In the UK, canine castration is one of the most common surgical procedures (Bushby & White, 2019). The majority of these are elective meaning the owner has chosen to have this procedure done.
In other countries in Europe, this is less common with countries such as Norway, the procedure is actually illegal unless it is deemed medically necessary. As a result, 93% of females & 99% of males remain intact in Scandinavian countries.
Here in the UK, we have a choice and the time of writing, more than half of owners are choosing to castrate their pet. Anecdotally, whilst talking to owners, many opt for this procedure because it is "the done thing" or because the vet suggested it. Others avoid the procedure as they feel a dog should "experience being a parent" for example. There are hundreds of reasons why people choose to/choose not to catrate their pet and it can be a complete minefield when deciding what to do.
The aim of this blog post is to outline all the scientific research that has been done into the female spay. This blog will only focus on females, with the male version already published in a separate post.
Disclaimer: I am not a veterinary professional; therefore, I can only comment on the findings of the research to hand. Whilst I have tried to use up-to-date papers, I may also reference some older papers if I feel this is appropriate. I cannot give a conclusive statement as to whether you should neuter or not or at what age to do this but I can present the current available literature on the subject. If you are interested in the research papers, a reference list is provided at the end.
NOTE: Numbers in brackets denotes the reference which can be found in the reference section at the end of the article.
For the purpose of the article, I use the terms "spay", "neuter", "intact" & " gonadectomy" interchangeably.
I have not been able to cover every consideration and topic in this area but have selected the main areas which come up in discussion and research most frequently.
A point to note before we start:
For anybody who has studied academia, you will know the pitfalls of research. Research has flaws due to methodology, samples and other variables. Throughout the article, I will attempt to pinpoint some obvious issues with particular research, but I will discuss this in general at the end.
Scientific research vs Anecdotal evidence
Here in the UK, we have a choice and the time of writing, more than half of owners are choosing to castrate their pet. Anecdotally, whilst talking to owners, many opt for this procedure because it is "the done thing" or because the vet suggested it. Others avoid the procedure as they feel a dog should "experience being a parent" for example. There are hundreds of reasons why people choose to/choose not to castrate their pet and it can be a complete minefield when deciding what to do. any biases or problems with the way the study was carried out and then approve this for publications. These are then published in scientific journals for other researchers, medical & veterinary professionals and others who are interested in academia to read and understand the latest knowledge in the field. Scientific research is not always conclusive, there are often limitations such as sample size, variables that could not be controlled for or something else may have affected the results which hasn’t been accounted for.
Anecdotal evidence is that has been collected in a more casual nature. For example, this may be someone’s personal experience. This is what is often seen in regards to castration on forums, website articles and Facebook pages. This will often sound like “well my dogs have never been neutered and …..” or “well my dogs have always been neutered and….”. We need to take great care with this kind of information. Because this is more about personal experience, it is not clear whether these dogs would have experienced anything different if their castration status had been different. These samples are usually only a few dogs and it is impossible to gauge what other factors were involved in these dogs lives to lead to the expressed outcome. Anecdotal evidence has its place. It can often be the basis that scientific research is based on. Enough people expressing a perceived link can often prompt a researcher to look into this in a more controlled way to see if there really is a link. The problem with anecdotal evidence is there is simply no way to know which anecdotal reports are accurate and which are not. It is not a reliable way to gain information about a medical & behavioural topic because there is no control of other potential factors when the observations were made. This means we cannot be sure what the cause of the observed outcome is.
Overpopulation is always an important consideration when considering a spay surgery but something I am not going to dwell on for too long here as this is something that can be prevented with some forethought and management. Accidental litters are something that needs to be avoided to prevent overpopulation and the overflow of rescue centres. It is difficult to find exact figures on the total number of dogs who are euthanised in kennels each year due to lack of space but take a look at rescue pages and websites from around the country and you will see the number of dogs in need of new homes. Many of these will be from dogs who were "oops" litters, possibly sold and then later ended up in a rescue home. For many rescues, kennels are full and there are waiting lists for dogs coming in. There are simply more dogs than there are homes for.
Many people worry that they are taking something away from their dog by not allowing them to be a parent. This isn’t how animals think. They have little concept of what could have been and for a dog who has never had a litter or been given an opportunity to tie with a bitch, they do not think that they are missing anything. Considering to breed has a huge financial consideration. Responsible breeding includes ensuring your dog will not pass on genetic conditions onto their offspring. Parents must have sound temperaments and a clean bill of health. Homes should be lined up and breeder contracts should include returning the dog if the buyer can no longer look after them. This allows breeders to keep track of their dogs and prevent them entering the rescue system.
Now I won’t get into the many different cost factors of breeding, this isn’t what this article is about BUT there is no denying that neutering completely eliminates the risk of any unexpected litters and therefore the financial fallout from this.
For those who do not intend to responsibly breed from their dog, the advice has always been to neuter to prevent the risk of such accidental litters. For multi-dog households which may contain an intact male & a bitch, careful management may be something that owners just don’t want to/are unable to deal with.
In this blog, I have not factored in the risks associated with mating, pregnancy, birth or anaesthesia risks of the spay itself. The blog will specifically focus on health & behaviour pros and cons after the procedure.
I am going to begin with the biggest medical considerations to spay/neuter.
Mammary Neoplasia is the most common forms of cancer seen in female dogs(1) with an annual incident rate of 16.8%(2). It has been found to affect intact females significantly more than spayed females(3) and is therefore a significant consideration when questioning whether (or when) to spay your bitch.
In a more recent studies, 72%-74% of cases of Mammary Neoplasia occurred in intact dogs and had normal oestrous cycles(4,5) with approximately 50-75% of these cases being malignant(2,5) .This figure did vary from study to study, but in each case Malignant tumours made up at least 48% of all diagnosed cases.
It was found that dogs spayed before the first heat, had a 0.5% risk of developing Mammary Neoplasia. This figure rose to 8% if spayed between the first and second heat and even higher to 26% if spayed after the second(6 ).
Age also played a significant factor in Mammary Neoplasia with frequency much lower for dogs under 6 years of age (2,5,7) with an increased risk for dogs over the age of 8(2,8). One researcher found that 60% of intact dogs over the page of 8 would be affected (4,5) . It was found however that neutering would have a beneficial effect on the risk for dogs up to the age of 9 years old(6). This means that if you chose to leave your dog intact until aged 9, they would still potentially benefit from the procedure in regard to neoplasia risk.
Why would spaying have a link to Mammary Neoplasia?
The reason for this likley lies in the role of oestrogen receptors in these kinds of tumours. 80% of mammary tumours have oestrogen receptors, progesterone receptors, or both(9).
In a study looking at both spayed and intact females, it was found that the oestrogen in intact females worked at a carcinogen for these tumours carrying oestrogen receptors. What is interesting is that for the tumours that didn’t carry oestrogen receptors, the oestrogen appeared to act as a protective factor. This may mean that whilst remaining entire may increase the risk of tumours possessing oestrogen receptors, it may add a protective factor for other forms of cancer.
Purebred dogs were significantly more at risk than crossbreeds across several studies with figures suggesting 80% of affected dogs were purebred. It should be noted here that it is possible more purebred dogs are simply being kept intact due to breeding or showing and this is leading to a skew in the results. In regard to specific breeds, each study had a list of breeds which had recorded greater incidents but these differences between studies. German Shepherds, Dobermanns, Staffordshire Bull Terriers & Rottweilers did appear across multiple studies (2,5,7,10.)
One final point to note here is that whilst the majority of studies did show an increased risk of mammary neoplasia in intact females, a very large systematic review (a detailed examination of as much research in the field as possible) found the verdict to remain inconclusive with no sound basis to advise either way for all research prior to 2012(11).
This review found that 9 studies in the field had a high risk of bias in their writing and a further 4 had a moderate risk of bias. Two studies they observed found there was no strong link at all between remaining intact and an increased risk, but instead that increased reports would likely be due to other factors such as genetics, breeds more likely to remain intact and other non-sex related cancer risks. It is also very difficult to predict whether an individual dog would have developed a mammary neoplasia, regardless of neuter status. Research post 2012 has also heavily cited some of these pieces and could be subject to bias also. This is something to keep in mind when drawing conclusions from all of the earlier research in this area.
As noted in the previous section, the risk of cancer in intact females has been suggested to be related to only those which have oestrogen or progesterone receptors. It is therefore important that we look at the risks of other forms of cancers to see if these are affected by neutering.
When looking at the research, the risk of other cancers including mass cell tumours, hemangiosarcoma and lymphoma was actually higher for those dogs who had been spayed. Dogs who were spayed after the age of 12 months appeared most at risk(12,13).
Dogs were categorised into “early neuter” – under 1 year old, “late neuter” – over 1 year or Intact. Hemangiosarcoma was diagnosed in 8% of late-neutered females and this was 4 times higher than early neutered females or those left intact. There were no cases of Mast Cell Tumours in any of the intact females, but this occurred in 6% of cases of late-neutered females(12). This suggests that whilst neutering does decrease the risk of some hormone related cancers, it actually increases the risk of other forms.
These findings were further supported by a series of breed specific studies which looked at many dogs over their lifetime(13). The aim of these was to reduce the number of variables by looking at large samples of the same breed.
One study looked at over 2000 Vizslas and categorised the neutered dogs into three groups.
- Dogs neutered before 6 months old
- Dogs neutered between 7 – 12 months.
- Those neutered after 12 months
All three of these groups were found to have a significantly higher chance of developing mast cell cancer and lymphoma compared to those who remained intact. Those neutered after 12 months were at the highest risk however for those dogs who were neutered earlier and diagnosed, these dogs tended to be diagnosed at a younger age.
In a separate study looking at Labradors & Golden Retrievers, a similar result was found. This study spanned 13 years and a large number of dogs. Differences were found between the breeds, with golden retrievers being much more affected than Labradors.
In golden retrievers, the risk of developing one of these was 3-4 times higher when the dog had been neutered(14). This does strongly suggest that there is a genetic and breed component to the significance of the difference. Golden retrievers are a breed that has been found particularly prone to cancer, with 65% of Golden retrievers reportedly dying from cancer(15).
Interestingly, whilst many studies supported these findings, a researcher looking specifically at lifespan found that intact females actually had a shorter lifespan than neutered females overall(15).
In addition, whilst intact males where less likley to develop cancerous tumours, those that did were more likley to develop higher grade tumours(16.)
The interesting thing with research in this area is that there will never be any conclusive answer. This is because there are so many variables between breed, age, obesity levels, food quality, environmental influences and more. It is therefore important that you consider this with all the research already presented, and the rest that will follow in the report. In this section in particular, most of the research focused on incident rates over severity and survival rates. This would be useful to delve into more. As mentioned in the research above, it is possible that whilst there is an increased risk of developing these forms of cancer when spayed, these could be less severe than they would have experienced if intact.
Cranial Cruciate ligament Injury
The Cranial cruciate ligament is a ligament that can be found in the knee joint (stifle) of dogs. It attaches the femur (thigh bone) to the tibia (shin bone)(17.)
One study compared the rate of cruciate ligaments of animals that have been spayed compared to those that remained intact. Records of 3218 dogs treated in one vet practice in a 2-year period were used. Females that had been spayed had a significantly higher prevalence than intact females across all breeds recorded with larger dogs also having an increased risk(18).
Most of the research available suggests that spayed dogs did have a higher prevalence of cranial cruciate ligament rupture(19,20), in some case this was a 2.1 times increase in odds for neutered females(21). One study even found no cases of intact females suffering in their 759 but 8% of all neutered females suffered(12). One researcher stated that the most common joint problem associated with early neutering was cranial cruciate ligament damage. It was found that 16% of females neutered before 1 year old suffered, whilst only 5% of intact dogs did(3).
It is currently unclear whether age of neuter had any effect. For several, at the time of ovariohysterectomy was not associated with prevalence(19) so it didn’t matter when the females were spayed, the risk was still increased. However, for others, early neuter, specifically below the age of a year, significantly increased the risk of future cruciate ligament repair being required(22).
Whist a large portion of the literature does point to spaying as a contributing factor to Cranial cruciate ligament injury, several researchers have noted that there is not sufficient evidence to claim this is the case(23).
Other factors such as obesity level, lifestyle & exercise habits at a young age were not always controlled for in these studies. An interesting point that has been raised is that insured dogs are actually 4 times more likley to be diagnosed(21). This is a really crucial point to consider and really demonstrates how association does not mean causation. Instead, what it does suggest is that there is another association to be made. This could be that those who have insurance are more likley to take their dogs to the vets. This could be that people who ensure their dogs are more likley to neuter their dogs or it could be that there is another factor at play which hasn’t been recorded. This is certainly an area that would benefit from further research to determine any relationship that has been shown in the research.
The reported incidence rate of hip dysplasia in the canine population is somewhere between 1-4%(24,25). This is a condition that is thought to have a high level of heritability and appears to affect male and female dogs equally, however some breeds are more prone(24).Other environmental factors have thought to play a role in the development including diet, exercise and living conditions making this a tricky area to discuss and pinpoint whether spaying has any significant effect on the diagnosis of hip dysplasia.
Several pieces of research have found that early spaying (particularly before the age of 7 months), leads to a marked increase in chance of the dog developing hip dysplasia(3,14).
The problem with a lot of the research in this field, however, is that there is no control for genetic interference. Hip scores and incidences of the disease in the dog’s family tree were not analysed and compared to the date so it is difficult to know whether the dog would have been affected anyway. Equally, environmental influences including the amount of exercise the dog received at different life stages, the living conditions and diet were not controlled for. This makes it incredibly difficult to draw any firm conclusions from this data.
One study looked at how feeding quantities affected hip dysplasia. It was found that dogs fed 25% less than the others actually had fewer signs of hip dysplasia(26). From other research in the area, it has been noted that female dogs can be more prone to obesity after a spay. It therefore may be the case that early spaying does not directly result in an increased risk of hip dysplasia, but instead early spaying increases the chance of a dog being obese and obesity increases the risk of hip problems.
Urinary Incontinence (UI)
It is thought that the overall prevalence of UI is 3.14% of the female population(34).
Urinary Incontinence occurred in 5 - 20% of spayed bitches. (35,36,37,38,39) compared to only approximately 1% of intact females. In a study of over 300,000 bitches, spaying was identified as a major factor for urinary incontinence. Spayed bitches were 3 times more likley to develop urinary incontinence than intact dogs(40).
Unlike some of the other medical considerations we have already looked at, whilst a few studies did suggest a link between early spay and an increased risk(41), a large amount of the research regarding urinary incontinence does not find any significant difference between the age of spay and the end result. Those who were spayed early were at no greater or lesser risk of developing UI than those who had been spayed later(36,42).
Instead, risk was relative to the size of the dog with heaver dogs being up to 7 times more likley to be affected that smaller dogs (36,43). In one study, bitches over 10kg were found to be 3.7 times more likley to suffer incontinence. Breeds with the highest risks varied but Rottweiler, Dobermann pinscher, Weimaraner 7 boxer were recurring breeds mentioned in those most at risk. Others mentioned included Irish setter, Dalmatian, old English sheepdog & Hungarian Viszla(44)
Why would this occur?
It is thought there is a relationship between continence and a specific group of hormones called gonadotrophin. These act on the testes and ovaries to increase the production of sex hormones. Spayed bitches have a huge reduction in these hormones which could explain the link between spaying and incontinence45.
After 5 weeks of treatment aimed at affecting these hormone levels, 70% of those with urinary incontinence improved, with 44% maintaining full continence. Unfortunately, the side effects affected 90% of those who underwent this treatment(47).
Out of 1853 records, seven studies were observed for their link between urinary incontinence and spay. Of these, 4 were found to be high of bias and a further 3 had risk of bias(46). This makes it incredibly difficult to draw any conclusions from this data.
Weight Gain & Diabetes
Weight gain has been associated with the spaying of a female dog (27) and in turn this has increased the chance of spayed females developing diabetes. One study however showed the link was weak, if there at all between diabetes & spaying directly. Diabetes is also something that occurs later in life 28 and therefore it can be tricky to ascertain if age of spay would have any affect.
Whilst spaying may increase the risk of obesity. It is important to ascertain why. Spaying was found to significantly reduce the daily energy requirement of dogs, which means that a lot of dogs became lazier. Overconsumption of food was also seen in dogs that were free fed compared to those that remained intact(29). This means that these issues are likley to be able to be managed if the owner makes small changes to feeding routine, diet and exercise for the dog.
Patellar luxation is thought to be a hereditary problem that both male and female dogs suffer with equally. Two studies showed spayed females had increased incidences. The results were taken from 119 vet clinics across England which ensures a good sample of different areas. However, these studies did not look at any genetic components or environmental factors33. This makes it very tricky for any clear conclusions to be made on this.
Pyometra is an infection of the uterus and is a life-threatening condition. It involves the womb filling with puss and left untreated, leads to death. This is often one of the biggest considerations when a person is deciding whether to spay their female dog. Pyometra has been directly related to the hormonal changes which occur during a heat cycle of a dog. Therefore, those who have been spayed are at no risk* of pyometra developing.
The risk of pyometra increases as the unspayed bitch ages. By the age of 10, almost 25% of unspayed females will have been affected by this. For females who had been pregnant in the past, the risk was significantly reduced in the four of the five breed samples studied (Rottweiler, Collie, Labrador & German Shepherd)(48)
Why would pregnancy be a protective factor?
According to two veterinary experts from the VCA,
“Pyometra is a secondary infection that occurs as a result of hormonal changes in the female's reproductive tract. Following estrus (heat), the hormone progesterone remains elevated for up to two months and causes the lining of the uterus to thicken in preparation for pregnancy. If pregnancy does not occur for several consecutive estrus cycles, the uterine lining continues to increase in thickness until cysts form within the uterine tissues (a condition called cystic endometrial hyperplasia). The thickened, cystic lining secretes fluids that create an ideal environment for bacterial growth. In addition, the muscles of the uterus cannot contract properly either due to thickening of the uterine wall or the high levels of the hormone progesterone. This means that bacteria that enters the uterus and fluids that have accumulated cannot be expelled.”49
Because a pregnancy has not occurred, the lining had continued to thicken anyway, increasing the risk of the cysts. Its important to note however that having a pregnancy does not always prevent this with Golden Retrievers in the study above having no significant difference in the number of dogs affected, regardless of whether they had been pregnant or not.(48)
Pyometra is something that could have an article all of its own. There are hundreds of studies looking at how different factors such as specific hormones, proteins, other medical issues etc can affect the severity, type & risk of development however the overwhelming research suggests that if a dog spayed, they are at no/very little risk of this being developed.
Therefore, for the purpose of this section, I am going to discuss the differences in age of pyometra developing, as this will affect the recommended age of spay when considering the other factors we have discussed so far.
Several studies suggest that dogs over the age of 6 are at a significantly higher risk of developing pyometra(50,52,53) but pyometra can occur any time after the first heat cycle(52). Whilst some dogs were diagnosed as early as 9 months old, the median age in one study of over 10,000 dogs was 9 years old(51). Dogs who has never been pregnant were seen as being at higher risk.
In another study, those over 6 years made up 70% of all cases of pyometra. Just over 16% of cases were seen in dogs 3-5 years old & the final 14% was seen in dogs under 3 years of age(54).
In this area of the research, all the findings agree this is predominantly seen in older, intact females. The mean age varies slightly between studies which is to be expected due to a difference in dogs used (breed, size, no. litters etc) but this ranges from aged 6- aged 9 in most of the studies.
Some larger breeds such as Dogue de Bordeaux & Bullmastiff were mentioned as having lower ages (3 years & 5 years respectively)(53), whilst some were recorded as older (Border collies, mean age of 10.3 years), but we must take care when assuming this would be the case for our own dogs of these breeds. Some breeds such as the spitz were found to have the highest incidence rate in one study, which it was suggested may be due to their susceptibility to a specific hormone disorder (diestrual hormone disorder), or it is possible that there is a genetic component also(54).
This is an interesting point to note as if pyometra has a large genetic factor, this suggests that knowing your dog’s ancestry and lines could help in determining your own dogs risk. In addition, breeders may be able to take this into account to reduce the number of incidences in their lines.
In rare cases, stump pyometra can occur in a spayed female. This is when a small segment of uterine tissue has been left during the dogs spay operation which then becomes infected. This is thought to be particularly dangerous because most people with spayed dogs are not looking out for the symptoms of Pyometra in a way perhaps the owner of an intact female is. A spayed dog with no remaining ovarian tissue is at no real risk of this. However, a stump granuloma is possible, which is an inflammation of any remnants of tissue. Preventative measures include ensuring all uterine tissue is completely removed and absorbable sutures are used(55).
Other Medical Findings
Up to 30% of dogs suffer with atopic dermatitis (a form of eczema)(30). There are already several causes that have been identified which include environmental factors. There are only a few studies which have assessed whether spaying can have any effect on the prevalence of this, but they were all reasonably large samples.
One study looked at over 22,000 dogs and found that spayed females were more frequently diagnosed with atopic dermatitis, autoimmune disorders, hypothyroidism, inflammatory bowel disease and lupus. Another looking at over 100,000 dogs found that spayed dogs were more likely to suffer with chronic kidney disease(34). This was further supported in other samples of dogs which also found cystitis was more prevalent in spayed females too(31). A smaller study of dogs could not find the same association(32)
Behaviour changes are often a significant factor when looking at whether to neuter a pet. Some individuals hope that this will change behaviour for the better, and others worry this will change a dog in a negative way.
In this section I will go through the research that has been done into this area. This does not mean that because you decide to spay your pet or choose to leave them intact, that you will see these statistics reflecting in your pet.
Particularly in relation to the female spay, I found this area lacking in research when it came to behavioural effects of the spay. This section is therefore shorter than I hoped, and I am hoping I can update this in the future as more research is done.
Saint Francis Service dogs (SFSD) takes a keen interest in the effects of early spay. Until 2013, most puppies were spayed at 4-6 months old. Organisations such as these have to be very careful about when they neuter their dogs to ensure they have the highest chance of success in their training. It was found that dogs who were neutered between the ages of 7-11 months had a significantly lower dismissal rate than those who had been neutered before 6 months of after 12 months. Dogs neutered at less than 7 months old had more than twice the risk of health-related dismissals than dogs neutered at any older age.
Dogs who were neutered under 5.5 months had fewer incidences of separation anxiety but a greater chance of aggression towards family and strangers(57).
Aggression is something that historically, people believed would be reduced with neuter particularly in males’ dogs. For those who have already read my male castration blog, you will know that this isn’t necessarily the case.
In a study of over 13,000 dogs, those who were neutered before the age of 6 months had significantly more incidences of aggression towards family members(58). These findings were echoed in a study looking specifically at English cocker spaniels. Whilst age was not controlled for, females who had been spayed prior to any signs of aggression were more likely to show aggression towards children in the household. Territory guarding behaviours were deemed the most common form of aggression seen in these dogs(59).
Again, these findings were replicated in several more studies, with aggression towards people being the most common form of aggression seen(60) .In these studies, this behaviour developed after spay. It is impossible to control for other environmental influences and therefore we must take this into account too.
One specific study looked at dogs who were already showing signs of reactivity prior to spay and also dogs who were not yet showing any reactive signs. (Reactivity in this study was defined as a negative response to a human approaching). For dogs who were not yet reactive, results suggested that spaying of a bitch may induce a reactivity problem. For dogs who were already reactive, this appeared to increase reactivity. An interesting point to note here is that we cannot be sure that those spayed were not going to become reactive anyway(61).