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Genetic Welfare Problems of Companion Animals

An information resource for prospective pet owners



Brachycephalic Airway Obstruction Syndrome (BAOS)

Related terms: Brachycephalic Obstructive Airway Syndrome (BOAS), Brachycephalic Airway Syndrome (BAS), Brachycephalic Airway Disease (BAD), Brachycephalic disease, Upper airway obstruction

Brief overview: The short-nosed shape of the Pug skull causes breathing difficulties and ‘knock-on’ effects, which cause chronic discomfort and respiratory distress, which can be life-threatening.

Summary of Information

(for more information click on the links below)

1. Brief description

Brachycephalic airway obstruction syndrome (BAOS) or Brachycephalic Obstructive Airway Syndrome (BOAS) occurs in all breeds with significant brachycephaly. Brachycephaly is abnormally short head shape (compared with the ancestral, natural, head shape of dogs) with, in some cases, greatly shortened upper jaws and noses. BAOS refers to the suite of respiratory problems associated with brachycephaly. Although the facial bones are shortened, the soft tissues inside remain unchanged in size and are thus squeezed into a substantially smaller space. This leads to narrowing, and increased resistance to airflow, in the nose and upper airways. BAOS describes the clinical signs due to these effects. The main congenital (present from birth) abnormalities are stenotic nares (abnormally narrow nostrils) and elongated and thick soft palate.  (Tracheal hypoplasia (abnormally narrow windpipe) is rare in the pug). The permanent narrowing and obstruction of the airways makes breathing much harder. In time, the effects of the  increased respiratory effort leads to secondary changes which further narrow the air passages, and may contribute to the collapse of the larynx (the voice box at the entrance of the windpipe). BAOS leads to snoring, respiratory noise, mouth breathing, and respiratory distress with rapid breathing and struggling for breath, and can lead to collapse and death. Dogs with BAOS are unable to take even moderate amounts of exercise, are very prone to heat stroke and have disrupted sleep.

2. Intensity of welfare impact

This is a major welfare problem and affected dogs are at risk of bouts of severe respiratory distress and fear, and these crises can be life-threatening eg they are especially at risk when exercising in hot weather. Even mildly affected dogs are likely to suffer disrupted sleep and are prevented from carrying out normal behaviours such as running and exercising because of breathing difficulties. It seems unacceptable that that this condition, even in its mildest form, could be considered normal.

3. Duration of welfare impact

BAOS affects animals from early in their lives, and is a life-long and progressively-worsening condition.

4. Number of animals affected

To an extent, all Pugs may be affected in some way by this condition, because of their severe brachycephalic conformation, but the severity of clinical signs may vary.

5. Diagnosis

Vets would suspect BAOS in any pug showing the typical signs, as it is so common in the breed. However, specific diagnosis of the severity of most of the abnormalities that together form the syndrome requires examination under anaesthetic, radiographs (x-rays) and possibly endoscopy (examination of the airways with a fibre optic tube). These procedures have to be performed under anaesthesia, which creates a dilemma as dogs with BAOS have a substantially increased risk of dying under anaesthesia because of their respiratory compromise.

6. Genetics

BAOS is caused by brachycephaly – abnormally short head shape. This inherited defect is a consequence of selection for this feature.

7. How do you know if an animal is a carrier or likely to become affected?

Probably all pugs are affected with BAOS to a greater or lesser extent, but the clinical signs will vary from mild to severe. Examination prior to purchase is essential, along with examination of its dam and sire. It is recommended that dogs showing any signs of BAOS, or whose parents have any signs or have had surgical procedures to alleviate the condition should not be purchased.

8. Methods and prospects for elimination of the problem

As BAOS is due to the brachycephalic head shape it seems unlikely that it will be possible to elimination the condition without changing head shape (and the breed standard). As all Pugs have this condition, to a greater or lesser extent, it seems that this would be likely to require out-crossing with non-brachycephalic breeds. Opinions differ as to whether it is ethically acceptable to breed animals whose welfare is likely to be compromised.


For further details about this condition, please click on the following:
(these link to items down this page)

1. Clinical and pathological effects

The term “brachycephaly” comes from the Greek words meaning short and head. The term applies to all breeds of dog (and cat) with short heads. Among the brachycephalic breeds are: pugs, English bulldogs, French bulldogs, Pekinese, Lhasa apsos, Shih tzus, and Boston terriers.

Brachycephaly affects the health and welfare of animals in various ways (Oechtering, 2010). It affects the upper airways, eyes, ears, gastro-intestinal tract, spine and the ability to give birth normally. Here we focus on just one subset of these problems – BAOS. BAOS is the collective term for the problems caused to the upper airways.

The brachycephalic head shape is due to an inherited defect in development of the bones of the skull (Stockard 1941). The head is a normal width but its length - especially of the muzzle - is reduced and in some breeds, like the Pug, extremely so (Oechtering et al, 2007; Oechtering, 2010). However, the soft tissues of the head are not correspondingly reduced in size and have to squeeze into a smaller space. This affects the passage of air into the lower airways and lungs (Harvey 1989). Pugs also exhibit other characteristic of an extreme brachycephalic conformation with a pronounced underbite and widely placed, shallow eye orbits.

Brachycephalic Airway Obstruction Syndrome (BAOS or BOAS) has resulted from breeding for a short face and head shape. Most brachycephalic dogs are affected by upper airway obstruction to some degree (Brown and Gregory 2005). The airways are narrowed in several places due to the abnormal face shape and also because of the 'knock-on' consequences on the linings of the airways, of chronic laboured breathing and the abnormal pressures associated with this. (Shell 2008). BAOS is the term given to this collection of anatomical abnormalities, the secondary pathological changes they cause, and the resulting clinical signs. The risk of BOAS increases sharply as relative muzzle length shortens and BOAS occurs in dogs whose muzzles comprise less than half their cranial lengths (Packer et al 2015).

There are a number of congenital (present from birth), anatomical abnormalities that form the basis of BAOS (Oechtering, 2010):

  • Stenotic Nares (abnormally narrow nostrils) – This is usually a bilateral condition (ie it affects both nostrils). It causes occlusion of the airways, increasing the effort needed to breathe in. The 'knock-on' consequences of this laboured breathing include pulling the soft palate deeper into the larynx causing further blockage to the airway (Shell 2008). Distorted nasal passages are also thought to cause some occlusion (Hendricks 1995.
  • Elongated soft palate – It is believed that the genetic defect responsible for shortening the bones of the nose does not correspondingly affect the soft tissues of the head, so relatively, the soft palate and tongue at the back of the mouth are disproportionately large (Venker-van Haagen 1995). In a normal dog of the same size (ie with a  normal head shape), the soft palate would be the correct length, but in the brachycephalic dog, it is over-large and over-long and  gets pushed backwards, partly obstructing the larynx (the opening to the lower airway) (Hendricks 1995). The abnormally large soft palate also increases airflow turbulence and rubs on adjacent tissues, such as the larynx, leading to inflammation and swelling of airway tissues (Torrez & Hunt 2006). For these reasons, the soft palate itself becomes swollen and further enlarged over time, further blocking the airways (Shell 2008).
  • Pharyngeal turbinates – These are bones of the nose that should be confined to the nose but which can  protrude back into the upper part of the throat in some brachycephalic dogs – especially pugs (Ginn et al 2008; Oechtering et al, 2007).
  • Trachea hypoplasia (Abnormally narrow windpipe relative to the size of dog) –Tracheal hypoplasia is often seen in conjunction with the above abnormalities in other brachycephalic breeds – but it is very uncommon in the pug. It can also be seen in association with other abnormalities not considered to be part of BAOS including megaoesophagus (abnormally enlarged oesophagus; Coyne and Fingland 1992). Views vary about the extent to which trachea hypoplasia plays a part in BAOS. It has been suggested that tracheal hypoplasia worsens the prognosis for dogs with BAOS (Orsher 1993, Monnet 2003) and predisposes to (increases the risk of) chronic or recurrent lower airway and lung infections (Harvey 1989). However, further evidence in the literature to support this was not identified by Pink et al (2006). As noted above, tracheal hypoplasia is very rare in pugs.
  • Laryngeal hypoplasia (abnormally small, underdeveloped larynx) - In this condition the cartilages that form the larynx are abnormally soft and underdeveloped in shape, and the abductor muscles that function to open the larynx do not operate properly (Venker-van Haagen 1995). This is a common cause of laryngeal collapse in brachycephalic breeds according to Venker-van Haagen (1995), though other authors do not discuss this condition as part of BAOS. Burbidge et al (1988) recorded a case of BAOS with laryngeal malformation and suggested that this may be seen in other cases of BAOS.

Affected dogs with extreme brachycephaly conformation usually have some combination of the above defects (Brown and Gregory 2005) and most have more than one of them (Fasanella et al 2010). They result in increased workload in breathing and, over time, to a progression of secondary problems. These include:

  • Collapse of the larynx – There has been debate about whether collapse of the larynx is secondary to the abnormally low  pressures in the pharyngolaryngeal (throat) region, as a result of the obstructions described above (Wykes 1991), or whether it is a primary problem which tends to accompany brachycephaly. It is believed that it may be a primary problem associated with brachycephaly in the pug (Oechtering, personal communication). Three stages of laryngeal collapse have been described:

    In stage I there is eversion of the laryngeal saccules (small sac-like structures in the side walls of the larynx) which turn inside out and balloon into the airway, adding to the obstruction.

    In stage II there is loss of rigidity and medial displacement of the cuneiform processes of the arytenoid cartilage (part of the cartilage that forms the support structure of the larynx) and they collapse inwards, and in stage III there is collapse of the corniculate processes of the arytenoid cartilages - this involves further collapse of the laryngeal structures blocking the airway (Pink et al 2006).


  • Bronchial collapse. Laryngeal collapse has been found to be significantly associated with bronchial collapse: the collapse of the branching airways in the lungs (De Lorenzi et al 2009). As with laryngeal collapse, this may not be secondary to pressure changes caused by BAOS, but be a primary problem.
  • Evertion and hypertrophy of the tonsils – Inflammation of the soft tissues in the throat leads to the tonsils enlarging, adding to the amount of tissue protruding into the pharynx (the back of the throat; Fasanella et al 2010).
  • Hypertrophy of the pharyngeal muscles – The muscles in the throat become larger and thicker as they are constantly working harder to keep the pharynx open to enable the passage of air into the lungs. However, this hypertrophy of the muscles further narrows the pharyngeal cavity.
  • Gastrointestinal problems: dysphagia, regurgitation, vomiting and acid reflux – Dysphagia is defined by Poncet et al (2005) as difficult or painful swallowing. It can be associated with regurgitation of oesophageal contents and vomiting. Dysphagia in brachycephalic dogs has been reported by many authors (e.g. Ducarouge 2002, Dupre and Freiche 2002, Koch and others 2003). Poncet et al (2005) also noted many reports of anatomical defects in the gastrointestinal tracts of brachycephalic dogs. For example, hiatus hernia (in which part of the stomach can pass up into the chest through a larger than normal hole in the diaphragm, and pyloric stenosis (in which the stomach exit is narrowed, preventing food from leaving the stomach normally). They found a correlation between the severity of BAOS and the severity of gastrointestinal disease and postulated that the respiratory diseases affected the severity of gastrointestinal signs and vice versa.
  • Heart Failure – This occurs as a consequence of inadequate oxygenation of the blood in the lungs, due to the airway obstructions. In response to inadequate oxygenation of the blood, the capillaries in parts of the lung that are poorly ventilated are constricted. Chronic (long term) vasoconstriction and increased blood pressure in the lungs increases the blood pressure in the right side of the heart and eventually can lead to right-sided heart failure (Monnet 2008).
  • Breathing problems during whelping -  Breathing difficulties may affect some brachycephalic bitches during whelping (giving birth) and may be a reason for elective or emergency caesarean section in these dogs (Harvey 1989).
  • Thermoregulation – The normal nose plays an important role in body temperature regulation in the dog. Evaporation from the extensive folded surfaces over the complex mesh of turbinate bones, enables cooling. In brachycephalic dogs, this function is severely disrupted resulting in severe difficulties in coping with heat – whether due to the warmth of the environment or resulting from exercise. As Oechtering (2010) states: ' This is why brachycephalic animals are particularly heat-sensitive, why many pant to no avail even at room temperature and without physical effort, and may need several hours to recover even after brief exercise.

Some of these conditions are progressive so the signs depend on the congenital anatomical defects present, the severity of each and how long the secondary conditions have been progressing. The primary congenital abnormalities cause the signs of BAOS in puppyhood. The mean age at which brachycephalic dogs are presented to vets for treatment of BAOS is 3 to 4 years of age (Monnet 2008). Laryngeal collapse is usually seen in older animals (Harvey 1982a).

In the mildest cases the clinical signs include snoring and respiratory noise (stridor) on inspiration (breathing in), (Brown and Gregory 2005, Fasanella et al 2010). In more severe cases signs include exercise intolerance , mouth breathing, gagging, restlessness, rapid breathing (tachypnoea), cyanosis (blue coloured membranes of the mouth – due to lack of oxygen in the blood), dysphagia, abnormal posture, and intermittent collapse (Shell 2008, Fasanella et al 2010). Vomiting and regurgitation are also frequently seen (Dupre 2008). Flatulence is common due to aerophagia (swallowing air; Harvey 1989).

Respiratory crises commonly occur in moderately to severely affected individuals, in which the animals rapidly develop respiratory distress, struggle to breathe, and may collapse and die. Severely affected individuals, with laboured breathing, stand with their elbows held away from their chests in an attempt to ease breathing (Brown and Gregory 2005). With the laboured breathing, there is over inflation of the chest and the abdomen is sucked in (Dupre 2008).

Dogs with more severe BAOS live a precarious existence and minor aggravations can lead to severe respiratory distress and crisis (Hendricks 1995). Stress, exercise, excitement, heat, and trips to the vet all act as aggravators (Hendricks 1995, Dupre 2008). These dogs are very prone to heat stroke (Hendricks 1995).

Hendricks (1995) examined sleep disorders in 20 English Bulldogs, another brachycephalic breed of dog that suffers from BOAS. All dogs over 2 weeks of age exhibited disordered breathing during sleep including pauses in breathing, and blood oxygen levels which dropped below normal many times per hour. It is likely that this happens in pugs also.

BAOS is a progressive disease. Some of the primary problems, such as stenotic nares and over-large soft palate can be helped with surgery. Tracheal hypoplasia is untreatable (Brown and Gregory 2005). Surgical treatment is a major intervention but without treatment moderate to severely affected dogs will suffer progressive respiratory distress, incapacity and ultimately death.

Some of the secondary problems can also be improved with surgery. Overall prognosis for dogs that have had surgical interventions varies; Reicks et al (2007), in a study of 62 cases from various brachycephalic breeds, reported a favourable outcome for all dogs needing surgery. Poncet et al (2005) found good or excellent improvement in 88% of the 51 brachycephalic cases studied. However, the outlook for dogs with BAOS depends on which primary and secondary conditions are present and how advanced they are.

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2. Intensity of welfare impact

BAOS has a major welfare impact since it not only reduces general quality of life but puts affected dogs at risk of death through major respiratory distress. Affected dogs suffer great distress (which may include anxiety or fear) when struggling to breath, and these crises can be induced by minor occurrences such as “a simple walk in humid weather” (Hendricks 1995). Without major surgical treatment such episodes can reoccur. Disrupted sleep patterns are probably common. Even those with mild to moderate BAOS are unable to exhibit, what for non-brachycephalic breeds of dogs, is considered normal behaviour such as running and resting comfortably. Overweight and obese dogs were considered to have an increased risk of exhibiting clinical signs of BOAS and other associated upper respiratory tract disorders (Packer et al 2015; O’Neill et al 2015).

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3. Duration of welfare impact

This is a life-long disorder, starting in puppies, progressively worsening throughout life and ultimately often shortening it (Dupre 2008). The average age of first presentation to vets for treatment, for all brachycephalic breeds, is 3-4 years (Monnet 2008). Even with surgical treatment life-expectancy can be compromised. Some authors record death or euthanasia due to BAOS in a relatively high percentage of cases.

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4. Number of animals affected

Brown and Gregory (2005) suggest most dogs with extreme brachycephalic conformation dogs are affected by upper airway obstruction to some degree. The severity of the problem depends on the number and severity of the anatomical abnormalities. In the study by Fasanella et al (2010) Pugs were the breed second most commonly affected with BOAS (after English bulldogs). In one study, BOAS was diagnosed by veterinarians in 88-91% of Pugs seen in a referral hospital population and a non-hospital population (eg breeders, first-opinion veterinary practices and rescue centres; Packer et al 2015), therefore giving a good estimate of the high prevalence of the condition in the breed. 

Stenotic nares - occur in 46 to 77% of dogs with BAOS (Lorinson et al 1997, Harvey 1982a, Fasanlla et al 2010).

Disproportionately large soft palate - 96 to 100% of brachycephalic dogs have an overlong soft palate (Harvey 1982b, Dupre 2008, Fasanella et al 2010).

Everted tonsils - were seen in 56% of dogs with BAOS (Fasanella et al 2010).

Nasopharyngeal turbinates - are found in some brachycephalics, most commonly pugs (Ginn et al 2008).

Laryngeal collapse is common in pugs with BAOS. Surgical treatment is more challenging than in larger dogs (Harvey 1982d). Fasanella et al (2010) reported that 66% of dogs with BAOS syndrome had everted laryngeal saccules.

Trachea hypoplasia is seen in some pugs in conjunction with BAOS (Eom et al 2008), but is generally believed to be rare in pugs.

Secondary problems are also common. The occurrence of laryngeal problems apparently varies from 30% to 64% (Harvey 1982 c, Harvey 1982 d, Dupre 2008). In one study of 73 cases of BAOS, 97% were found to have oesophageal, gastric or duodenal anomalies and 74% had gastrointestinal problems classed as moderate or severe (Poncet et al 2005). In one study of extreme brachycephalic breeds, including the Pug, 22% (132 of 600) of dogs had at least one upper respiratory tract disorder e.g of the nares or trachea and 68 dogs (11.3%) had upper respiratory tract disorders at multiple sites (O’Neill et al 2015). Of 200 Pugs, 53 dogs (26.5%) had at least one upper respiratory tract disorder, and 26 (13.0%) had disorders at multiple sites – this was the highest prevalence of the three extreme brachycephalic breeds tested (Bulldog, French Bulldog, Pug; O’Neill et al 2015).

It seems likely that all pugs have BAOS to some extent, although some people may consider them “normal”. It is increasingly recognised that there is widespread acceptance of some of the clinical signs of BOAS as “normal” for the breed. The owners of over half of dogs diagnosed with BOAS at a referral centre stated that their dog did not have breathing problems (Packer et al 2012). This normalisation phenomenon may blind owners and veterinarians to the extent of the problem in affected animals and lead to under-reporting and under-diagnosis.

Harvey (1989) states “The breathing problems caused by these abnormalities [BAOS] are so commonly recognised by breeders of bulldogs and other short-faced breeds that some carry oxygen cylinders with them to shows, and routinely arrange for caesarean section birth of puppies so as not to cause asphyxiation of the whelping bitch“.

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5. Diagnosis

BAOS will be suspected in any brachycephalic dog showing the characteristic signs, because it is so common. Stenotic nares can be diagnosed on clinical examination of the conscious animal, but definitive diagnosis of elongated soft palate, tonsillar hypertrophy and laryngeal collapse depends on examination under anaesthetic. Diagnoses of tracheal hypoplasia, right-sided heart failure and confirmation of disproportionately large soft palate are made with radiography (x-rays) under anaesthesia and gastrointestinal complications may be investigated using endoscopy and biopsies, again under anaesthetic (Monnet 2008). Anaesthetics carry substantially increased risks during recovery for BAOS sufferers and most authors only recommend anaesthesia for diagnosis when corrective surgical treatment can also be carried out at the same time, under the same anaesthetic, so that these risks during recovery are reduced.

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6. Genetics

Brachycephalic head shape has long been thought to be the result of an inherited defect (Stockard 1941). Recently the region of the canine genome associated with brachycephaly has been identified and 2 particular genes have been implicated. The exact gene or genes are currently unknown (Bannasch et al 2010). This inherited defect defines all brachycephalic breeds and without this defect these breeds would no longer exist in their current form.

Pugs have been shown to have narrow tracheas, thus a genetic component is assumed, though currently the specific genetics of this condition are unknown (Eom et al 2008).

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7. How do you know if an animal is a carrier or likely to become affected?

The pug breed is currently defined by its brachycephalic head shape – all pugs are affected most have some degree of BAOS (Brown and Gregory 2005), so it is extremely difficult to identify a puppy that will be free of this condition. Anyone wishing to obtain a pug should ensure that it and its sire and dam are free of obvious signs of BAOS. Ensuring they have not had correction surgery for any of the primary or secondary conditions is also vital.

Currently, identifying individuals that have had previous surgery for these conditions is difficult because scars on the nostrils may not be easy to see and the other surgical scars are hidden inside the throat. Because of this, Harvey (1989) had a policy of simultaneous neutering all animals that had corrective surgery. It is not known if others have adopted this approach but it is certainly not universal. Because demand would sustain continued production of affected animals, puppies with signs of BAOS should not be purchased and, if signs appear after purchase, the puppy should be returned to its breeder.

Dogs which have shown any signs of BAOS or have had corrective surgery for BAOS should not be used for breeding (see below).

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8. Methods and prospects for elimination of the problem

Clear identification and neutering of all dogs which have had surgical interventions seems vital (but must not be a constraint to prompt treatment). However, it is the brachycephalic head shape that underlies the primary anatomical abnormalities that cause BAOS. The practices of Caesarean section and artificial insemination that allow some brachycephalic dogs to reproduce have enabled selection for even more extreme versions of this defect (Bannasch et al 2010). If dogs with signs of BAOS were not used for breeding, the remaining gene pool might be so small (because so many are affected) that the breed would be unsustainable so it appears that outcrossing with non-brachycephalic would be necessary. Opinions differ as to whether it is ethically acceptable to breed animals whose welfare is likely to be compromised.

As Oechtering (2010) states: ' It is high time for a radical rethink in brachycephalic breeding. Brachycephaly is a pure manmade disease.'

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9. Acknowledgements

UFAW is grateful to Rosie Godfrey BVetMed MRCVS,  David Godfrey BVetMed FRCVS, Dr Emma Buckland (BSc PhD), Dr David Brodbelt (MA VetMB PhD DVA DipECVAA MRCVS) and Dr Dan O’Neill (MVB BSc MSc PhD  MRCVS) for their work in compiling this section and to Dr Gerhard Oechtering of Leipzig University for his very helpful comments and contributions (UFAW is the sole author of any remaining inaccuracies).

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10. References


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Wykes P (1991) Brachycephalic airway obstructive syndrome. Problems in Veterinary Medicine  3 (2), p 188-97

© UFAW 2016

Credit for main photo above:

By Beckilee (Own work) [CC0], via Wikimedia Commons