This is the largest study to date using primary-care veterinary data that specifically aimed to report on SBT disorder predisposition and protection and used an underlying comparator group that reflected the entire remaining population of dogs under veterinary care. The study characterised the demography and health of a large cohort 22,333 dogs of which 1304 were SBTs under primary veterinary care in the UK. This has enabled reporting of relative predispositions to, and protections from, disorders, as well as absolute prevalence values using methods that have previously been restricted due to the inability to access the large data resources required for such analyses. This approach can now also be extended to explore the health of many other breeds.
SBTs were no more likely to have at least one disorder than non-SBTs (OR 0.90; p = 0.102). At a specific-level of diagnostic precision, SBTs had higher odds of 4/36 (11.1%) disorders compared to non-SBTs and had reduced odds of 5/36 (13.9%) disorders. There were 27/36 (75.0%) disorders with no significant difference in odds between SBTs and non-SBTs. At a grouped-level of diagnostic precision, SBTs had higher odds of 2/32 (6.3%) disorders and had reduced odds of 5/32 (15.6%) disorders compared to non-SBTs. There were 25/32 (78.1%) disorders with no significant difference in odds between SBTs and non-SBTs. Although severity and duration of disorder predispositions and protections should be taken into account when fully interpreting these results, these immediate results suggest no evidence that SBTs have higher overall health problems compared to the remainder of the general UK canine population. Despite this overall conclusion, the current study did identify some specific predispositions and protections that are very relevant to the breed.
The current study hypothesised that SBTs have higher odds of aggression compared with non-SBTs. This is an important question to answer because the perception of differential risk of aggression between SBTs and other breeds has a strong impact on the potential to rehome SBTs . However, SBTs showed no significant difference in odds for aggression compared to non-SBTs (OR 1.09; p = 0.644). This contrasts with previous reports that SBTs showed higher levels of aggression compared to other dog breeds [6, 7], although many of these reported predispositions were documented over 20 years ago. It is possible there has since been a shift in breed behaviour, with more SBTs kept as family pets, rather than as a status symbol as they have been historically, reducing levels of aggression in the breed . The current study findings suggest that potential owners visiting rehoming centres should avoid preconceptions about the breed behaviour, as given responsible ownership and an enriched environment SBTs can make a suitable family pet . It should be noted that all forms of aggression were grouped and evaluated as a single disorder. The study design used for the current study based on a spectrum of disorders managed in primary-care practice precluded extraction of sub-categories of aggression with deeper context. However, future studies with specific focus on aggression could evaluate different forms of aggression within different dog breeds to gain a deeper understanding of these issues.
SBTs showed predisposition to seizure disorder at a specific-level of diagnostic precision (OR 2.06), with prevalence of 1.38%. SBTs were previously overrepresented in a UK based study of 1260 epileptic dogs . However, previous research based on primary-care data did not identify a significantly increased or decreased risk of epilepsy of unknown origin in SBTs compared with crossbreeds . A further report with similar methodology identified SBTs at decreased risk (OR 0.72) of seizure disorder compared with Labrador Retrievers . The contrasting findings in the current study may be due to the different case definitions and comparative populations used. The current study classified disorders according to their most precise diagnostic term, thus SBTs diagnosed with seizure disorder did not encompass those dogs with a more precise term, such as epilepsy. Therefore, the findings might suggest breed diagnostic differences rather than a true predisposition and should be interpreted with some caution. In addition, 18 SBTs were recorded with seizure disorder, therefore the relatively small number of cases identified may artificially inflate the odds ratio. SBTs have previously been reported as predisposed to L-2HGA, an inherited metabolic disorder which can result in seizures . None of the 18 cases mentioned L-2HGA as a cause of seizure disorder, which may reflect the rarity of L-2HGA, however only 1/18 (5.6%) SBTs were tested based on the EPRs. There is no known treatment for L-2HGA and seizures are usually well controlled with anti-epileptic drugs . Therefore, given that diagnosis of L-2HGA doesn’t necessarily alter treatment outcome, it might be that identification of this genetic mutation is a higher priority in SBTs used for breeding to reduce risk of affected offspring.
SBTs showed predisposition to atopic dermatitis at a specific-level of diagnostic precision (OR 1.88). This concurs with previous research that reported SBTs as the breed with the fifth-highest incidence of atopic dermatitis in insured dogs in Sweden (8.0 cases per 1000) . Despite the predisposition identified, the prevalence of atopic dermatitis in the current study was relatively low (1.99%), compared with a US estimate of 8.7% in dogs overall . Atopy is complex in its aetiology and diagnosis [35, 36] and therefore the true prevalence may be higher than reported here and thus the disorder should be considered important for the breed due to the associated negative impacts on quality of life . At a grouped-level of diagnostic precision, SBTs showed predisposition to skin disorders (OR 1.18). Whilst this would include atopic dermatitis, and possible alternative diagnostic terms used for atopy, SBTs have previously been identified with predisposition to juvenile-onset demodicosis [38, 39]. Demodicosis did not feature within the top 30 specific disorders in SBTs or non-SBTs, however may have been included within skin disorders at a grouped-level.
SBTs showed predisposition to skin masses at a specific-level of diagnostic precision (OR 1.80) and masses at a grouped-level (OR 1.51), with prevalence of 3.76 and 8.05% respectively. A previous study based on UK primary-care data did not find a significant difference in skin mass prevalence between SBTs and crossbreeds, however confounding factors were not accounted for in this previous study and a different comparator group was used . SBTs have documented predisposition to mast cell tumours (MCTs), which most commonly present as skin masses [40,41,42,43]. Although predisposition to neoplasia was not identified in the current study, it might be that a proportion of the skin masses were MCTs, but were not investigated further and so a definitive diagnosis not reached. SBTs have previously reported predispositions to gastric carcinoma , mammary carcinoma , and had the eighth-highest proportional mortality from neoplasia among pedigree breeds . Therefore, it is possible that the predisposition to masses identified in SBTs, but not neoplasia, is reflective of diagnostic differences between breeds.
SBTs had 1.76 times the odds of stiffness at a specific-level of diagnostic precision compared with non-SBTs, with a prevalence of 1.15%. This finding should be interpreted with some caution as no significant predispositions were identified in similar categories, including lameness, osteoarthritis and musculoskeletal disorders. Stiffness is a clinical sign commonly associated with musculoskeletal disorders, but is not a formal biomedical diagnosis in itself . Therefore, the predisposition to stiffness reported may be a reflection of the different language or diagnostic criteria used by veterinarians between different dog breeds. It may also indicate that musculoskeletal disorders in SBTs appear relatively mild, therefore “stiffness” may not be categorised at a further diagnostic level. Indeed, veterinarians rated Pitbull breed types with low pain sensitivity compared with other breeds and gave lower ratings for Pitbull breed types than the general public . Given the prevalence of elbow dysplasia in SBTs previously reported as 31.3–33.3% [48, 49] and the documented stoicism of the breed , it might be that “stiffness” is indicative of an underlying condition that may warrant further investigation.
The UK KC offers eye testing and DNA tests for HC-HSF4 and L-2HGA in SBTs, inherited conditions to which the breed is reportedly predisposed . These conditions were not identified in sufficient numbers to be included in the current study. This may reflect a true rarity of these disorders, or it could be that they are just not routinely tested for. Further studies evaluating the prevalence of these disorders would be useful to determine the value of genetic testing in the breed.
SBTs showed protection to patella luxation at a specific-level of diagnostic precision (OR 0.15), with prevalence of 0.15%. This is in agreement with previous research based on primary-care data in which SBTs had 0.5 times the odds of patella luxation compared with crossbreeds . As a protected breed, SBTs could be used as a model to explore conformational and other factors that may assist to develop strategies to reduce the prevalence in other higher-risk breeds, such as the Bichon and Bulldog.
SBTs showed protection to enteropathy (OR 0.78, prevalence 7.29%) and anal sac disorder (OR 0.48, prevalence 2.84%) at a grouped-level of diagnostic precision and anal sac impaction at a specific-level (OR 0.53, prevalence 2.61%). This is similar to previous research based on primary-care data in which prevalence of enteropathy and anal sac disorder were lower for SBTs compared with crossbreeds . Although enteropathy is a broad term, soft faeces and inflammatory bowel disease have been associated with anal sac disease , therefore enteropathy and anal sac disorders are somewhat interlinked and may share some of the same risk factors, including diet [52, 53]. It might also be that reduced risk of anal sac disorders is linked to conformation, with SBTs being a highly muscular breed . Further research exploring the mechanisms underlying these protective effects in SBTs may benefit the at-risk breeds in the population.
SBTs showed protection to dental disorders at a grouped-level of diagnostic precision (OR 0.45) and periodontal disease (OR 0.41) and retained deciduous tooth (OR 0.19) at a specific-level. This is in agreement with previous research based on a primary-care population in which SBTs had a lower prevalence of dental disorder compared to crossbreeds (3.0% versus 9.8%) . Although SBTs had protection to dental disorders in the current study, they still had a relatively high prevalence of such conditions (7.82% for dental disorders and 6.52% for periodontal disease). Exploration of the reasons why SBTs are protected in comparison to other breeds may enable these protective factors to be increased so that the breed prevalence decreases further, and equally that the health and welfare of other breeds might also be improved.
SBTs showed protection to heart disease at a grouped-level of diagnostic precision (OR 0.33) and heart murmurs at a specific-level (OR 0.33), with prevalence of 1.15 and 0.84% respectively. This is in agreement with previous research in which prevalence of heart disease and heart murmur were lower for SBTs than for crossbreeds under primary-care in the UK . In addition, SBTs have previously been reported at lower odds (OR 0.25) of degenerative mitral valve disease compared with crossbreeds . It might be that the historic use of SBTs as a fighting dog and “muscular, active and agile” appearance  has resulted in residual protection to cardiac disorders even in today’s dogs. Therefore, this could suggest some value in using SBTs in cross breeding programmes to outcross other breeds affected with specific cardiac predispositions .
SBTs showed protection to upper respiratory tract disorder at a grouped-level of diagnostic precision (OR 0.49), with prevalence of 1.69%. To the authors’ knowledge this protection has not been documented previously, therefore future work may explore this novel finding in greater detail. SBTs have been reported as at-risk to Brachycephalic Obstructive Airway Syndrome (BOAS) [56, 57], which can be characterised by increased and/or abnormal upper respiratory tract noise . BOAS was not one of the most common disorders recorded in SBTs or non-SBTs in the current study, however is thought to be underreported in veterinary clinic records due to psychological desensitisation via normalisation in commonly affected breeds . SBTs have been documented as having “noisy breathing” , however it may be that this is “accepted” by owners and veterinary professionals as a typical breed characteristic rather than the possibility of associated underlying pathology. Given the popularity of brachycephalic (“flat-faced”) dogs, but their intrinsic health and welfare issues , breeding away from this phenotype in SBTs should be encouraged.
The limitations of this study include those reported in previous VetCompass publications based on similar methods that applied retrospective analysis of primary-care EPR data [2, 62]. Additionally, the current study did not account for differences in severity and duration between disorders, which could provide further insights into the nature and ranking of breed predispositions . Comparing the relative number of predispositions to protections does not necessarily reflect breed health without a measure of severity, however the study findings highlight the types of conditions SBTs are predisposed to and protected from.
A number of the predispositions identified, including seizure disorder, skin mass, stiffness and mass and the protections, including heart murmur, were the residual groupings when a more precise diagnosis was not reached. Therefore, these findings need to be interpreted with some caution, as they may reflect diagnostic differences between SBTs and other dog breeds rather than a true predisposition or protection. The greatest confidence may be given to predispositions identified at the greatest level of diagnostic precision, namely atopic dermatitis.
Although dogs were classified using a dualist system as either ‘SBT’ or ‘non-SBT’, the high proportion (26.8%) of crossbreeds among the non-SBTs might have caused some dilution of the phenotype given that some of these crossbreds may have included some SBT parentage. In addition, SBTs can be difficult to identify based on appearance alone, with some disagreement in classification of SBTs and Pit Bulls (based on photographs) reported between UK and US shelter workers . However, difficulty in breed classification is not necessarily confined to one specific breed type, therefore it is unlikely misclassification was unidirectional.
This study used multiple comparisons. Strict adherence to a cut-off P-value of 0.05 to infer significance for multiple comparisons can lead to a Type 1 error of accepting false positive results. Furthermore, the small number of cases in some disorders reported might have led to a type 2 error of accepting false negative results . We recommend that readers do not rely on the P-values of odds ratios alone, but consider the confidence intervals and prevalence percentages when interpreting the current results . Consequently, the individual results for each of the disorders assessed should be interpreted as exploratory rather than confirmatory.