Factors associated with mortality of geriatric horses in the United Kingdom.

Abstract: Geriatric horses (aged≥15 years) now represent a substantial proportion of the equine population, yet mortality rates for aged horses in the UK have not previously been described. Although post-mortem studies have provided some data regarding specific causes of death, "old age" is a common owner-reported reason for euthanasia of adult horses, indicating further elucidation of reasons for death or euthanasia is required for the geriatric equine population. The objective of this cohort study was to describe mortality rates, causes of death or euthanasia and factors associated with mortality in geriatric horses and ponies. Veterinary registered horses randomly selected for a cross-sectional questionnaire survey on geriatric health were enrolled in the cohort study (n=908). Follow-up information was obtained over an 18-month period via telephone questionnaires and mortality questionnaires, providing data regarding reasons for death or euthanasia, were completed for 118 cases of mortality. Overall and stratified mortality rates were calculated and Cox proportional hazards models were used to investigate risk factors associated with mortality. The majority (94%) of mortalities were euthanised, most frequently due to lameness (24%) and colic (21%). Veterinary advice was important in owner decision making regarding euthanasia of cases of colic or other acute illnesses, while poor quality of life was considered an important factor where euthanasia was due to chronic diseases or lameness. Overall mortality rate was 11.1 (95% C.I. 9.2-13.2) per 100 horse-years at risk, with the mortality rate of animals >30 years of age over five times the rate in horses aged 15-19 years. On multivariable analysis, increasing age was associated with increasing mortality and Cob/Cob crossbreeds and Thoroughbred/Thoroughbred crossbreeds had an increased risk of mortality. Horses considered to be underweight had a greater risk of mortality than those in good condition. Increasing number of owner-reported clinical signs and the degree to which pain was reported to limit normal daily activities were associated with increased risk of mortality. The results of this study provide useful information about rates and factors associated with mortality in geriatric horses in the UK, which should be relevant to veterinary surgeons involved in the treatment of aged horses.