Science Digest: What is the prevalence of and risk factors for BOAS in different brachycephalic breeds?

20 April 2026

Most research on brachycephalic obstructive airway syndrome (BOAS) to date has focused on the three most popular brachycephalic dog breeds – French Bulldogs, Bulldogs and Pugs. Many other dog breeds can be considered to be brachycephalic, yet there is currently limited evidence on the prevalence in other breeds and how BOAS varies between the wide range of phenotypes of different breeds.¹

A new study published in PLOS ONE by researchers at the University of Cambridge investigated the frequency and severity of BOAS in 14 breeds and the key conformational and modifiable risk factors associated with more severe disease.¹

A total of 898 dogs were recruited into the study of the following breeds: Affenpinscher, Boston Terrier, Boxer, Cavalier King Charles Spaniel, Chihuahua, Dogue de Bordeaux, Griffon Bruxellois, Japanese Chin, King Charles Spaniel, Maltese, Pekingese, Pomeranian, Shih Tzu and Staffordshire Bull Terrier. These were selected to capture a broad range of brachycephalic phenotypes, including both extreme and more moderate conformations.

Dogs were recruited from owners who were willing to attend an appointment at Queen’s Veterinary School Hospital in Cambridge, or from dog shows or breed-specific health testing days. Dogs were evaluated for BOAS using the existing respiratory function grading criteria, and signalment factors and conformation measurements were recorded to evaluate the key risk factors for BOAS both across and within each breed. Within each breed, the proportion of Grade 0 (clinically unaffected) dogs was compared to that of the three popular brachycephalic breeds – Pugs (7%), French Bulldogs (10%), and Bulldogs (10.9%).

Almost every breed had some detectable level of breathing abnormality. Two breeds were identified as being at high risk of BOAS: the Pekingese and Japanese Chin, with only 10.9% and 17.4% being Grade 0, respectively. Both of these breeds had comparable rates of BOAS to the Pug, French Bulldog and Bulldog.

Five breeds were defined as having a moderate risk of BOAS, with between 25–50% of dogs being Grade 0: the Griffon Bruxellois, Boston Terrier, Dogue de Bordeaux, King Charles Spaniel and Shih Tzu. The remaining breeds had over 50% of the sample population assessed as Grade 0, and have a mild risk of BOAS: Staffordshire Bull Terrier, Cavalier King Charles Spaniel, Chihuahua, Boxer, Affenpinscher and Pomeranian. Only the Maltese and Pomeranian had no dogs with clinically significant disease (Grade 2 or 3). Within the Maltese, only one dog was identified as a Grade 1, and due to a smaller sample size, this breed was excluded from the singular breed conformation risk factor analysis.

Increased BOAS severity was associated with lower craniofacial ratio (more extreme facial hypoplasia), nostril stenosis, and higher body condition score. However, the model of these three conformation factors together only accounted for 20% of the variation in BOAS status across the different breeds. These conformational risk factors can be useful for both breeders and prospective owners in selecting dogs that are less likely to be affected by BOAS, and inform breed standards.

A limitation of the study is that the models for conformational risk factors didn’t include data for the three popular brachycephalic breeds (the Pug, French Bulldog and Bulldog) and that the data for the BOAS grades in these three breeds was taken from a study published in 2016, and therefore might not reflect the BOAS grade distribution of today’s population. The dogs sampled also may not be a true representation of the pet population, as the study relied on owners volunteering to attend an appointment or participate in shows or health testing days, which might have introduced sampling biases.

Further research into the internal anatomical lesion sites, genetics and histopathology of airway tissues could improve the understanding of the underlying disease mechanisms and help to explain variation between individual dogs affected by BOAS.

Take home message

BOAS severity varies widely between brachycephalic breeds. Pekingese and Japanese Chin are high risk for BOAS, while the Maltese and Pomeranian were the only breeds with no dogs with clinically significant disease. More extreme brachycephaly, nostril stenosis and high body condition score are associated with increased BOAS risk.

Reference

¹Tomlinson F, Liu NC, Sargan D & Ladlow JF (2026) A cross-sectional study into the prevalence and conformational risk factors of BOAS across fourteen brachycephalic dog breeds. PLOS ONE. 21(2): e0340604. https://doi.org/10.1371/journal.pone.0340604