Timing of the surgery
One of the most common (and controversial) questions posed by clients is “What is the best age to neuter my pet?”. Despite the frequency with which it is asked, it is often one vets are cautious to give a definitive answer to, as it is frequently loaded with strong opinions previously voiced to the owner by media, friends and breeders whose advice may be conflicting. As a profession are nervous of giving an opinion that we feel is not strongly backed by science – and the science is lacking. In Cedarmount, we have however kept bang up to date with the current literature and current thinking. In light of recent findings, national guidelines have been updated (late 2020) to reflect current research, and we have adopted these in the main. Please note that as the research and literature continue to change, these guidelines may also be updated in the future.
The following table is drawn from the data outlined below, but mainly based on the work of Hart & Hart. Dog sizes relate to expected adult bodyweight. The chart is intended as guidance only and obviously the decision rests with the owner, having taken these and all other factors specific to the individual – we strongly recommend you talk to us to discuss your own dog specifically, and don’t forget to ask about the gastropexy option also: What is this?
Quick reference chart: Spay and Neuter Timings Dogs/Bitches
*** national guidelines suggest there may be no problems neutering/spaying as young as 3mths – at Cedarmount we prefer to allow a little more time and have adopted 5 mths as earliest routine spay/neuter age, and only for small dogs whose adult body weight will be less than 20 kg
- We prefer to spay/neuter as early in the “Reasonable Choice” recommendation as possible
- We prefer not to spay bitches on heat or in a false pregnancy – so if we are delaying the spay then we believe the ideal window to choose is 3-4 weeks, or 3-4 mths post-season (last bleed).
- If a dog is naturally fearful, we prefer to defer neutering until after puberty – it is believed this allows a flush of testosterone to assist in combatting nervousness.
The commonly perceived risk areas that will be covered below are those most frequently voiced by owners (and vets)
Urinary sphincter mechanism incompetence (USMI) and neutering age
Urinary incontinence acquired in adult life is a common condition in the female canine (Beauvais et al., 2012b) and has long been anecdotally associated with spaying before the first season. A 1998 study showed an incidence rate of USMI of 0.22% in entire females vs 1.74% in spayed females (Thrusfield et al., 1998). It is speculated this is due to an alteration in hormonal control of the external urethral sphincter (Hotston Moore, 2017). However the evidence supporting the association between USMI and age of neutering remains surprisingly sparse. A systematic review published in JSAP in 2012 (Beauvais et al., 2012b) found only three studies which qualified under Cochrane guidelines, finding that “… Of the remaining three studies, which were at moderate risk of bias, there was some weak evidence that neutering, particularly before the age of three months, increases the risk of urinary incontinence. However, overall the evidence is not consistent nor strong enough to make firm recommendations on the effect of neutering or age at neutering on the risk of urinary incontinence.” A 2011 case-controlled study found no correlation between age of neutering and USMI (de Bleser et al., 2011). So why the perceived link? Byron’s 2017 study did show that bitches with an adult weight of >25kg did show a higher incidence of USMI if neutered early (at less than one year of age) with risk increasing for each month earlier that they were neutered, with no such correlation in animals with an adult weight of <25kg (Byron et al., 2017). They also noted that USMI was generally more common in this size of bitch regardless of neutering. As for the specific timing compared to first oestrus, the Beauvais study found no link (Beauvais et al., 2012b). It is highly likely that obesity is a confounding factor – USMI increases with obesity, and weight gain may be associated with neutering (Hotston Moore, 2017). Added to this we must consider that certain breeds are known to have a genetic predisposition to USMI (Holt and Thrusfield, 1993) – with Rottweilers, Dobermanns, Irish Setters and Boxers having a higher risk. So what should we advise with regards to USMI and neutering? Alasdair Hotston Moore advises that the overall evidence is weak, “…in general, no clear evidence exists between age at neutering and USMI, therefore [we] cannot recommend an age at which neutering should take place…” although he does clarify that given the findings of weak evidence in larger breeds (>25kg), these may have some protective effect if neutered later than 12mths, and he also speculates that delayed neutering may be prudent in specifically vulnerable breeds. Studies of paediatric neutering do not recommend spaying bitches before 12 weeks (Spain et al., 2004).
Joint disease and neutering age
A recent study by Hart & Hart (Hart et al., 2020) assessed incidence of joint disease against neutering age for dogs treated at the Veterinary Medical Teaching Hospital at the University of California, Davis. Mixed breed dogs were retrospectively divided into groups dependent on adult bodyweight, and only animals whose age at neutering was known (or entire animals) were included, so disease incidence could be assessed relative to neutering status. This was a large study over twenty years. Alongside the incident of various cancers (see below), the study assessed the incidence of joint conditions including CCL disease, hip dysphasia and elbow dysplasia in the group. The results showed no significant change in the incidence of joint disease for dogs with an adult bodyweight of less than 20kg dependent on age at neutering. However, in dogs in the adult bodyweight group 20-30kg, neutering before the age of 12 months was associated with a rise in the incidence of the joint diseases measured from 3% to 5% in males, and 4% to 10% (neutered <6mths and 12% (6-12mths) in females. There was no significant difference if the animals were neutered later than 12 months of age. In the group of 30-40kg adult bodyweight, overall incidence of joint disease in intact males was 8%. This rose to 17% (neutered <6mths) and 11% (6-12mths). In females the incidence in intact females was recorded at 0%, but rising to 10% (<6mths) and 23% (6-12mths). Again neither group showed a significant increase in risk over the intact group if neutered after 12 months of age. In the giant group (40kg+), the incidence in males was 9% (intact), rising to 28% (<6mths), and 11% (6-12mths). It must be recognised that this study did only assess mixed breed dogs, but given its size, these results are useful for shaping advice. It is notable that some of these conditions are considered congenital, and often present before the age of 12 months, so it may be that it is clinical severity and diagnosis rather than true incidence that is affected. This would still be of signficance. These studies also allowed for changes in bodyweight that may be a result of neutering and therefore a confounding factor – the incidence of joint conditions was found to be independent of body condition score (Torres de la Riva et al., 2013).
Neoplasia and neutering age
Always with neoplasia, prevention is preferable to treatment, and neutering has long been associated with certain reductions in incidence of some tumours. Obviously tumours of the gonads are hopefully prevented with surgery, and especially in the case of cryptorchidism, neutering is recommended specifically for this reason. However hormonal influence of distant neoplasms is a more complex area, and harder to find reliable data for. The traditional view with regards to the protective benefit of neutering early to give protection against mammary malignancies was that spaying before the first season led to 0.5% of the population incidence, after only one oestrus the incidence was 8%, and two or more seasons before neutering gave 26% of the risk of unspayed females (Schneider et al., 1969). However this, and other similar studies, are now nearly fifty years old. A recent study on a canine population in Italy attributed a decrease in the incidence of mammary neoplasia in the second half of their study to an increase in earlier neutering in the area, but this was speculation and not analysed statistically (Merlo et al., 2008). In 2012 Beauvais et al performed a systematic review of the relationship between neutering and mammary neoplasia, published in JSAP, and found that many of the studies on which our advice is based did not give reliable result when the Cochrane guidelines were applied (Beauvais et al., 2012a). Their conclusion, once all studies that showed a possibility of unconscious bias were removed, was that “…there is some evidence to suggest that neutering bitches before 2.5 years of age is associated with a considerable reduction in the risk of malignant mammary tumours, and that this risk may be reduced further by neutering before the first oestrus … [however] the strength of this evidence is weak”. Other neoplasms may also show a link to neutering – male dogs who are neutered can show a slight increase in malignancies of the prostate (Heuter (Heuter, 2008) suggests an overall incidence of 0.6% for prostatic neoplasia, so an increase in this disease should not affect decisions in a major way) and urinary bladder (Bryan et al., 2007, Congress, 2012) despite being at a reduced risk of benign prostatic hyperplasia (Bigliardi and Ferrari, 2011). The findings of a retrospective study of the medical records of golden retrievers (Torres de la Riva et al., 2013) demonstrated an increase in the incidence of haemangiosarcoma in females of that breed neutered later than 12 months of age, but an decrease in the incidence of MCT in entire females.
Also early neutering is shown to delay growth plate closure (Salmeri et al., 1991), so may theoretically increase the risk of osteosarcoma (which is linked to adult height in dogs), but this is unproven statistically (Congress, 2012). However in his report to WSAVA congress, Gerry Polton made a special mention of Rottweilers with regard to osteosarcoma – pointing out that results from a large database in the USA showed that the risk of osteosarcoma in Rottweilers at large was 12.5%, rising to 25% in neutered animals, so this point should be discussed with owners of that specific breed, and may affect advice. Interestingly, a study of over 600 Rottweilers (Cooley et al., 2002) found that overall incidence of OSA rose from 8% incidence in entire animals to 28% in neutered males and 21% in neutered females, but that despite this the overall longevity of the total group was higher for the neutered animals than the entire ones (This finding was reflected in a study of 655 golden retrievers at necropsy – neutered animals had a higher incidence of cancer, yet on average lived longer than entire females (Kent et al., 2018)). The UCLA study (Hart et al., 2020) found no clear overall link between neutering age and the incidence of mast cell tumour, lymphoma, haemangiosarcoma or mammary cancer, but the study only included animals up to the age of 11, after which these diseases are most commonly seen. Clearly this is a complex area with multiple factors affecting the behaviour of different tumours in different ways, and often a clear line is hard to take, and depends on breed and relative risk.
Behaviour and neutering age
Changes in behaviour, and “maturity” are often cited as reasons for and against neutering (Downes et al., 2015), but as these changes are highly subjective, evidence either way is largely circumstantial and anecdotal (Congress, 2012). Any behaviours that are directly hormonally caused (such as calling in female cats, pseudopregnancy in bitches) should resolve after neutering. Those that are hormonally driven behaviours (such as male dogs straying to search for mates, or humping cushions) may be improved, but if they become learned behaviours may not. Aggression is more likely to be the result of social factors rather than pure testosterone in males, and in cases of fear aggression may be worsened in the case of neutering (Voith, 1979). The effect of neutering as a preventative measure for future behaviour is therefore not supported by evidence, and its role in treating existing behaviours is complex and should ideally be discussed with a behaviourist first.
Paediatric neutering (dogs)
Paediatric neutering was first introduced in the USA by rehoming centres who had noted that puppies and kittens they rehomed had, despite their advice, only a 40% compliance with their neutering program. They trialled early neutering so animals could be sterilised before rehoming, reducing unwanted population growth. Benefits reported are – better compliance, quicker (andtherefore lower complication rate) surgical procedures, and lower costs (Howe, 1997). Short-term results showed no increased risk factors compared to the population (Howe, 1997). In the longer term, Spain et al advise that they would support paediatric neutering for male dogs, but due to an increase in long-term USMI, would not recommend it in females under 12 weeks of age. Although delay in growth plate closure has been shown in animals neutered early, actual physical skeletal and behavioural development were no different in animals neutered at 7 weeks or 7 months (Salmeri et al., 1991).
Although “what age should I neuter my pet” is such a simple and sensible question, it’s clear that the evidence is vague, and in some cases contradictory. It seems that a “one size fits all” policy is not what is appropriate. We should consider the benefits and drawbacks of all options and fit them to the species and breed of our patients, as well as the wishes of the client – it is important to find out why they wish their pet to be (or not to be) neutered – are they more focussed on reproductive control or neoplasia risk?
Sensible Guidelines (dogs):
Male dogs – neutering should be discussed from a behavioural point of view. There is no medical reason why smaller dogs should not be neutered from 7 weeks, although delay for other reasons (such as physical appearance, socialisation before surgery) to an older age is perfectly reasonable. Any age from 12 weeks upwards seems sensible with consideration of vaccine status. With respect to breeds with larger adult size, dogs with an expected adult bodyweight of 20-40kg may benefit from neutering being delayed until beyond 11 months of age due to increased risk of joint disorders, and those over 40kg may benefit from delay until 23 months of age. Howe (Howe, 2015) reports an increase in longevity for neutered male dogs of 13.8% overall.
Female dogs – neutering before 12 weeks is not advised with respect to USMI, and neutering before 2.5 years may well provide benefits in protection against mammary malignancies. Therefore any time in the interval of 3-30 months is sensible. In animals with an expected bodyweight of over 20kg, delaying neutering until beyond 11 months of age may reduce the incidence of joint disease. Howe (Howe, 2015) reports an increase in longevity for neutered female dogs of 26.3% overall.
Exceptions – special exceptions should be made in Rottweilers, where the incidence of osteosarcoma should be discussed before neutering, and golden retrievers (males would benefit from the later neutering suggested for their size, whereas in females, neutering at 12 months of age would balance the benefit of younger neutering against HSA, but allow for a reduced incidence of joint disorders compared to juvenile neutering). Also, in female canines of breeds predisposed to USMI neutering beyond 11 months of age may be preferable; in bitches with juvenile vaginitis (these should be allowed a first season as this usually resolves the condition); and of course in animals which are selected for breeding.