Table A1.

Potential canine risk factors for association with HRI in UK dogs [16].

Potential Risk Factor for HRI	Variable Definition	Justification
Breed type	Categorical variable including all named breed types (including both KC recognised purebred and non-KC recognised purebred) and designer hybrid types with contrived names (e.g., Cockapoo, Labradoodle, Lurcher) with ≥5 HRI cases and/or ≥5000 dogs in the overall study population. All remaining dogs were assigned to grouped categories of “other purebred”, “other designer cross” or “non-designer crossbred”.	Chow Chow, Bulldog, French Bulldog, Dogue de Bordeaux, Greyhound, Cavalier King Charles Spaniel, Pug, English Springer Spaniel and Golden Retriever breeds have all be identified as having greater odds of HRI in UK dogs [16]. Labrador Retriever was used as the comparator for this variable as they were the largest breed type in the denominator population (after crossbred) so enabled high statistical power to explore breed risks [38,40].
Purebred	Categorical variable grouping all dogs of recognisable breeds as “purebred”, all recognisable designer crossbreeds as “designer cross” and the remaining dogs as “crossbred”.	Purebred dogs are more likely to have an exaggerated conformation such as brachycephaly, thick coat, or giant body size, limiting their ability to thermoregulate [50]. A higher percentage of purebred dogs presented with heatstroke to one veterinary hospital [34].
Skull shape	Purebred dogs were categorised by skull shape into three groups, “brachycephalic”, “mesocephalic” and dolichocephalic” (see Supplementary note 1 for breeds by category). Designer crossbred dogs including a brachycephalic breed were classified as “brachycephalic cross” and all other dogs listed as crossbred or unrecorded breed were classified as “skull shape unrecorded”.	Surface areas of the nasal turbinates and effective ventilation provide the mechanism to enable evaporative heat loss through panting, thus brachycephalic dogs have reduced heat dissipation mechanisms [15,18,50,64].
Adult bodyweight	Adult bodyweight was defined as the mean of all bodyweight (kg) values recorded for each dog after reaching 18 months old. Bodyweight (kg) was then categorised into seven groups (<10, 10– < 20, 20– < 30, 30– < 40, 40– < 50, ≥50), dogs under 18 months or with no recorded adult bodyweight were classified as “unrecorded”.	Small breeds of dog are reported to have decreased risk of HRI [18], dogs with greater body mass have been reported to develop higher post exercise body temperatures [19].
Bodyweight relative to breed/sex mean	A categorical variable grouping dogs with a mean adult bodyweight “equal or above” or “below” the mean adult bodyweight for their breed and sex (calculated using the overall VetCompass study population). An “unrecorded” variable included all dogs with no adult bodyweight or labelled as crossbred.	Increased bodyweight can be due to increases in either lean muscle mass, or body fat. Obesity limits heat conduction and radiation from the skin and can limit effective cooling via respiration [50], overweight animals overheat faster and take longer to cool [65]. Dogs with greater lean body mass developed higher post-exercise temperatures than lighter dogs [19].
Sex/neuter	Dogs were classified by sex and neuter status into five categories (female entire, female neutered, male entire, male neutered) with “unrecorded” was used to group any dogs with no recorded sex or neuter status.	Male dogs develop higher body temperature post exercise [19,20], and are over-represented in cases of heatstroke presenting to veterinary hospitals [37,66,67].
Age	The age variable described the age of the dog at the end of the study period (31st December 2016) for non-case dogs, or the age at the first HRI event for 2016 incident HRI cases. Age (years) was categorised into eight groups (<2, 2– < 4, 4– < 6, 6– < 8, 8– < 10, 10– < 12, ≥12) with “unrecorded” for any dogs with no date of birth recorded in the EPR.	Older animals are more likely to have pre-existing conditions that limit effective heat dissipation such as heart disease, or respiratory diseases e.g., laryngeal paralysis [64].