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Genetic Welfare Problems of Companion Animals

An information resource for prospective pet owners


Boston Terrier

Brachycephalic Airway Obstruction Syndrome (BAOS)

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

Outline: The short-nosed shape of the Boston terrier skull causes breathing difficulties and ‘knock-on’ effects which cause chronic discomfort and prevent it exercising normally. 

Summary of Information

(for more information click on the links below)

1. Brief description

Brachycephalic Airway Obstruction Syndrome (BAOS) occurs in all dog breeds with brachycephaly Brachycephaly is characterised by greatly shortened upper jaws and noses, but the soft tissues of the nose and throat remain unchanged, thus these tissues are squeezed into a substantially smaller space. This leads to narrowing and increased resistance in the respiratory system. BAOS describes the clinical signs seen due to these effects. The main congenital abnormalities (abnormalities present from birth) seen are stenotic nares (abnormally narrowed nostrils), excessively long soft palate in relation to head shape and tracheal hypoplasia (abnormally narrowed windpipe). The permanent narrowing and obstruction of the airways makes breathing harder. The constantly increased respiratory effort leads to secondary changes which further narrow the air passages, including collapse of the larynx (the voice box, the opening to the windpipe). BAOS leads to snoring, respiratory noise, mouth breathing, 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 constantly disrupted sleep.

2. Intensity of welfare impact

This is a major welfare problem with bouts of severe respiratory distress Even mildly affected dogs are likely to suffer disrupted sleep and are prevented from carrying out normal canine behaviours such as running and exercising due to their respiratory compromise.

3. Duration of welfare impact

It is a progressive, life-long condition. BAOS can affect immature dogs under 6 months of age.

4. Number of animals affected

Probably all Boston terriers have this condition to some degree.

5. Diagnosis

Vets would suspect BAOS in any showing the typical signs, as it is so common. However, specific diagnosis of most of the abnormalities, that together form the syndrome, requires examination under anaesthetic, plus radiographs (x-rays) and possibly endoscopy (examination with a fibre optic tube inside the dog). 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. This inherited defect is a consequence of the appearance and breed standards of all brachycephalic breeds.

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

Probably all Boston terriers are affected with BAOS to a greater or lesser extent. Examination of any puppy prior to purchase is essential, along with its dam and sire. To avoid the risk of perpetuating the welfare problems associated with this condition, 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 completely linked to the brachycephalic head shape, it seems unlikely that it will be possible to elimination the condition from Boston terriers without changing the conformation (and the breed standard). As all Boston terriers have this condition, to a greater or lesser extent, this would mean out-crossing to 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. Some brachycephalic breeds include Pugs, English bulldogs Pekinese, French bulldogs, Lhasa Apsos, Shih Tzus, Boston terriers and, in cats, Persians.

The brachycephalic head shape is due to an inherited defect in the development of the bones of the skull (Stockard 1941). The head is of a normal width but the length is markedly reduced. However, the soft tissues of the head are not similarly reduced in size and have to be squeezed into a smaller shortened space. This affects the passage of air into the lower airways and lungs (Harvey 1989).

Brachycephalic Airway Obstruction Syndrome (BAOS) is a result of breeding practices that have selected for a shortened facial appearance. Most brachycephalic dogs are affected by upper airway obstruction to some degree (Brown & Gregory 2005). The airways are narrowed in several places due to the shortened facial features and the resultant increased airflow resistance, so affected dogs have to use excessive effort to breathe (Shell 2008). BAOS is the term given to this collection of anatomical effects, the secondary anatomical changes they cause and the clinical signs that result.

There are a number of congenital (present from birth), anatomical abnormalities that form the basis of BAOS and which contribute to the breathing problems:

  • Stenotic Nares (abnormally narrow nostrils) – This is usually a bilateral condition (occurs in both nostrils), and when present, adds to the airway occlusion, increasing the inspiratory effort needed (the effort needed to breath in). This extra effort has a further impact of pulling the soft palate deeper into the larynx(the opening to the airway) and blocking the airway opening further (Shell 2008). Distorted nasal passages are also thought to cause some occlusion of the larynx (Hendricks 1995)
  • Elongated soft palate – It is believed that the genetic defect responsible for shortening the nose does not affect the soft tissues of the head, the impact of which means that relatively, there is excess soft palate (and tongue) at the back of the mouth (pharyngeal cavity) (Venker-van Haagen 1995). In fact, in a dog of the same size with a normal head shape, the soft palate would be the correct length, but in the brachycephalic dog, the overlong soft palate get pushed backwards, partly obstructing  the larynx (the opening to the airway) (Hendricks 1995). An elongated soft palate also increases airflow turbulence and rubs against other tissues, such as the larynx, leading to inflammation and swelling of airway tissues (Torrez & Hunt 2006). As part of this inflammation, the soft palate itself becomes swollen and thickened over time, further blocking the airways (Shell 2008).
  • Trachea hypoplasia (Abnormally narrow windpipe relative to the size of dog) –Tracheal hypoplasia is often seen in conjunction with the above abnormalities. It can also be seen associated with other abnormalities not considered to be part of BAOS including megaoesophagus (abnormally enlarged oesophagus) (Coyne & Fingland 1992). Views vary as to how much 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 causes chronic or recurrent lower airway and lung infections (Harvey 1989).
  • Laryngeal hypoplasia (abnormally small, underdeveloped larynx) - In this condition the cartilages that form the larynx are soft and underdeveloped in shape, with abductor muscles that don’t function properly to open the larynx (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 to 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.

Individual dogs usually have some combination of the above defects (Brown and Gregory 2005) and most have more than one defect (Fasanella et al 2010). These anatomical defects increase the respiratory system’s workload in affected dogs leading, over time, to a progression of secondary problems. These include:

  • Collapse of the larynx - The increased negative pressure created in the pharyngolaryngeal (throat) region, as a result of the obstructions described above, ultimately results in distortion and collapse of the larynx (Wykes 1991). Three stages of laryngeal collapse have been described:

    In stage I laryngeal collapse the laryngeal saccules (small sac-like structures in the larynx) balloon and turn inside out (evert). This adds to the airway 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 form the support structure of the larynx) which 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 (collapse of bronchi – branching airways in the lungs) (De Lorenzi et al 2009).
  • Evertion and hypertrophy of the tonsils – Inflammation of the soft tissues in the throat area 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. They state that it can be associated with regurgitation (bringing up contents of the oesophagus – often saliva in BAOS) and vomiting (bringing up stomach contents). Dysphagia in brachycephalic dogs has been reported by many authors (eg Ducarouge 2002, Dupre and Freiche 2002, Koch and others 2003). Poncet et al (2005) also noted many reports of anatomical defects in the gastrointestinal tract of brachycephalic dogs. For example, hiatus hernias (where part of the stomach can pass up into the chest through a larger than normal hole in the diaphragm, and pyloric stenosis (where the stomach exit is narrowed stopping food leaving the stomach normally). They found a correlation between the severities of BAOS and of the gastrointestinal disease. They postulated that the severity of respiratory signs affected the severity of gastrointestinal signs and vice versa.
  • Heart Failure – This occurs secondarily to inadequate oxygenation of the blood in the lung, due to the constant airway obstructions. The body detects that the blood is inadequately oxygenated and narrows (vasoconstricts) the capillaries in parts of the lung that are poorly ventilated. 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 affect some brachycephalic bitches during whelping (giving birth) have been identified by Harvey (1989) and are a reason for caesarean section being performed in these dogs. 

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

Clinical signs in the mildest cases consist of snoring and respiratory noise on inspiration (breathing in), also called stridor (Brown & Gregory 2005, Fasanella et al 2010). Signs in more severe cases include exercise intolerance (inability to cope with exercise), 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 due to respiratory compromise 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 moderate to severely affected individuals, in which the animals rapidly develop respiratory distress – struggling to breathe - and can collapse and die. Severely affected individuals with laboured breathing, stand with their elbows held away from their chest in an attempt to ease it (Brown & Gregory 2005). With laboured breathing, over inflation of the chest is seen (Dupre 2008).

Dogs with more severe BAOS live a precarious existence and minor aggravations can lead to severe respiratory distress and a crisis (Hendricks 1995). Stress, exercise, excitement, 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. All dogs over 2 weeks of age exhibited sleep-disordered breathing including pauses in breathing and blood oxygen levels dropping below normal many times per hour. It is likely that this happens in Boston terriers as well.

BAOS is a progressive disease. Some of the primary problems, such as stenotic nares and elongated soft palate can be helped with surgery. Tracheal hypoplasia is untreatable (Brown & 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. However, the outlook for dogs with BAOS depends on which primary and secondary conditions they have at the time of treatment and how advanced they are. Overall prognosis for dogs that have had surgical interventions varies; Reicks et al (2007), in a study of 62 cases in various brachycephalic breeds, reported a favourable outcome for all dogs needing surgery. Poncet et al (2006) found good or excellent improvement in 88% of the 51 brachycephalic cases studied.

The prognosis and complexity of the surgery needed also varies depending on the exact characteristics of the disease in each individual dog.

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2. Intensity of welfare impact

BAOS has a major welfare impact. Dogs affected by BAOS suffer great distress (which may include apprehension 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 these crises can reoccur and can be life-threatening. Disrupted sleep patterns because of brachycephalic respiratory compromise 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.

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3. Duration of welfare impact

This is a life-long disorder, starting in puppies, progressing throughout life and ultimately often shortening life (Dupre 2008), with the average age of presentation to vets for treatment, for all brachycephalic breeds, being 3-4 years of age (Monnet 2008). Even with surgical treatment life-expectancy can be compromised with some authors recording 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 brachycephalic dogs are affected by upper airway obstruction to some degree. In the study by Fasanella et al (2010) Boston terriers were the breed third most commonly affected with BAOS (after bulldogs and pugs). The degree of problem depends on the number and severity of the anatomical abnormalities that are present in any individual dog.

Stenotic nares occur in between 46 and 50% of dogs with BAOS (Lorinson et al 1997, Harvey 1982a, Fasanella et al 2010).

Elongated 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 bradycephalics, but not commonly in Boston terriers (Ginn et al 2008)

Laryngeal collapse is common in Boston terriers with BAOS. For small dogs such as Boston terriers to have laryngeal correction is more surgically challenging than in larger dogs (Harvey 1982d). 66% of dogs with BAOS syndrome had everted laryngeal saccules in the study by Fasanella et al (2010).

Trachea hypoplasia is seen in some Boston terriers in conjunction with BAOS (Coyne & Fingland 1992).

Secondary problems are also common.  The reported occurrence of laryngeal problems varies from 30% to 64% (Harvey 1982 c, Harvey 1982 d, Dupre 2008). In one study of 73 cases of BAOS, on presentation, 97% had oesophageal, gastric or duodenal anomalies and 74% had gastrointestinal problems classed as moderate or severe  (Poncet et al 2005).

So it seems likely that all Boston terriers have BAOS to some extent, though some people may consider them “normal”.

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

Vets will suspect the syndrome in any brachycephalic dog showing appropriate signs, because it is so common. Diagnosis of stenotic nares can be made on clinical examination, with a definitive diagnosis of elongated soft palate, tonsillar hypertrophy and laryngeal collapse being made on examination under anaesthetic. Diagnoses of tracheal hypoplasia, right sided heart failure and confirmation of elongated palate are made with radiography (x-rays) under anaesthesia and gastrointestinal complications are investigated and confirmed via 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 gene defectsare 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.

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

As the Boston terrier is defined by its brachycephalic head shape – all Boston terriers are affected by the inherited defect which causes brachycephaly. Most brachycephalic animals 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 Boston terrier should ensure that it and its sire and dam are free of signs of BAOS and that they have not had correction surgery for any of the primary or secondary conditions.

Currently, identifying individuals that have had previous surgery 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 problem Harvey (1989) had a policy of simultaneous neutering all animals that had corrective surgery. It is not known if other veterinary surgeons have had this policy but it is certainly not universal. Any puppy which shows signs of BAOS should not be purchased; 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 bred from.

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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. However, it is the brachycephalic head shape that brings with it 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 extreme versions of this defect (Bannasch et al 2010). If dogs with signs of BAOS were not bred from the remaining gene pool of animals in this breed is likely to be unsustainable so it appears that outcrosses to non-brachycephalic breeds would be necessary. However, opinions differ as to whether it is ethically acceptable to breed animals whose welfare is likely to be compromised.

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9. Acknowledgements

UFAW is grateful to Rosie Godfrey BVetMed MRCVS and David Godfrey BVetMed FRCVS for their work in compiling this section.

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10. References

Bannasch D, Young A, Myers J, Truvé K, Dickinson P et al (2010) Localization of Canine Brachycephaly Using an Across Breed Mapping Approach. PLoS ONE 5(3): e9632. doi:10.1371/journal.pone.0009632

Brown D and Gregory S (2005) Brachycephalic Airway Disease. In: BSAVA Manual of Canine and Feline Head, Neck and Thoracic Surgery. Eds, D Brockman & D Holt. BSAVA, Cheltenham. pp 84

Burbidge HM, Goulden BE and Dickson LR (1988) Surgical relief of severe laryngeal malformation in an English Bulldog. New Zealand Veterinary Journal.36: 29-31

Coyne B and Fingland R (1992) Hypoplasia of the tracheal in dogs: 103 cases (1974–1990). Journal of the American Veterinary Medical Association, Volume 201: 768–772

Ducarouge B (2002) Le syndrome obstructif des voies respiratoires supérieures chez les chiens brachycéphales. Etudes clinique à propos de 27 cas. Doctoral thesis, Lyon

De Lorenzi D, Bertoncello D and Drigo M (2009) Bronchial abnormalities found in a consecutive series of 40 brachycephalic dogs. J Am Vet Med Assoc, Vol. 235, No. 7, p835-40.

Dupre G (2008) Brachycephalic Syndrome: New Knowledge, New Treatments. Presentation at WSAVA Congress, Dublin, Ireland, 20-24th August 2008 (On-line). Available at http://www.vin.com/proceedings/Proceedings.plx?CID=WSAVA2010&Category=&PID=56236&O=Generic. Accessed 20.7.10

Dupre G and Freiche V (2002) Ronflements et vomissements chez les bouledogues: traitement médical ou chirurgical? Proceedings of the AFVAC Annual Congress. Paris, France, November 10, 2002. pp 235-236

Fasanella FJ, Shivley JM, Wardlaw JL and Givaruangsawat S (2010) Brachycephalic airway obstructive syndrome in dogs: 90 cases (1991–2008). Journal of the American Veterinary Medical Association 237: 1048-1051

Ginn JA, Kumar MSA, McKiernan BC and Powers BE (2008) Nasopharyngeal Turbinates in Brachycephalic Dogs and Cats. Journal of the American Animal Hospital Association 44: 243-249

Harvey C (1982a)Upper airway obstruction surgery 1:Stenotic nares surgery in brachycephalic dogs. Journal ofAmerican Hospital Association 18: 535-537

Harvey C (1982b)Upper airway obstruction surgery 2:Soft palate resection in brachycephalic dogs. Journal of the American Animal Hospital Association 18: 538-544

Harvey C (1982c) Upper airway obstruction surgery 3:Everted laryngeal saccule surgery in brachycephalic dogs.Journal of the American Animal Hospital Association 18: 545-547

Harvey C (1982d)Upper airway obstruction surgery 4:Partial laryngectomy in brachycephalic dogs. Journal of theAmerican Animal Hospital Association 18: 548-550

Harvey C (1989) Inherited and congenital airway conditions. Journal of Small Animal Practice 30: 184-187

Hendricks J (1995) Recognition and treatment of congenital respiratory tract defects in brachycephalics. In Bonagura, J. (Ed) Kirk’s Current Veterinary Therapy XII small animal practice. Philadelphia: WB Saunders

Koch D, Arnold S, Hubler M and Montavon P (2003) Brachycephalic syndrome in dogs. Compendium on Continuing Education for the Practicing Veterinarian 25: 48-55

Lorinson D, Bright R and White R (1997) Brachycephalic airway obstruction syndrome – a review of 118 cases. Canine Practice 22: 18-21

Monnet E (2003) Brachycephalic airway syndrome. In: Slatter D (Eds) Textbook of Small Animal Surgery. Philadelphia: WB Saunders

Monnet E (2008) Brachycephalic airway syndrome (CVC Proceedings). (On-line). Available at http://veterinarycalendar.dvm360.com/avhc/Medicine/Brachycephalic-airway-syndrome-Proceedings/ArticleStandard/Article/detail/587147. Accessed 19th July 2010

Orsher RJ (1993) Brachycephalic airway disease. In Bojrab M (Ed) Disease Mechanisms in Small Animal Surgery. Philadelphia: Lea and Febiger

Pink J, Doyle R, Hughes J, Tobin E and Bellenger C (2006) Laryngeal collapse in seven brachycephalic puppies. Journal of Small Animal Practice 47: 131–135

Poncet C, Dupre G, Freiche G, Estrada M, Poubannet Y and Bouvy B (2005) Prevalence of gastrointestinal tract lesions in 73 brachycephalic dogs with upper respiratory syndrome.Journal of Small Animal Practice 46: 273–279

Reicks TW,Birchard SJ and Stephens JA (2007) Surgical correction of brachycephalic syndrome in dogs: 62 cases (1991–2004). Journal of the American Veterinary Medical Association 2301324-8

Shell L (2008) Brachycephalic Airway Syndrome. (On-line). Available at http://www.vin.com/Members/Associate/Associate.plx?DiseaseId=564. Accessed 16.7.10

Stockard  (1941)Wistar Institute Monograph. The genetic and endocrine basis for differences in form and behaviour as elucidated by studies of contrasted pure line dog breeds and their hybrids. Animal Anatomical Memoirs No 19: The Wistar Institute

Torrez C and Hunt G (2006) Results of surgical correction of abnormalities associated with brachycephalic airway obstruction syndrome in dogs in Australia. Journal of Small Animal Practice 47: 150–154

Venker-van Haagen A (1995) Diseases of the throat. In Ettinger S and Feldman E (Eds) Textbook of Veterinary Internal Medicine. Philadelphia: WB Saunders

Wykes P (1991) Brachycephalic airway obstructive syndrome. Probl Vet Med 3: No. 2, p 188-97

© UFAW 2011

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