Impact of Facial Conformation on Canine Health: Brachycephalic Obstructive Airway Syndrome

TLDR

Domestic dogs exhibit extreme morphological diversity due to selective breeding for aesthetic traits, and brachycephaly—shortened facial skeleton—has become common in breeds such as Bulldogs, Pugs, and French Bulldogs, leading to the chronic respiratory disorder Brachycephalic Obstructive Airway Syndrome. This study seeks to quantitatively link brachycephaly to BOAS risk to inform healthier breeding practices. Across 700 dogs and a confirmatory cohort of 154 brachycephalic dogs, BOAS risk rises sharply as muzzle length shortens, occurring only when the muzzle is less than half the cranial length, with thicker neck girth and obesity further elevating risk, underscoring the need for breeders to avoid extreme conformation.

Abstract

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.