Background: Desexing dogs is promoted for population control, preventative healthcare, and behavior modification. Common methods are orchiectomy and ovariectomy/ovariohysterectomy. GnRH superagonist implants are available in some areas. Alternative methods like vasectomy and salpingectomy/hysterectomy are uncommon. The terminology used to describe desexing is inconsistent and contradictory, showing a need for the adaption of standardized terminology. Population Control: Surprisingly, empirical studies show no effects of desexing on population control in companion and shelter dogs despite desexing being consistently recommended in the literature. There is evidence for a population control effect in free-roaming dogs, where desexing also has benefits on zoonotic disease and bite risk. Population control in free-roaming dogs is mostly correlated with female, not male desexing. Health and Lifespan: Desexing affects numerous disease risks, but studies commonly neglect age at diagnosis and overall lifespan, age being by far the most important risk factor for most diseases. We argue that lifespan is a more important outcome than ultimate cause of death. A beneficial effect of desexing on lifespan is consistently demonstrated in females, while evidence for a beneficial effect in males is inconsistent. Studies are likely biased in desexing being a proxy for better care and desexed dogs having already lived to the age of desexing. Desexing reduces or eliminates common life-limiting diseases of the female reproductive system such as pyometra and mammary tumors, while no analogous effect exists in males. Disease risks increases across sexes and breeds include cruciate ligament rupture, various cancers, and obesity. Urinary incontinence risk is increased in females only. Various other disease risk changes show considerable variability between breeds and sexes. Behavioral Effects: Desexed males show reduced libido, roaming, conspecific mounting, and urinary marking frequency, as well as reduced male dog-directed aggression in a majority of males desexed for behavioral reasons. There is a detrimental effect on the risk and progression of age-related cognitive dysfunction. Desexed dogs may be less likely to cause bite injuries across sexes. The evidence for other effects such as human-directed aggression, human or object mounting, resource guarding, or shyness and anxiety is inconsistent and contradictory. There are few studies specific to females or individual breeds. Conclusions: The evidence for a beneficial effect of desexing is stronger in female than in male dogs; however, there is significant variation between breeds and sexes, and more research is needed to further elucidate these differences and to arrive at individualized evidence-based recommendations for clinical practice.

Background: The privately owned companion dog is an emerging model in comparative medicine, notably because it shares the human environment including its risk factors, is affected by many analogous age-related diseases, receives comparable medical care, and has excellent veterinary medical data available.Past studies of dog lifespan have used academic, corporate or insurance data. While independent primary care data exist for the UK, none have as of yet been published for the US. This study analyzed data from three independent primary care US veterinary hospitals and identified factors that influence lifespan and mortality in a cohort ofn = 20,970 privately owned dogs using Kaplan-Meier survival estimators and Cox Proportional Hazards modelling, including body size as a covariate.Results: As previously reported, body size was negatively correlated with lifespan. Gonadectomy was associated with a longer lifespan, with the effect being stronger in females than in males. This lifespan advantage was conserved in gonadectomized female dogs that lived to at least ages 5 and 8 years. We did not find significant differences in lifespan between purebred and mixed breed dogs; however, breeds with larger effective population sizes and/or lower inbreeding coefficients had median survival times 3–6 months longer than breeds with smaller effective population sizes or higher inbreeding coefficients, indicating that these measures of genetic diversity may be affecting breed lifespans. We also found that dog breeds belonging to the “Mountain” ancestral group had median survival times that were 3.5–4.6 years shorter than other purebred dog groups, which remained significant even when correcting for body size.Conclusions: Our findings show that it is possible to obtain and analyze data from independent veterinary clinics in the US, an approach that could be useful for studies of comparative epidemiology under theOne HealthandOne Welfareparadigms. We also show that the lifespan effects of gonadectomy are not identical between the sexes and should be investigated separately by sex in future analyses. More research is needed to further clarify the influence of age at gonadectomy, as well as the factors leading to the observed differences in lifespan in the “Mountain” ancestral group and in dog breeds of varying inbreeding coefficients and effective population sizes.