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Dogs which are overweight or which have narrowed nostrils or a wider, shorter head shape are more likely to suffer from the serious breathing condition, Brachycephalic Obstructive Airway Syndrome (BOAS), according to new research from the Cambridge Veterinary School. In some breeds, shorter tails and thicker necks represented an additional risk factor.

The study, published today in PLOS One, found that BOAS varies considerably, in prevalence and severity, between flat-faced dog breeds but also within each breed.

BOAS is a chronic disease associated with short-skulled or flat-faced dogs. Lesions within the upper respiratory tract result in airway narrowing. This often leads to noisy breathing but can also impact a dog’s ability to exercise, sleep and cope with heat or stress.

“BOAS exists on a spectrum. Some dogs are only mildly affected, but for those at the more severe end, it can significantly reduce quality of life and become a serious welfare issue,” said Dr Fran Tomlinson, from the Cambridge Veterinary School, who co-led the study.

“While surgery, weight management and other interventions can help affected dogs to some degree, BOAS is hereditary, and there is still much to learn about how we can reduce the risk in future generations.”

While previous research into BOAS has focused on the three most popular ‘flat-faced’ dog breeds in the UK – the French Bulldog, the Pug and Bulldog – this study investigated a further 14 breeds:

The 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 the Staffordshire Bull Terrier.

The University of Cambridge-led study, which involved nearly 900 dogs, found that 12 out of the 14 breeds studied had some detectable level of breathing abnormality. The researchers identified two breeds at high risk of BOAS. 89% of Pekingese and 82% of Japanese Chin were found to be affected, rates comparable with Pugs, French Bulldogs and Bulldogs.

Five breeds were found to be at moderate risk of BOAS – the King Charles Spaniel, Shih Tzu, Griffon Bruxellois, Boston Terrier, and the Dogue de Bordeaux – with half to three-quarters of the dogs studied being affected.

Staffordshire Bull Terriers, Cavalier King Charles Spaniel, Chihuahua, Boxer, and Affenpinscher were found to be at mild risk, with only half of dogs having some degree of noisy breathing and only a few animals with clinically significant disease. None of the Pomeranian or Maltese dogs studied were found to be clinically affected.

Method

The researchers assessed almost 900 dogs during individual appointments at the Queen’s Veterinary School Hospital in Cambridge, at dog shows and at breed-specific health testing days.

The UK Kennel Club and the University of Cambridge run a Respiratory Function Grading Scheme to assess French Bulldogs, Bulldogs and Pugs, which is used in many countries around the world. The researchers adapted this assessment to enable them to study the 14 additional breeds.

The team assessed the dogs’ breathing before and after a 3-minute exercise test, grading any upper respiratory noises and signs of difficulty or discomfort. Dogs that completed the exercise test with no breathing discomfort or respiratory noise detected were classified as unaffected, while dogs that displayed any upper airway noises were classified from mild to severe BOAS.

Head shape

‘Brachycephalic’ dog breeds are generally associated with shortened muzzles and are described as being ‘flat-faced’ or as having ‘facial hypoplasia’. But the authors of this study caution that dogs which have a relatively wide skull in comparison to its length, such as the Staffordshire Bull Terrier, can also be considered brachycephalic.

The researchers found that dogs with wider shorter head shapes – those with a lower craniofacial ratio – were more likely to have BOAS.

“Our findings show that the relationship between relative muzzle length and BOAS risk is more complex than is commonly assumed,” said Dr David Sargan.

“The King Charles Spaniel, also known as the English Toy Spaniel in the US, is an extremely flat-faced breed so you might expect to find it in a higher risk group. However, 40% of the animals we assessed were unaffected by BOAS.”

Nasal passages

Nostril ‘stenosis’, or narrowing, has previously been reported as a key risk factor for BOAS. This study provides more evidence to support this. The researchers found that prevalence of this problem varied substantially between different breeds and was significantly associated with BOAS risk.

The two breeds found to be high risk for BOAS, the Pekingese and Japanese Chin, had high rates of nostril narrowing. Only around 6% and 18% of dogs respectively had open nostrils. The Griffon Bruxellois and the Boston Terrier, breeds at moderate risk of BOAS, were also more likely to have restricted nostrils than the other breeds.

Tails, necks, body ratios and weight

In both the Shih Tzu and Staffordshire Bull Terrier breeds, the researchers found that shorter tails were associated with an increased risk of BOAS. For the Staffordshire Bull Terriers, those with longer tails were roughly 30% less likely to have BOAS, and affected dogs had tails 1.5 cm shorter on average. 

In the Boston Terrier and Staffordshire Bull Terrier breeds, they found that dogs with proportionately thicker necks were more likely to be affected. Neck girth ratio has previously been noted to be a significant factor in the risk of BOAS in Bulldogs and French Bulldogs.

Dr Jane Ladlow, who co-led the study said: “Considering the close genetic relationship between Staffordshire Bull Terriers, Boston Terriers and the bulldogs, it isn’t surprising that they share this link between neck thickness and BOAS.”

In the Chihuahua and King Charles Spaniel breeds, the researchers found that dogs with relatively longer bodies with a shorter height were more likely to affected by BOAS.

The researchers found that being overweight was a significant risk factor for the Cavalier King Charles Spaniel, the Shih Tzu and the Affenpinscher.

“Weight loss could be used as a management tool to reduce the risk of BOAS in these three breeds, as it is in the Pug,” said Jane Ladlow.

Implications and applications

The researchers hope that this study will lead to more ‘flat-faced’ dogs being tested and encourage greater engagement on BOAS and other health issues faced by these breeds.

Dr Ladlow said: “Being aware of risk factors can be useful for both breeders and prospective owners in selecting dogs which are less likely to be affected by BOAS. Knowledge of these risk factors can also help to inform judges in deciding which features are detrimental to health so that factors associated with BOAS are not rewarded in the show ring, particularly as winning dogs can become popular sires.”

The researchers point out that weight, nostril narrowing and craniofacial ratio only accounted for 20% of variation in BOAS status across the different breeds. For now, a breathing assessment remains the most accurate way to determine BOAS status and therefore which dogs should be selected for breeding, or whose welfare would benefit from veterinary intervention.

“Each individual breed has its own risk profile for BOAS and different factors affecting this,” said Dr Fran Tomlinson. “Our findings support a breed-specific approach when tackling the reduction of this disease on a population level.”

The Veterinary School team wish to acknowledge the help they have received from dog owners in carrying out this work. Funding was provided by The Royal Kennel Club Charitable Trust.

Reference

F. Tomlinson, N-C. Liu, D.R. Sargan, J.F. Ladlow, ‘A cross-sectional study into the prevalence and conformational risk factors of BOAS across fourteen brachycephalic dog breeds’, PLOS One (2026). DOI: 10.1371/journal.pone.0340604

Scientists have identified a further twelve dog breeds as being at risk of Brachycephalic Obstructive Airway Syndrome – a condition that can cause serious breathing problems – including the Pekingese, Shih Tzu, Boston Terrier, Staffordshire Bull Terrier, Cavalier King Charles Spaniel, Chihuahua and Boxer.

Our findings support a breed-specific approach when tackling the reduction of this disease Fran TomlinsonFran TomlinsonFour Boston Terrier study participants with (from left to right) Dr David Sargan, Dr Fran Tomlinson and Dr Jane Ladlow, from the Cambridge Veterinary School

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In a huge global study led by University of Cambridge researchers, a single group of bacteria - named CAG-170 - has repeatedly shown up in high numbers in the gut microbiomes of healthy people.

CAG-170 is a group of gut bacteria known only from their genetic fingerprints - scientists have never been able to grow most of them in the lab.

Using diverse computational approaches, the team looked for CAG-170’s genetic fingerprint in gut microbiome samples from over 11,000 people across 39 countries.

They found the level of CAG-170 present was consistently higher in healthy people than those with diseases including inflammatory bowel disease, obesity and chronic fatigue syndrome.

Further analysis of CAG-170 revealed they have the capacity to produce high levels of Vitamin B12, and enzymes that break down a wide range of carbohydrates, sugars and fibres in our gut.

The researchers think it’s likely the Vitamin B12 supports other species of gut bacteria, rather than supporting the humans whose guts it is being produced in.

The findings mean that CAG-170 could, in the future, be used as an indicator of our gut microbiome health. They also open the door to developing new probiotics specifically designed to support and maintain healthy levels of CAG‑170 in the gut.

Dr Alexandre Almeida, a researcher in the University of Cambridge’s Department of Veterinary Medicine who led the work, said: “Our work has revealed that CAG-170 bacteria - part of the ‘hidden microbiome’ - appear to be key players in human health, likely by helping us to digest the main components of our food and keeping the whole microbiome running smoothly.”

He added: “We looked at the gut microbes of thousands of people across 39 countries and 13 different diseases including Crohn’s and obesity. We consistently found that people with these diseases had lower levels of CAG-170 bacteria in their gut.”

The study is published today in the journal Cell Host & Microbe.

Discovering the ‘hidden microbiome’

The study builds on Almeida’s previous work to create a comprehensive reference catalogue of all the genomes in the human gut microbiome, called the ‘Unified Human Gastrointestinal Genome catalogue’. He used an approach called ‘metagenomics’ – essentially, analysing the genomes of all the microbes in the gut in one go, and then teasing these apart into the genomes of individual species.

This revealed over 4,600 bacterial species, including over 3,000 that hadn’t previously been seen in the gut before - indicating the extent of the ‘hidden microbiome’ waiting to be explored. The catalogue provided ‘reference genomes’ for each species, including CAG-170: these are like fingerprints that the researchers can now look for in other gut microbiome samples.

“Our earlier work revealed that around two-thirds of the species in our gut microbiome were previously unknown. No-one knew what they were doing there – and now we’ve found that some of these are a fundamental and underappreciated component of human health,” said Almeida.

Three lines of evidence

The team looked at over 11,000 samples of human gut bacteria from people across 39 countries – primarily in Europe, North America and Asia. These were from healthy people, and people with 13 different diseases including Crohn’s, colorectal cancer, Parkinson’s and multiple sclerosis.

By comparing each sample to the Unified Human Gastrointestinal Genome catalogue the researchers noticed that CAG-170 bacteria are the part of the ‘hidden microbiome’ most strongly linked with health - and this is consistent across countries.

In a second approach, the team computationally analysed the full make-up of the gut microbiome of over 6,000 healthy people to investigate which species had the strongest potential to keep the gut ecosystem in check. Of all the bacteria in the ‘hidden microbiome’, CAG-170 were again most consistently associated with health.

In a third analysis they measured the level of CAG-170 present in the gut microbiomes of people with dysbiosis – a condition where the gut microbiome is out of balance. This revealed that lower levels of CAG-170 in the gut are linked with a higher likelihood of having a gut imbalance. Dysbiosis is linked with many long-term conditions including irritable bowel syndrome, rheumatoid arthritis, and anxiety and depression.

Therapeutic possibilities

The billions of bacteria that make up our gut microbiome represent around 4,600 different species. The composition is different in all of us, but the function is the same: to keep our body running smoothly.

Scientists hope that by understanding more about what a healthy gut microbiome looks like, they can see how it changes in people with specific diseases - and try to correct it using approaches including tailored probiotics. The new study is an important step towards making this a reality.

“The probiotic industry hasn't really kept up with gut microbiome research - people are still using the same probiotic species that were being used decades ago. We’re now discovering new groups of bacteria like CAG-170 with important links to our health, and probiotics aimed at supporting them could have a much greater health benefit,” said Almeida.

Until now, scientists studying the gut microbiome have focused their attention on the bacteria within it that can be grown, and therefore studied, in the lab. Most of the CAG-170 gut bacteria are not in this category - so scientists will need to figure out ways to grow and test them, in order to translate these findings into new potential therapeutics.

Reference: da Silva, A.C. et al: ‘Meta-analysis of the uncultured gut microbiome across 11,115 global metagenomes reveals a candidate signature of health.’ Cell Host & Microbe, February 2026. DOI:10.1016/j.chom.2026.01.013

An understudied group of bacteria in our gut microbiome appears to play a central role in keeping us healthy, according to researchers at the University of Cambridge.

These are a fundamental and underappreciated component of human health.Alexandre AlmeidaThom Leach, Science Photo Library on GettyGut microbes

The text in this work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License. Images, including our videos, are Copyright ©University of Cambridge and licensors/contributors as identified. All rights reserved. We make our image and video content available in a number of ways – on our main website under its Terms and conditions, and on a range of channels including social media that permit your use and sharing of our content under their respective Terms.

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