In the UK and USA, veterinary associations strongly encourage owners of dogs and cats to have their companion animal neutered, promoting it as an element of responsible pet ownership. In contrast, vets in continental Europe have traditionally been more reluctant to neuter companion animals, especially dogs. One potential area of disagreement concerns effects on the health of the animals. This paper reviews some of the medical benefits and adverse effects of neutering, and considers confounding factors.
Techniques and Timing
Hormonal products can be used for temporary (chemical) sterilization, but are not commonly used long term, due to concerns over potential adverse effects, and issues of convenience, cost and effectiveness. Surgical neutering achieves permanent control of reproduction, but carries the risk of minor (e.g., inflammation at the surgical site) or major side effects (e.g., bleeding, infection, or death). Surgical risks are greater for females, as the procedure is more invasive. Ovariohysterectomy is most common, but has no reported advantages over ovariectomy, which is a simpler and less invasive surgery. Minimally invasive laparoscopic techniques further decrease morbidity. Male animals are typically castrated by surgical removal of the testicles, although vasectomy is also an option for dogs. Complication rates vary with the procedure: A study of 1,016 dogs and 1,459 cats undergoing elective surgery reported post-operative complications in 6.1–19.4% of dogs and 2.6–12.2% cats, most of which were minor.1
Conventional neutering is traditionally performed at around 6 months of age in cats and bitches, with male dogs castrated a few months later. However ‘early’ neutering at 6–16 weeks is increasingly promoted, particularly by charities, for population control. Initial concerns that neutering animals at such a young age would carry increased perioperative risks have proven unfounded, and no significant differences in short- or long-term health issues were reported in a recent study of 800 shelter kittens, randomly assigned to be neutered at 8–12 weeks vs. 6–8 months.2
Gender, Species and Breed-Specific Concerns
The current literature contains hundreds of studies on the effects of neutering, and the results are often conflicting. This may be as a result of different methodologies, for example grouping subjects based on gender but mixing breeds; reproductive status is often inferred based on age, which may be inappropriate; and descriptors such as ‘early’ and ‘prepubertal’ are not synonymous. Despite this, the relationship between neutering and conditions such as obesity is relatively consistent, neutering being a predisposing risk factor, but seeming to be mitigated to a degree if the neutering occurs at a younger age. In contrast, studies on the relationship between neutering and overall longevity have produced conflicting results. In many cases, the interactions are complex, with gender, species, breed and the timing of neutering (with respect to the animals’ reproductive status), all having an influence.
Female dogs (bitches): Neutering prevents complications from pregnancy and decreases the risk of diseases of the reproductive tract, such as pyometra. In contrast, the risk of developing urinary incontinence, especially in large breeds, is increased. As an example, a recent study reported a significantly higher incidence of incontinence in female German Shepherds neutered <1 year of age (7.2%) compared to intact females (0%).3
The relationship between neutering and the occurrence of neoplasia is more complex. Neutering was believed to reduce the risk of mammary tumours especially if performed before the first season; however, a systematic review by Beauvais et al.4 found limited evidence to support this. More recently, Hart et al.3,5 reported mammary cancer rates of 0–4.1% in entire bitches and up to 5% in bitches neutered between 2–8 years of age. However Hart et al.5 highlighted the importance of breed, reporting that neutering Labrador Retrievers of either sex at any stage had little effect on increasing cancers, while neutering a female Golden Retriever >6 months of age increased the risk of one or more cancers to three to four times that of intact females. Similarly, female Rottweilers, neutered at <1 year of age were three times more likely than intact females to develop osteosarcoma (OSA),6 while neutering did not increase the risk of OSA in German Shepherds.3
Similar breed- and age-specific effects have been reported for musculoskeletal disease: Hart et al.5 reported the incidence of joint disease in intact female and male Labrador Retrievers and Golden Retrievers to be around 5%: neutering at <6 months of age nearly doubled the risk of suffering joint disease in the former, and increased it 4–5 fold in the latter.
Overall, however, it appears that the long-term health-related welfare of bitches is likely to be improved, or at least not reduced, by neutering.
Female cats (queens): Neutering prevents the health risks associated with pregnancy, which is more likely to occur in free-roaming cats, compared to bitches. Although mammary tumours are much rarer in cats than dogs, they are also much more likely to be malignant, and neutering <6 months results in a 91% reduction in the risk of mammary carcinomas.7 In contrast to bitches, neutering does not increase the risk of urinary incontinence in queens. The main health related welfare problem for neutered cats (of both sexes) is a substantially increased risk of obesity, with neutered cats being significantly more likely to become obese than entire cats,8 and as a result being at significantly greater risk of becoming diabetic. Obesity can to some extent be avoided, however, and so on that basis at least, neutering is likely to improve, or at least not reduce, health-related welfare in queens.
Male dogs: Surgical complications of neutering are rare, but there are risks of significant negative long-term consequences for health-related welfare. Castration of male dogs removes the risk of testicular disease and reduces the risk of androgen-dependent diseases such as perineal hernias. However neutering significantly increases the risk of prostate and bladder cancer, and although prostate cancer is rare in male dogs (0.2–0.6% incidence), it is almost always malignant.9
Castration also significantly increases the risk of lymphosarcoma, with Hart et al.5 reporting a 4% incidence in entire males, compared to 11.5% in neutered males; and Torres de la Riva et al.10 reporting that ‘early’ neutered male Golden Retrievers had three times the frequency of lymphosarcoma of intact males. Similarly, male Rottweilers, neutered at <1 year of age are four times more likely than intact males to develop osteosarcoma (OSA).6 Thus, the costs of routinely neutering male dogs in terms of the increased risk of very serious diseases probably outweigh the benefits.
Male cats (toms): In contrast to dogs, both testicular and prostatic diseases are rare in toms, thus neutering has little direct impact on these aspects of health. Some studies report that neutering delays growth plate closure, while others suggest it does not, but none have identified a clinical problem as a result. The main welfare issue for the tomcat itself is, as with female cats, a significantly increased risk of obesity and of diseases associated with obesity, such as diabetes. Despite this, tomcats are frequently neutered to make them easier to live with by reducing, to variable degrees, urine spraying, aggression and roaming. Overall, neutering is unlikely to significantly affect health-related welfare in toms.
The health effects of neutering vary depending on factors such as the gender, breed, species and age/reproductive stage at which the neutering occurs.
Evidence suggests that, overall, bitches may benefit from neutering in terms of reducing their risk of serious diseases, but the same is not true of male dogs, where neutering increases long-term health risks. There is less evidence of impact of neutering on the health and welfare of cats.
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2. Porters N, Polis I, Moons CPH, Van de Maele I, Ducatelle R, Goethals K, et al. Relationship between age at gonadectomy and health problems in kittens adopted from shelters. Veterinary Record. 27 March 2015; doi: 10.11361 vr.102678.
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9. Bryan JN, Keeler MR, Henry CJ, Bryan ME, Hahn AW, Caldwell CW. A population study of neutering status as a risk factor for canine prostate cancer. The Prostate. 2007;67(11):1174–1181.
10. Torres de la Riva G, Hart BL, Farver TB, Oberbauer AM, Messam LLM, Willits N, et al. Neutering dogs: effects on joint disorders and cancers in Golden Retrievers. PLoS ONE. 2013; doi.org/10.1371/journal.pone.0055937.