The epidemiology of mammary tumours in insured female dogs in Sweden

Mammary tumour (MT) is the most common neoplasia in bitches. This study explored the incidence and cause-specific mortality rates of and risk factors for MT in dogs insured by Agria Djurförsäkring in Sweden, 2011-2016. The study population included just over 600,000 dogs, and 13,109 females had veterinary care claims for MT. The incidence rate in females was 157 cases per 10,000 dog-years at risk (95 % confidence interval (CI): 154-160). The breeds at highest risk of MT were the Kerry blue terrier (relative risk (RR) 4.72, 95 % CI 2.58-7.92) and English springer spaniel (RR 3.32, 95 % CI: 3.02-3.63), while the breeds at lowest risk were the German spitz (RR 0.06, 95 % CI: 0.00-0.33) and English bulldog (RR 0.07, 95 % CI: 0.00-0.37). The median age at first MT diagnosis was 8.94 years (interquartile range (IQR) 7.43-10.5). The Dogue de Bordeaux (median age at diagnosis 5.30 years) and Irish wolfhound (6.42 years) were youngest at diagnosis, and the Miniature dachshund (10.9 years) and West Highland white terrier (11.0 years) were oldest. Higher odds of MT were found in bitches with a history of pseudopregnancy (OR 2.10, P < 0.001) and acute mastitis (OR 4.93, P < 0.001). In total, 486 bitches died of MT-related causes at a median age of 9.32 years (IQR 8.37-9.99). The Large Munsterlander had the highest risk of MT-related death (RR 40.3, 95 % CI: 8.28-118.4). In conclusion, MT mainly affected middle-aged to older bitches. Large breed-related differences in the risk of MT and age at diagnosis were found.