Imagine this clinical scenario: a client brings in their three-year-old male neutered Pug because he is displaying severe dyspnoea and tachypnoea. The weather is sunny (23 degrees Celsius), and the patient was moderately exercised at midday with the owner’s other dog, a Labrador.
After you have stabilised and treated the patient, the owner asks why the Pug needed emergency treatment when their Labrador was unaffected. From your own experience, you suspect that brachycephalic dogs are at higher risk of developing heatstroke but decide to do some research to ensure that your view is supported by the evidence.
Heat-related illness (HRI) describes a spectrum of heat-induced conditions that range from the mild form (traditionally called heat stress), which can respond to cooling and rehydrating, to the severe, life-threatening disorder (traditionally called heatstroke) characterised by brain damage and multiple organ failure (Hall et al., 2020).
The evidence
Four papers were critically reviewed. Two were retrospective cohort studies (Hall et al., 2020 and Segev et al., 2015a), one was a prospective cohort study (Segev et al., 2015b) and one was a case control study (Davis et al., 2017). All of the studies reviewed investigated the factors leading to heat-related illness in dogs. They all included a mixture of brachycephalic and non-brachycephalic breeds and considered brachycephaly as a factor in cases of heat-related illness.
All [the studies] included a mixture of brachycephalic and non-brachycephalic breeds and considered brachycephaly as a factor in cases of heat-related illness
Hall et al. (2020) examined a database of 905,543 dogs from a veterinary practice group. The authors found 1,222 patients that had experienced 1,259 incidents of HRI over a two-year period. A risk factor analysis was carried out for each HRI case, assessing breed type, skull shape, adult body weight, sex, neuter status and age.
Segev et al. (2015a) examined the medical records of 126 dogs that were presented to a hospital with HRI. The outcomes studied were objective assessment of signalment, vital signs on presentation, history of exertional/environmental heatstroke, mortality and results from diagnostic testing.
Segev et al. (2015b) examined 30 dogs that presented with HRI, as well as a control group of 13 healthy staff-owned dogs. The outcomes studied were objective assessment of signalment, clinical signs, renal biomarkers and fatalities.
Davis et al. (2017) examined 105 client-owned dogs, 52 of which were brachycephalic. Each dog was exposed to two environmental conditions (hot and cold) using a climate control system under laboratory conditions. This was designed to test their tolerance to hot and cold conditions. The outcomes studied were objective measures of respiratory pattern, body temperature, tidal volume and breathing cycle duration.
Limitations of the evidence
The overall strength of the evidence was moderate, and each study had some limitations.
Hall et al. (2020), which was based on patient records from multiple practices, had the potential for observer, recall and recording biases, missing data and no record of the prevalence of brachycephaly in the study population. Clinical signs used to identify HRI were broad, no standardised definition for HRI was used and brachycephaly was classified according to breed type (not individual patient skull morphology).
Segev et al. (2015a) did not provide a sample size calculation, and the sample was drawn from one referral population of animals. The study excluded dogs euthanised on financial grounds or where the owner declined treatment and dogs with concurrent conditions. There was the possibility of recall or observer bias, and limited data were available regarding breed classification and signalment.
Segev et al. (2015b) had a high likelihood of selection bias and the potential for recall/observer bias. There was a small sample size and limited details on methodology. The study excluded dogs euthanised on financial grounds or where the owner declined treatment and dogs with concurrent conditions. There was no record of the prevalence of brachycephaly in the population for comparison. Limited data were available regarding breed classification and signalment.
Davis et al. (2017) did not provide the sample size calculation. Findings in the laboratory environment may not apply to naturally occurring HRI, and stress or other factors may have affected the results. No dogs with brachycephalic obstructive airway syndrome (BOAS) were included, and brachycephaly was assigned according to breed type rather than morphology.
Summary of findings
The findings of all four studies provide moderate evidence to suggest that brachycephaly is a risk factor in the development of HRI. Hall et al. (2020), Segev et al. (2015a) and Segev et al. (2015b) all found that brachycephalic dogs were over-represented in presentations of HRI, while Davis et al. (2017) found that brachycephalic dogs were more affected by exposure to heat stress than non-brachycephalic dogs.
The findings of all four studies provide moderate evidence to suggest that brachycephaly is a risk factor in the development of heat-related illness
Hall et al. (2020) found that skull shape is a significant risk factor for HRI, with brachycephalic dogs more likely to develop exertional, environmental and vehicular HRI compared to mesocephalic dogs. The paper also reported that French Bulldogs have a significantly higher odds ratio of environmental, exertional and vehicular HRI compared with Labrador Retrievers.
Segev et al. (2015a) found that brachycephalic dogs were disproportionately affected by HRI: 29 percent of the dogs studied were brachycephalic, compared with 16 percent of the general population presented to the hospital. However, the authors did not detect any significant difference in mortality between brachycephalic and non-brachycephalic dogs.
Segev et al. (2015b) found that 33 percent (10/30) of all HRI-affected dogs studied were brachycephalic, with the most commonly affected breeds being Boxers and French Bulldogs.
Davis et al. (2017) found that brachycephalic dogs had a significantly increased respiratory rate and a significantly greater increase in body temperature when exposed to a hot environment under laboratory conditions. Additionally, 10.4 percent of the brachycephalic dogs studied were unable to complete the hot environment trial due to respiratory distress.
Conclusion
The studies collectively provide a moderate strength of evidence to support the hypothesis that brachycephalic dogs are at an increased risk of heat-related illness.
Heat-related illness is a multifactorial condition and that age, body condition score and skull morphology are also likely to be significant in [its] development
Further studies are required to increase the strength of the evidence, and the following would be beneficial in this regard: studies that include systematic reviews with meta-analysis, the impact of BOAS, and specific investigations of skull morphology instead of breed classification. It is also worth noting that HRI is a multifactorial condition and that age, body condition score and skull morphology are also likely to be significant in the development of HRI.
| The full Knowledge Summary can be read in RCVS Knowledge’s open access journal Veterinary Evidence. |
Disclaimer
The application of evidence into practice should take into account multiple factors, not limited to individual clinical expertise, patient’s circumstances, owner’s values, the individual case in front of you, the availability of therapies and resources, and the country, location or clinic where you work.
Knowledge Summaries are a resource to help reinforce or inform decision making. They do not override the responsibility or judgement of the practitioner to do what is best for the animal in their care.
