Page 42 - WSAVA2018
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 25-28 September, 2018 | Singapore
WSV18-0098
GENETICS
WHAT PRACTITIONERS SHOULD KNOW ABOUT THE GENETICS OF BRACHYCEPHALY
J. Bell1
1Tufts Cummings School of Veterinary Medicine, Deptatment of Clinical Sciences, N. Grafton- MA, USA
WHAT PRACTITIONERS SHOULD KNOW ABOUT THE GENETICS OF BRACHYCEPHALY
Jerold S Bell DVM, Adjunct Professor of Clinical Genetics
Tufts Cummings School of Veterinary Medicine, N. Grafton, MA USA
jerold.bell@tufts.edu
Part of the welfare considerations practitioners deal with in our patients is the expression of extreme phenotypes or morphology. The “more is better” rule has led to extremes in size (large and small), hair, skin folds, angulation, and the most frequently seen is extreme brachycephaly.
Public preference for dogs and cats with pushed-
in noses and bulging eyes has created a breeding environment that has crossed a tipping point in predisposing to disease. Brachycephalic obstructive airway syndrome (BOAS) is a disorder of breathing difficulty in short-snouted and “bully” breeds (especially the popular English Bulldogs, French Bulldogs and Pugs).1 In Persian and Himalayan cats, BOAS causes chronic sinusitis.2 These dog and cat breeds can also have issues with skin fold dermatitis, corneal ulceration, globe proptosis, dental malocclusion, and dystocia.1
Of the anatomical components of BOAS, stenotic nares and hypoplastic trachea are inherited. The length of the soft palate and therefore soft palate/epiglottis overlap is also inherited. Soft palate thickness (which exacerbates BOAS) is caused by hyperplasia secondary to chronic turbulence, as are everted laryngeal saccules. Laryngeal collapse and hiatal hernia (with regurgitation) are secondary to the effects of negative inspiratory pressure from restricted airflow.
THE INCIDENCE OF BOAS IS RAPIDLY INCREASING
BOAS is becoming a greater clinical issue due to the rapidly increasing popularity of the susceptible breeds. Registrations of French Bulldogs have increased
3,000% over the past 10 years.1 As the clinical morbidity of BOAS does not usually manifest until middle age, these rapidly expanding young breed populations will continue to increase their presentation with symptomatic BOAS. Another welfare issue involves the increase of brachycephalic dogs relinquished to shelters and rescue organizations, and the difficulty rehoming dogs with breathing difficulties.
Compounding the matter is the fact that up to 60% of owners do not see their dogs (or cats) as being impaired by BOAS. The “normalization” of brachycephalic stridor also extends to veterinarians: “That’s just how Pugs breath.” However, BOAS dogs are regularly seen – especially in warm weather - with dyspnea, exercise intolerance, heat intolerance, abnormal and increased respiratory noise, sleep disorders, cyanosis, syncope and death.
Individual dogs in breeds with extreme brachycephaly die younger (median longevity 8.6 years) than dogs
in moderate and non-brachycephalic breeds (median 12.7 years). A higher proportion of deaths in extreme brachycephalic breeds are due to upper respiratory disease (16.7%).3
PHENOTYPIC SCREENING AGAINST BOAS
The genetics of brachycephaly are present in all members of the involved breeds. Therefore the question becomes, “What is genetically different between dogs in these breeds exhibiting BOAS and those that do not? Part of this question also involves identifying diagnostic criteria that separates BOAS affected and non-affected dogs.
A group at the Royal Veterinary College in Cambridge, UK led by Dr. David Sargan has developed a closed chamber for Whole-Body Barometric Plethysmography (WBBP). A dog’s airflow is measured at rest and after controlled activity. Physiological measurements of airflow volume and pattern consistently separate dogs and breeds with BOAS versus those without BOAS. Based on WBBP, approximately 50% of the study dogs
in the three extreme brachycephalic breeds were BOAS affected.1While WBBP is not practical for widespread screening, its results have allowed studies of other measurements that differentiate between BOAS+ and BOAS- dogs. The data show that roughly 50% of BOAS variation is genetic and 50% is environmental; including exercise, feeding, temperature and humidity. There are several recorded measurements that are significant in different breeds and sexes. However stenotic airways and body condition score (obesity) are the most significant factors correlated to the clinical presentation of BOAS across all breeds.4Stenotic nares are primarily caused by the enlargement of the alar folds that impinge into the nasal opening. Veterinary surgeons demonstrate that surgically correcting stenotic nares relieves most dogs of the clinical signs of BOAS. Surgery removes
the lateral alar cartilage. The rule of thumb is to open the nares to a cumulative 1/3 the width of the nose. According to WBBP almost all dogs improve their BOAS index after nares surgery and 40% become BOAS- non- affected.1
Dr. Rowena Packer at the Royal Veterinary College developed a measuring system for stenotic nares.
She states that each breed has very specific nares conformation, and breed-specific values would need to be established for setting breeding goals. Studies show
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43RD WORLD SMALL ANIMAL VETERINARY ASSOCIATION CONGRESS AND 9TH FASAVA CONGRESS






































































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