Simple Summary: Desexing is a general term for interventions suppressing fertility in dogs, most commonly by surgically removing the testes or ovaries ("gonadectomy"). Desexing is promoted for population control, health benefits, and behavior modification. Surprisingly, the existing evidence shows no effect of desexing on population size in companion or shelter dogs; however, an effect has been shown for desexing female free-roaming dogs. Desexing has consistently been shown to change various health risks, including a reduction in pyometra and mammary tumor risk, as well as an increased risk of cranial cruciate ligament rupture, several forms of cancer, and obesity in both sexes. Other health effects vary considerably between breeds and sexes. A lifespan advantage in desexed dogs has consistently been shown in females, while the evidence is inconsistent in males, and the effect is smaller in studies that found one. There is more literature on behavioral effects in males than in females, and the evidence suggests reduced libido, roaming, conspecific mounting, and urinary marking in a large percentage of gonadectomized males, and reduced male dog aggression in a majority of males gonadectomized because of behavioral problems. The decision whether to desex dogs needs to be individualized based on the available evidence.
Abstract: 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 Animals 2019, 9, 1086 2 of 28 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.