Flat-faced dogs: study reveals genetic link to breathing problems

Owners of flat-faced dogs often hear the same reassurance: snoring and laboured breathing are simply characteristics of the breed.

Until now, there has been no clear figure for how much of that breathing difficulty is inherited. The answer appears to be high enough for breeders to make realistic choices that produce healthier dogs.

Breathing problems in flat-faced dogs

The condition is clinically known as brachycephalic obstructive airway syndrome (BOAS). It can cause noisy breathing, an inability to tolerate exercise and difficulty cooling down. In severe cases, dogs may collapse or die.

Flat-faced dogs experience the syndrome at much higher rates than other breeds, and obesity makes the symptoms worse. What had been missing was an understanding of the genetic contribution.

The new study was led by Dr Joanna Ilska, a geneticist at the Royal Kennel Club (RKC). Her team used information from a screening programme developed by the club with the University of Cambridge.

Data from thousands of dogs

Researchers examined records for 1,474 Bulldogs, 1,917 French Bulldogs and 900 Pugs. Each had undergone a standardised exercise test, with scores ranging from zero to three. Dogs scoring two or three were considered clinically affected.

The clinical rates were lower than expected: approximately 15.6% of French Bulldogs, 18.9% of Bulldogs and 19.8% of Pugs were classified as affected. Earlier reference work had placed the figures above 50%.

The researchers suggest two possible explanations. Their sample was five to ten times larger than those used in previous studies. Obesity was also less common than in earlier datasets, although it is not clear whether that reflects better management or sampling bias.

The role of genetics

Across the three breeds, the study found that genetics accounted for between 21% and 49% of the variation in dogs’ respiratory scores. That means breathing function can be influenced through selective breeding rather than left to chance.

A similar pattern emerged for body weight and nostril stenosis, the narrowing of the nostrils that restricts airflow before it reaches the throat. Between 31% and 39% of the variation in nostril width was inherited.

The traits were linked across all three breeds. Narrower nostrils were associated with poorer breathing scores, while dogs with poor scores tended to be heavier.

An earlier paper had identified narrow nostrils and high body condition as risk factors. The new analysis shows that the connection is also reflected in the genome.

The link between facial structure and function

One finding surprised the team. Although French Bulldogs had the lowest BOAS rate of the three breeds, just 6% had nostrils graded as open. All the others fell somewhere on the stenosis scale.

Narrow nostrils increase the odds of BOAS by 2.6 times, while obesity raises the risk by 3.8 times. Yet the breed with the worst nostrils still had the lowest clinical rate.

The researchers see this as evidence that the relationship between facial structure and function goes beyond any one trait. Several features contribute to the syndrome, working together rather than separately.

Limited data for pet dogs

Participation in screening adds another dimension to the picture. After seven years of the programme, only a small proportion of dogs from the three breeds have been assessed.

Testing among Pugs peaked in 2023 at 3.1% of dogs born that year. For French Bulldogs, the rate has remained between 0.4% and 0.9%. Bulldog participation is somewhat higher, but still in the low single digits.

Dogs from show-line backgrounds are tested much more frequently. Those bred for the pet market are largely missing from the data, and when their results are recorded, their scores are slightly worse than average.

Stricter breeding standards are needed

That gap is starting to narrow in at least one setting. From 2025, every Bulldog, French Bulldog and Pug entered at Crufts must have a respiratory test on record.

Dogs with the most severe scores are excluded from the show ring. By 2026, those graded moderately affected will also be barred from the UK’s largest dog show.

A separate study puts almost one in six UK dogs in the flat-faced category, offering an indication of the scale of the welfare issue.

“Our findings provide clear evidence that respiratory health in these breeds is influenced by genetic differences between dogs, and importantly, that improvement is achievable through selective breeding,” said Ilska.

The study recommends tougher breeding standards. More than three-quarters of tested dogs already pass the breathing scale, and removing borderline cases from breeding pools could speed up progress.

What changes now

Before this study, there was no within-breed estimate of the genetic contribution to BOAS in the three signature flat-faced breeds. Those figures are now available, drawn from the largest dataset ever assembled to investigate the question.

Selecting for better breathing is no longer merely a hypothesis. It is a measurable tool, with effect sizes that breeders can use in their planning.

That changes what breeders can be expected to deliver, as well as what buyers can ask before paying a deposit.