13 Mar 2023
Kit Sturgess MA, VetMB, PhD, CertVR, DSAM, CertVC, FRCVS draws on published literature to try to evaluate the benefits and risks of neutering versus not in cats and dogs.
Kit Sturgess
Job Title
Image: © Khaligo / Adobe Stock
Medical professionals have a complex and difficult line to navigate having both responsibilities to individuals to deliver them the best care available and wider roles that involve herd health, the environment, impact on other species and so on.
This has been brought into sharp focus with the recent COVID-19 pandemic, where decisions on health of the population have superseded personal freedoms. The public has had to engage with risk-benefit decisions most recently with regards to vaccine safety – what is an acceptable level of significant complications compared to the benefits of achieving population immunity, opening up the economy, addressing other medical issues, improving mental health and the protection it provides an individual against serious disease?
In veterinary medicine, attitudes differ around the world on whether all cats and dogs should be neutered (apart from those specifically selected for breeding) and whether the benefits that this brings to the individual, the population and society outweighs the costs to the individual – for example, exhibiting natural behaviours, anaesthetic and surgical risks, increased incidence of obesity and so on.
Population control can be also delivered by using contraception as a reversible control of reproduction. Drugs are available that have been demonstrated to effectively suppress oestrus or spermatogenesis for varying periods of time in dogs and cats; side effects are rare.
These drugs include:
Currently their use is relatively limited for specific purposes, usually for relatively short periods of time. Long-term use has been associated with potential side effects, including development of cystic endometrial hyperplasia, mammary nodules, mammary swelling and lactation in bitches. Such products are, therefore, not recommended unless there is an intention to breed from that individual at a later date; regular monitoring is important where repeated treatments are given.
In the future it is hoped that products will become available that deliver safe, effective, non-surgical lifetime suppression of reproductive capability following a single non-surgical procedure leading to lower costs and risks and the ability to mass deliver with limited training.
Efforts are ongoing, with most focus on immunocontraception as well as research into targeted cytokines and gene silencing. To date, successful products have not been produced.
Some work has been undertaken investigating sclerotization methods in male dogs.
This article will focus on surgical sterilisation and the published literature to try to answer the question as to the benefits and risks of neutering versus not neutering, and the strength of evidence to support them. Method of surgical neutering remains the subject of debate – particularly whether laparoscopic ovariectomy is superior to open surgical ovariectomy or ovariohysterectomy. The literature on cats and dogs is significantly different and warrants being discussed separately.
Cats are induced ovulators and reach sexual maturity, depending on the season and to some extent breed, as early as four months of age. An estimated 106.4 million cats were in Europe in 2019 (Statista, 2022a). In the UK, for example, Cats Protection (2020) has estimated that around 10.2 million owned cats exist in the UK. In addition, in the UK as many as a million cats may be feral, in shelters or stray.
Approach to sterilisation and population control in these groups has a different purpose, with the welfare impact of being unneutered higher in the feral and stray group. Within the feral group, cats can be classified as unowned and unsocialised, with population control being delivered by a trap, neuter, return (TNR) policy, whereas the unowned socialised cats (community cats) require engagement with that community to facilitate neutering (McDonald et al, 2018).
The number of feral kittens is likely to have increased significantly during the CoVID-19 pandemic as veterinary capacity has decreased and TNR programmes have had to be suspended serving to demonstrate how quickly cat populations will increase if sterilisation does not take place.
Beyond population control, the primary benefit of ovariohysterectomy is decreased incidence of mammary neoplasia, with intact queens reported to have seven times the risk of developing mammary tumours compared with neutered females (Dorn et al, 1968). This is of particular concern in cats as 95% of mammary tumours are malignant.
The author’s impression is that pyometra in elder, intact queens is also common, with neutering removing this risk. Parturition has a risk associated with it; a survey by Sparkes et al (2006) reported that 8% of pedigree cats required caesarean section. Dystocia risk is in domestic shorthair cats is about one third that of pedigree cats, with 2% of caesarean sections resulted in death of the queen (Holst et al, 2017).
The primary benefits of castration of tomcats are behavioural in the form of aggression to other cats and reduced territorial marking with foul-smelling urine. Increased fighting not only leads to injuries, but increases the transmission of blood-borne disease such as FIV.
Neutering has the short-term risks of anaesthesia and surgery, with the major surgical complications being bleeding, damage to adjacent organs (bowel and bladder penetration) and ureteral obstruction, leading to hydronephrosis.
Anaesthetic complications are relatively rare; a survey of death within 24 hours of sterilisation by Levy et al (2017) at a high volume spay-neuter clinic documented 34 cats out of 71,557 procedures (0.048%), with risks being twice as high in females compared to males.
Despite the sterilisation of cats being perceived to be a quicker and surgically less demanding procedure, mortality rates were five times higher than in dogs (four fatalities among 42,349 procedures [0.009%]).
As a population control measure, strong advocacy exists recommending routine prepubertal neutering in cats (less than four months) rather than the more “traditional” neutering at six months.
Studies published to date have shown no significant difference in pain scores or recovery times in cats undergoing prepubertal neutering (Porters et al, 2015a) with no additional long term consequences demonstrated (Howe et al 2000; Porters et al 2015b). Evidence also exists that post-neutering weight gain is less marked following prepubertal neutering (Allaway et al, 2017).
Beyond the immediate perioperative risk, the major consequence of sterilisation is the development of obesity associated with a decline in metabolic rate (Mitsuhashi et al, 2011; Belisto et al, 2009). This risk has been recognised by nutrition companies that have responded with diets with lower energy density and that promote satiety.
Also, an important role for the veterinary team exists to ensure owners are reminded of the risks at the time of the procedure and at postoperative checks. This can also be used as an opportunity to promote appropriate diets and portion control, as well as increased activity through playing and environmental enrichment – particularly in indoor only cats.
Delayed physeal closure of long bones in neutered cats has been documented, but this has not been associated with increased limb length or higher fracture incidence (Perry et al, 2014; Uçmak et al, 2015).
Concerns have also been raised about the increased risk of urinary obstruction in castrated male cats due to a less developed and narrower urethra, but this is not supported by the evidence (Olson et al, 2001). However, obesity does increase the risk of urethral obstruction (Jukes et al, 2019).
The dog population in Europe in 2019 was estimated at 87.5 million (Statista, 2022b), but establishing the number of feral dogs in a particular country or region can be difficult.
Responsibility for stray dogs varies across Europe. In the UK, stray dogs are the responsibility of local authorities, which dealt with an estimated 70,000 cases in 2018-19 of which around 50% were returned to their owners and the remainder rehomed with only a small percentage (2%) euthanised (Dogs Trust, 2020).
The average inter-seasonal interval in bitches is around six months, with most dogs having clear signs that they are in oestrous with bleeding and behavioural changes, which serves to limit the risks of unplanned litters compared to cats.
In the UK, at least, it is difficult to establish whether a dog overpopulation issue exists, with strays and rescued dogs being cared for by charities representing around 1% to 2% of the population compared to 9% to 10% for cats. These figures suggest the current level of neutering occurring in the UK is being relatively effective in managing overpopulation, with the majority of litters produced by owned dogs being planned.
Neutering is, therefore, going to have the highest impact in the stray/free-roaming population as evidenced by studies from US.
The benefits of neutering the owned population is harder to evidence as the majority of dogs are neutered – hence, understanding the impact of a large population of owned, unneutered dogs is challenging.
The decision to neuter female dogs is further complicated by discussions on the benefits and risks of ovariectomy versus ovariohysterectomy and a laparoscopic versus open abdominal approach. In male dogs, discussions also exist around scrotal versus prescrotal castration and when scrotal ablation is appropriate.
Table 1 summarises the current data on benefits and risks of neutering in dogs, with an estimate of the strength of evidence to support that benefit/risk. A number of review articles summarising these data in more detail are available (Kustritz, 2018; Urfer and Kaeberlein, 2019; Hart et al, 2020a).
Establishing causation associating neutering with a particular outcome requires high-quality, prospective studies involving large numbers of patients. To date, many of the veterinary studies are relatively small, retrospective and involve specific breeds or populations. Care must therefore be taken when extrapolating such data to the larger dog population.
A number of studies have looked at lifespan in dogs with the balance of evidence indicating that neutered dogs live longer than entire dogs, although cause and effect is not identified nor is the impact of the timing of neutering.
A study of 70,574 deceased dogs from US teaching hospitals (Hoffman et al, 2013) found that neutering increased male life expectancy by 13.8%, while it increased female life expectancy by 26.3%. Analysis of data from the VetCompass (private veterinary practices in the UK) came to similar conclusions (Hoffman et al, 2018).
The primary reported benefits of spaying bitches are decreased incidence of mammary neoplasia and pyometra. Reported incidence of mammary neoplasia in bitches is 3.4%, with malignancy rate of about 50%.
The most recent systematic review of the literature (Beauvais et al, 2012) concluded that the evidence of the benefit of spaying for mammary tumour protection is weak – especially with respect to the timing (before or after the first season).
However, the weight of clinical experience and insurance data – for example, a study (Jitpean et al, 2012) of Swedish dogs showed 13% of 260,000 insured intact female dogs had developed mammary tumours by 10 years of age – tend to indicate that neutering carries a significant benefit in reducing the incidence of mammary tumours.
As with any surgery, a perioperative risk of death exists, but this appears to be low in dogs (Levy et al, 2017) at around 1/1,000 surgeries. Other perioperative risks that are less well defined include infection, haemorrhage, ureteric obstruction by inclusion in a ligature, acute kidney injury, damage to other internal organs and surgical equipment (particularly swabs) being left in the abdominal cavity.
The incidence of dystocia in dogs is heavily breed dependent, with some breeds – for example, bulldogs – rarely being able to give birth without caesarean section. Caesarean section is required in about 50% of dogs with dystocia (O’Neill et al, 2019) with a reported fatality rates following caesarean section are around 3% (Conze et al, 2020) – 30 times higher than the fatality rate associated with spaying.
Negative impacts of neutering include:
A large number of different studies have reported the impact of neutering on the risk of specific diseases in named breeds or unique populations. These studies are too numerous to discuss here but are summarised by Urfer and Kaeberlein (2019) and Hart et al (2020b).
For example, in a study by Edmunds et al (2021) looking at osteosarcoma, the incidence of cases compared to non-cases in the cross-bred population was 0.15% compared to 1.9% in Rottweilers. In the total population, 59 cases were in castrated male dogs, with 259,460 non-cases compared to 574 cases in entire male dogs with 209,982 non-cases.
This data suggests that for specific breeds, castration is a clinically relevant risk for the development of osteosarcoma.
Despite relatively extensive literature, causation is unknown and some of the effects found may be secondary – for example, orthopaedic disease occurring secondary to an increase in obesity associated with neutering. However, an alternative explanation could be increased incidence of orthopaedic disease results in less activity – and, therefore, an increased prevalence of obesity.
On balance, for the majority of bitches, neutering benefits outweigh the potential immediate and future harms.
In male dogs, castration decreases the incidence of testicular neoplasia and prevents the development of, or resolves, benign prostatic hypertrophy. Similarly, with regular inspection of the testicles, the vast majority of testicular neoplasia will be cured at the time of diagnosis by castration.
Detriments are similar to those reported for bitches. At this time, therefore, no compelling medical reasons support castration of male dogs early in life.
The major reason for most owners to castrate their male dog is to control testosterone-dependent behaviours, such as mounting, roaming and urine-marking (summary in Urfer and Kaeberlein, 2019).
Castration has been demonstrated to decrease these behaviours by 50% to 70%, but the literature is not consistent, with little observed effects of castration in free-roaming dogs and some studies showing increased incidence of mounting behaviour of inanimate objects in castrated dogs.
Castration tends to decrease aggressive and dominance behaviours such as biting, fighting and boldness, but may increase fearful behaviours. However, the benefit of castration where these behaviours are already established is less evidenced.
For male dogs, where regular checkups and veterinary care is available, the benefits or routine castration – particularly in large and giant breeds – are unlikely to outweigh the harms, particularly with respect to obesity. If obesity risks can be appropriately managed with education, client support and suitable nutrition then the global benefit/risk analysis is more equivocal.
All of the current evidence in cats points to the benefits of sterilisation for population control with prepubertal neutering – especially in the shelter environment, further reducing the incidence of unplanned pregnancies.
Apart from a low perioperative mortality rate, no significant evidence of detrimental impacts on growth, development or disease prevalence in neutered compared to unneutered cats has been shown and significant advantages exist in neutering in terms of lowering the incidence of mammary tumours, pyometra, fighting and transmission of blood-borne infectious disease.
The major consequence of neutering is decreased metabolic rate, leading to an increased tendency to obesity. This risk can be effectively managed through education of owners (for example, promoting exercise – especially of indoor cats) and control of calories fed. A wide variety of diets are available, with lower nutrient densities to facilitate weight control in neutered cats.
This makes it relatively straightforward to develop consistent advice and practice policy on the benefits and timing of neutering of cats.
Overall, the individual benefits of neutering female dogs – with rare exceptions – outweighs any potential risks. The discussion in male dogs is more nuanced, making it difficult to develop a clear and concise practice policy that can be delivered by all members of the veterinary team, meaning that scheduling a pre-surgical consultation to discuss the issues involved and give advice may be appropriate.
Neutering has clear advantages in managing inheritance of genetic defects, including undesirable physical and behavioural characteristics as well as health and longevity benefits. However, the benefits in terms of population control are less clear and documented negative associations exist with neutering – particularly obesity, incidence of certain neoplasms, orthopaedic disease and urinary incontinence. More of the negative associations of neutering affect large and giant breeds.
As uncastrated dogs do not have malodorous urine as tomcats do, the value of routine castration is less clear as the risks – particularly of obesity – may outweigh the benefits, emphasising the role that veterinary professionals have in giving clear nutritional advice to owners following neutering.
As most dogs do not have dog flaps and it is easier to make a garden dog proof, the risks of roaming in entire male dogs are less than for outdoor cats.
For the author, the most compelling reason to castrate male dogs is the increased lifespan reported, but further work needs to be conducted to ensure this effect is real and applies to all breeds, and to evidence the optimum time for surgery.
However, clear societal benefits exist from pet owners considering routine neutering, if breeding from their pets is not a priority, and veterinary practice has a significant role to play in discouraging irresponsible breeding practices.
Taking in all the pros and cons of neutering male dogs, the author’s own male dog (a border terrier) is neutered with strict portion control to maintain an appropriate bodyweight.
