This is a rather strange one (and I have to say I think it’s rather unfortunate it falls over Valentine’s Day!), but neutering is a major issue in animal health. There are three reasons for this – it’s widely carried out, there are population-level health benefits, but there are both advantages and disadvantages to neutering on the individual pet level.

In this blog, I’m going to take a good look at neutering, primarily in dogs (because we’ve got more evidence for dogs than other species), but with occasional reference to other small animals (e.g. cats and rabbits). The evidence base on what is considered “good practice” is always changing, so I’m going to I’m going to try and address some of the myths surrounding neutering, and try and take a dispassionate look at the advantages and disadvantages, based on the best evidence currently available.

What is neutering?

Neutering is a general term meaning any surgical procedure that irreversibly removes both fertility and the production of sex hormones (testosterone in males, oestrogen and progesterone in females).

In males, the procedure is castration (or orchiectomy), and involves surgical removal of both testicles – the main site of testosterone production and the only place where sperm can be made.

In females, neutering is also known as spaying; however, there are two different procedures possible. The “traditional” approach is the ovariohysterectomy, where the ovaries and uterus are removed. However, increasingly, a “lap spay” is being performed – an ovariectomy where the ovaries are removed but the uterus is left in situ. In dogs, it is well demonstrated that the effects are exactly the same as a traditional spay, with no increased risk of pyometra, and healing is faster – although it is a longer and usually more expensive operation. It should be noted that in rabbits, this is not routinely performed as it hasn’t yet been proven to eliminate the risk of womb cancer (uterine adenocarcinoma).

3 Myths About Neutering

People sometimes get very worked up about neutering… unfortunately, a sensible discussion is often obscured by some widely held myths.

1)        Vets only recommend neutering because of the money

This is untrue for several reasons… Firstly, vets are professionals, and the vast majority of us are very careful not to allow financial concerns to influence their professional judgement. Secondly, most vets aren’t on commission – it doesn’t make any financial difference to us whether we recommend neutering or not (you may not realise it, but most vets are employees of the practice, on a fixed salary). Finally, however, in many if not most practices, neutering doesn’t actually make a significant amount of money, as almost all practices discount it more or less to cost price.

2)        Neutering is harmful to an animal’s self-esteem

As far as we know, animals (unlike humans) are unaware of whether they are entire or neutered. “Self-esteem” appears to be a largely human preoccupation, and one which animals do not associate with reproductive status.

3)        Neutered dogs are always healthier

No, they aren’t; likewise, entire dogs aren’t always healthier either! Neutering does alter the risks of different diseases, but whether or not they’re healthier depends on the individual and their unique health status. Neutering is a procedure with many health benefits, but also some disadvantages, and the decision should always be made for that specific individual.

What are the advantages?

The advantages to neutering are well recognised and discussed; the most important ones include:

Loss of reproductive capacity – given that two cats, with unlimited resources, could have 40,000 offspring in 7 years, the potential for poor welfare (e.g. starving kittens) if reproduction continues unchecked is vast. For some species the capacity is even higher – 2 rabbits in the same scenario could have 184,000,000,000 descendents in the same time! This is why almost all shelter and rehomed animals are routinely neutered – to limit the excess population.

Behavioural changes – male dogs are less likely to roam looking for females, tomcats are less likely to spray and fight, bitches no longer have prolonged and sometimes messy seasons, and queens no longer call. In addition, fighting between males (all species) and between unrelated females (in rabbits) is significantly reduced, and sexual behaviours that many humans find objectionable (mounting, humping, and masturbation) are all dramatically reduced in males neutered before puberty or in early adolescence. These changes are all to our advantage, and often make neutered animals more socially acceptable pets, less likely to be abandoned or rehomed.

Reduced risk of certain diseases – this is perhaps the best argument for neutering pet dogs, cats and rabbits. In particular, we think of…

  • Pyometra in bitches – studies suggest that one in four (or maybe even more) entire bitches will develop a pyometra (womb infection) by the age of 10. This is a severe and potentially fatal condition, and is entirely preventable through neutering.
  • Mammary tumours (breast cancer) is relatively common in entire bitches and queens (female cats), but neutering exerts a very strong protective effect in both species – a bitch neutered before her first season has a 200 fold reduction in the risk, although the longer she stays entire, the less benefit she gets.
  • As many as 80% of entire does (female rabbits) will develop malignant cancers of the uterus; neutering eliminates the possibility of the disease entirely.
  • Testicular tumours in male dogs (the second most common group of tumours according to some sources) – no testicles = no tumours!
  • Prostatic diseases such as benign prostatic hypertrophy, prostatitis and prostatic abscesses are virtually unknown in neutered males of any species, as is the anal cancer in dogs “perianal adenocarcinoma”.

Increase in lifespan – the best data we have is for dogs, where the evidence shows that a neutered female lives as much as 26% longer than if she were entire. For males, the increase in lifespan is a little less, with castrated males living about 14% longer; however, the majority of this increased risk is due to road traffic accidents, presumably as a result of roaming after bitches and overconfidence (both testosterone-mediated behaviours), and if these are excluded, there is no significant difference in lifespan between neutered and entire.

What are the disadvantages?

There have been a lot of studies into possible disadvantages and even health risks from neutering. Unfortunately, this area of study is still in its infancy, and many of the findings are still rather uncertain as yet. However, the evidence does suggest that there are possible health threats as a result of neutering:

Behavioural changes – neutered male dogs are much less self-confident and appear to be at a significantly increased risk of fearfulness, especially fear-mediated aggression. Some studies suggest that increased aggression might be a consequence of neutering in bitches, but this is not proven.

Increased risk of certain conditions – these may be breed specific; however, there is fairly solid data for an increase in risk of:

  • Osteosarcoma – a type of malignant bone cancer; the risk is reportedly doubled in neutered animals. However, in most breeds, it’s fairly uncommon, and neutering only increases the average risk by 6 cases in 10,000; it may also be less of a risk if animals are neutered after skeletal maturity. That said, if a dog is of a breed with an unusually high genetic risk of this cancer, it might be worth an discussion.
  • Haemangiosarcoma (a malignant tumour of blood vessels) and transitional cell carcinoma (bladder cancer); a number of studies show an increased risk in neutered animals. However, others do not, and it is likely that genetic factors are at least as important (if not more so) as neutering status.
  • Obesity – neutered animals burn fewer calories!
  • Cruciate Ligament injuries – this may be linked to obesity, but some evidence suggests that neutering may be an independent risk factor, especially in Golden Retrievers.
  • Diabetes mellitus in dogs and cats (but probably again due to the obesity issue) and Feline Lower Urinary Tract Disorder in female cats (again, obesity is probably the main mechanism, although there may be others)
  • Urinary Obstruction in males is well known to be more likely in neutered cats, especially if castrated before puberty, due to the narrower urethra.

Should I have my pet neutered?

Ultimately, whether or not to neuter a pet MUST be a decision made for that individual animal, taking into account their specific health status, risks and their lifestyle. That’s a conversation you need to have with your vet – don’t get all your information from biased online sources!

How can I find out more?

There are a number of papers on this subject, and a LOT of website articles – however, I’d strongly advise going to the original research or at least scientific review papers rather than website opinion pieces. The reason is that a lot of the research is very specific, based on a small number of dogs from an even smaller number of breeds. As a result, it can be difficult to generalise reliably, and it’s useful to understand the limitations of any studies.

My recommendations would be:

  • The American Veterinary Medical Association’s article “Reference Point: Determining the optimal age for gonadectomy of dogs and cats”, which is a really good overview of the research and its significance. It is dated 2007 and some more recent studies are therefore not reported, but it’s available online here.
  • A Rutgers University literature review, also from 2007, here.
  • The famous (or notorious!) Golden Retriever paper about joint disease and cancer can be read in full here, just remember, it’s very specific to a fairly limited population, and may not be entirely representative of other breeds.
  • The more recent PLOS One paper on lifespan and disease risk, from 2013, is an excellent summary of the disease risks in neutered vs entire animals, containing data from 70,574 dogs of 185 different breeds. You can read the article here.