The domestic dog may be the most morphologically diverse terrestrial mammalian species known to man; pedigree dogs are artificially selected for extreme aesthetics dictated by formal Breed Standards, and breed-related disorders linked to conformation are ubiquitous and diverse. Brachycephaly–foreshortening of the facial skeleton–is a discrete mutation that has been selected for in many popular dog breeds e.g. the Bulldog, Pug, and French Bulldog. A chronic, debilitating respiratory syndrome, whereby soft tissue blocks the airways, predominantly affects dogs with this conformation, and thus is labelled Brachycephalic Obstructive Airway Syndrome (BOAS). Despite the name of the syndrome, scientific evidence quantitatively linking brachycephaly with BOAS is lacking, but it could aid efforts to select for healthier conformations. Here we show, in (1) an exploratory study of 700 dogs of diverse breeds and conformations, and (2) a confirmatory study of 154 brachycephalic dogs, that BOAS risk increases sharply in a non-linear manner as relative muzzle length shortens. BOAS only occurred in dogs whose muzzles comprised less than half their cranial lengths. Thicker neck girths also increased BOAS risk in both populations: a risk factor for human sleep apnoea and not previously realised in dogs; and obesity was found to further increase BOAS risk. This study provides evidence that breeding for brachycephaly leads to an increased risk of BOAS in dogs, with risk increasing as the morphology becomes more exaggerated. As such, dog breeders and buyers should be aware of this risk when selecting dogs, and breeding organisations should actively discourage exaggeration of this high-risk conformation in breed standards and the show ring.

Selection for brachycephalic (foreshortened muzzle) phenotypes in dogs is a major risk factor for brachycephalic obstructive airway syndrome (BOAS). Clinical signs include respiratory distress, exercise intolerance, upper respiratory noise and collapse. Efforts to combat BOAS may be constrained by a perception that it is 'normal' in brachycephalic dogs. This study aimed to quantify ownerperception of the clinical signs of BOAS as a veterinary problem. A questionnaire-based study was carried out over five months on the owners of dogs referred to the Queen Mother Hospital for Animals (QMHA) for all clinical services, except for Emergency and Critical Care. Owners reported the frequency of respiratory difficulty and characteristics of respiratory noise in their dogs in four scenarios, summarised as an 'owner-reported breathing' (ORB) score. Owners then reported whether their dog currently has, or has a history of, 'breathing problems'. Dogs (n = 285) representing 68 breeds were included, 31 of which were classed as 'affected' by BOAS either following diagnostics, or by fitting case criteria based on their ORB score, skull morphology and presence of stenotic nares. The median ORB score given by affected dogs' owners was 20/40 (range 8-30). Over half (58%) of owners of affected dogs reported that their dog did not have a breathing problem. This marked disparity between owners' reports of frequent, severe clinical signs and their perceived lack of a 'breathing problem' in their dogs is of concern. Without appreciation of the welfare implications of BOAS, affected but undiagnosed dogs may be negatively affected indefinitely through lack of treatment. Furthermore, affected dogs may continue to be selected in breeding programmes, perpetuating this disorder.

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Concern has arisen in recent years that selection for extreme facial morphology in the domestic dog may be leading to an increased frequency of eye disorders. Corneal ulcers are a common and painful eye problem in domestic dogs that can lead to scarring and/or perforation of the cornea, potentially causing blindness. Exaggerated juvenile-like craniofacial conformations and wide eyes have been suspected as risk factors for corneal ulceration. This study aimed to quantify the relationship between corneal ulceration risk and conformational factors including relative eyelid aperture width, brachycephalic (short-muzzled) skull shape, the presence of a nasal fold (wrinkle), and exposed eye-white. A 14 month cross-sectional study of dogs entering a large UK based small animal referral hospital for both corneal ulcers and unrelated disorders was carried out. Dogs were classed as affected if they were diagnosed with a corneal ulcer using fluorescein dye while at the hospital (whether referred for this disorder or not), or if a previous diagnosis of corneal ulcer(s) was documented in the dogs’ histories. Of 700 dogs recruited, measured and clinically examined, 31 were affected by corneal ulcers. Most cases were male (71%), small breed dogs (mean± SE weight: 11.4±1.1 kg), with the most commonly diagnosed breed being the Pug. Dogs with nasal folds were nearly five times more likely to be affected by corneal ulcers than those without, and brachycephalic dogs (craniofacial ratio <0.5) were twenty times more likely to be affected than non-brachycephalic dogs. A 10% increase in relative eyelid aperture width more than tripled the ulcer risk. Exposed eye-white was associated with a nearly three times increased risk. The results demonstrate that artificially selecting for these facial characteristics greatly heightens the risk of corneal ulcers, and such selection should thus be discouraged to improve canine welfare.