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Fig 1.

Examples of different degrees of nostril stenosis in pugs, French bulldogs, and bulldogs.

The following descriptions were adapted from a previously published figure by the authors (Fig 1 in Liu et al. 2016): “Open nostrils: nostrils are wide open; mildly stenotic nostrils: slightly narrowed nostrils where the lateral nostril wall does not touch the medial nostril wall. Immediately after the exercise tolerance test (ETT), the nostril wings should move dorsolaterally to open on inspiration; moderately stenotic nostrils: the lateral nostril wall touches the medial nostril wall at the dorsal part of the nostrils and the nostrils are only open at the bottom. Immediately after the ETT, the nostril wings are not able to move dorsolaterally and there may be nasal flaring (ie, muscle contraction around the nose trying to enlarge the nostrils; severely stenotic nostrils: nostrils are almost closed. The dog may switch to oral breathing from nasal breathing with stress or very gentle exercise such as playing.” (Liu et al. 2016).

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Table 1.

Descriptions of the soft tape measurements.

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Fig 2.

Demonstration of the soft tape measurements.

Nine measurements were made with a firmly held soft tape measure with the dog standing and at rest: Skull length (SL), cranial length (CL), snout length (SnL), neck length (NL), body length (BL), eye width (EW), and skull width (SW). The detailed definitions of the measurements are shown in Table 1. The photos of the dogs were taken from two of the study dogs by the authors.

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Table 2.

Signalment and the proportions of the subjects with stenotic nostrils and different functional grades.

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Fig 3.

Scatter plots show the inter-observer reproducibility of the conformational soft tape measures.

The diagonal red line indicates perfect agreement. ICC, intra-class correlation coefficient; CFR, craniofacial ratio; EWR, eye width ratio; SI, skull index; NGR, neck girth ratio; NLR, neck length ratio.

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Table 3.

The breed-specific models predicting the probability of having brachycephalic obstructive airway syndrome (BOAS).

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Table 3 Expand

Fig 4.

Stacked bar charts demonstrate the relationship between BOAS functional grade and body condition score (BCS), and the degree of nostril stenosis.

The y-axis shows the percentage of dogs of that functional grade showing each degree of BCS or nostril stenosis. Note the clear trends of an increased proportion of obese dogs and dogs with moderately or severely stenotic nostrils with an increase in functional grade. In French bulldogs, there were over 10% of Grade III dogs that had BCS of 3. The underweight condition was attributed to BOAS-related frequent regurgitation.

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Fig 5.

Univariate logistic regression plots demonstrate the trend of the five conformational ratios against the estimated BOAS probability.

The x-axis is the numeric data of each ratio in percentage; the y-axis is the estimated probability of BOAS. CFR, craniofacial ratio; EWR, eye width ratio; SI, skull index; NGR, neck girth ratio; NLR, neck length ratio.

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Fig 6.

The predictive performance of the breed specific models.

(A) boxplots show the distributions of the estimated probability of being BOAS-affected. The dotted line at 50% of the probability represents the raw predictive cut-off. For pugs and French bulldogs, the cut-off values can be adjusted to improve the specificity of the models; (B) receiver operating characteristic (ROC) curves show the predictive performance of the breed specific models. Area under the curve (AUC) was computed with its 95% confidence interval in the bracket.

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Table 4.

Validation of the predictive models using BOAS index.

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Fig 7.

An illustration demonstrates the relationship between the external craniofacial ratio (CFR) measurement and the corresponding internal anatomical structures of the upper airway.

The realistic anatomical illustration was made according to a computed tomographic 3-dimensional rendering image of a French bulldog. The illustration was reprinted from the Cambridge BOAS research group website ( under a CC BY license, with permission from the group in the University of Cambridge, original copyright 2016.

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