterinarian, followed by prescription eye medications.”
Deep ulcers. “Many deep ulcers also require grafts of conjunctival tissue to strengthen the cornea.”
What happened in practice. In the 2017 study, 17.0% of dogs had surgery, 7.4% were referred to a specialist, and the ulcer contributed to the decision to euthanise 10 dogs. Pain was recorded in 46.2% of cases.
Why rechecks matter. An ulcer can change quickly, so your vet may want to see the eye again within days. Keep those appointments, even if your dog seems more comfortable.
Our note: Two lines here are ours, not sourced: “Don’t put old eye drops in it” and “your vet may want to see the eye again within days”. Some eye drops can make an ulcer worse, so use only drops your vet has given you for this problem.
5. What we could not verify
| Commonly said | What we found |
|---|---|
| “A watery eye will clear up on its own” | Not safe to assume. A painful eye can be an ulcer or glaucoma. You can’t tell them apart at home. |
| “Ulcers are always caused by infection” | No. Merck says most are caused by injury, with infection often following. |
| “Only flat-faced dogs get ulcers” | No. Any dog can get one. Flat-faced types had about 11 times the odds, and spaniel types about 3 times. |
| “An ulcer always heals in a week” | Shallow ones usually do. Merck describes slow-healing and recurrent ulcers, especially in older dogs. |
| “Leftover eye drops are fine to use” | We found no source we read on this. Don’t use any drops without your vet. |
Sources
- O’Neill DG, Lee MM, Brodbelt DC, Church DB, Sanchez RF. Corneal ulcerative disease in dogs under primary veterinary care in England: epidemiology and clinical management. Canine Genet Epidemiol 2017: VetCompass, 104,233 dogs
- O’Neill DG, Mustikka MP, Brodbelt DC, et al. Conformational eyelid disorders in dogs under primary veterinary care in the UK: epidemiology and clinical management. PLoS One 2025: VetCompass
- Merck Veterinary Manual: Disorders of the Cornea in Dogs (pet owner version; Gelatt, DACVO; last updated Sept 2024)
Always confirm with your vet before acting on anything on this page.