Is there a ‘best’ time for neutering dogs or should we be neutering them at all?

Robert Falconer-Taylor BVetMed, DipCABT, MRCVS, Val Strong MSc and Professor Peter Neville – Centre of Applied Pet Ethology (COAPE)

Part 1

For clarity, in this article the term neuter is used to mean a surgical procedure carried out in order to reproductively sterilise both male and female dogs. The term castration is used to mean the removal of male dogs testicles. The term spay is used to mean the removal of female dogs ovaries alone (ovariectomy), or the surgical removal of female dogs ovaries and uterus together (ovariohysterectomy).

Those working with dogs professionally including trainers, behaviourists, breeders and vets often hold strong views on neutering much of which is probably based on anecdote, opinion and handed down tradition with little solid evidence for or against. The purpose of this article is not to make any recommendations for neutering or against it, but to present a summary of some of the science we do have on the pros and cons of neutering dogs.

There have been enormous advances in anaesthetic techniques over the last decade or so; nonetheless there will always remain inherent risks in all surgical procedures including the neutering of dogs. However, left unneutered and to their own devices dogs will be dogs and there are also inherent risks in procreation, especially in pregnancy. Among mammals dogs show the greatest phenotypic diversity on the planet and with dogs of every size and shape it comes as no surprise that dystocia is a very common complication for some breeds, most of which require a caesarean section to deliver the puppies. Unlike routine spaying, a whelping cannot be a precisely planned and prepared-for event and therefore the surgical procedure will be inherently more risky for the bitch and of course the puppies. In terms of the overall welfare of the dog population the most compelling reason for supporting neutering is the number of dogs being abandoned in UK. A 2011 report compiled by the charity Dogs Trust came up with a figure of 126,176 dogs per year, a staggering 345 dogs every day. With figures like these there is no place any more in our society for the ‘casual litter of pups’ and the sentiment so often expressed by our children ‘Please Mummy, can we have just one litter of puppies?’.

Around the world attitudes and approaches to neutering of dogs vary enormously. Here in the United Kingdom and in the United States neutering is a routine part of (responsible) dog ownership and in the UK we neuter more of our pets than anywhere else in the world. In other European countries like Germany and the Scandinavian countries (Norway, Sweden, Denmark, Finland and Iceland) neutering pets is considered a mutilation and is illegal. In addition early neutering of puppies at 8 weeks old is routine in the United States, whereas here in the UK the British Veterinary Association state: “at the current time there is insufficient scientific data available to form a position on the early neutering of dogs and bitches.”

In the UK about 54% of dogs are neutered. When it comes to professional advice about the ‘best’ time to have their dog neutered, there is currently no clear consensus within the veterinary profession, which has given rise to owners across the country being given a range of opinions. One survey found that about 72% of vets ‘always’ recommend spaying if the dog is not to be used for breeding (the other 28% recommending it ‘mostly’, ‘sometimes’ or ‘rarely’). And 16% of vets ‘always’ recommend spaying before the first season (the other 84% recommending it ‘mostly’, ‘sometimes’, ‘rarely’ or ‘never’). According to another source most veterinary practices recommend neutering of both sexes between ‘6 months and 2 years of age’. Interestingly vets practicing in urban areas neuter more dogs and at a younger age than those in rural areas who are more likely to recommend a bitch ‘has her first season’’ before spaying.

There is varied and often conflicting information on the influence of neutering on dogs’ behaviour and the development of behaviour problems. This is in part because how dogs interact and behave is very much shaped by their circumstances and environment. But in addition there have been no epidemiological studies specifically looking at both normal and abnormal behaviour in large populations of dogs over their lifetimes. The only data we have are from relatively small owner-questionnaire surveys focusing on specific behaviour problems, such as aggression, and data collected by behavioural referral clinics, for example in university departments, neither of which properly represent the normal dog population at large. However, we can still draw some useful information from the data we do have. There have been many studies focussing on aggression in dogs and most of these make a causal link with intact male dogs. Several studies have reported significant reductions in roaming (e.g. 90%), dog-dog aggression (e.g. 60%) and mounting (e.g. 80%) after castration. In the same studies this pattern is reversed for female dogs where there is a higher incidence of aggressive behaviour among spayed dogs compared with intact bitches. There is also some evidence that spaying bitches that have already shown aggression towards their owner before six months old exacerbates the problem.

This is supported by a study that showed that neutered German Shepherd Dogs were more reactive around unfamiliar people and dogs than intact German Shepherd Dogs. Other studies have shown that castrated dogs are more excitable than entire dogs and neutered dogs are more ‘active’ than intact dogs.

There are no studies in dogs investigating the role of sex hormones in the development of the brain in puppies, but we do have many studies in other mammals. What we do know is that, beyond their role in sex and sexual behaviour, both male and female sex hormones are crucially important in the emotional development of the young. We also know that many of the ongoing day-to-day cognitive functions in adults such as learning and memory, attention and processing of emotional events are modulated by the sex hormones. What effects, if any, neutering may have on dogs’ cognitive performance is currently not known. A study on age-related cognitive impairment comparing populations of older neutered and intact dogs suggested that circulating testosterone in entire male dogs has a protective role in the progress of the disease. No conclusions could be drawn for females, as there were not enough intact dogs in the study.

Several studies suggest a link between neutering, including age of neutering, and an increased likely-hood of ‘at risk’ dogs developing hip dysplasia that requires some form of treatment. At risk dogs include the larger breeds where the estimates of occurrence have been quoted as high as 75%, but of course around 5% of these dogs will show physical signs of lameness. Hip dysplasia affects less than 1% of small breeds. The reason why neutering can have a negative effect on hip dysplasia outcomes is not clear, but it may be an indirect effect as a result of weight gain in some dogs later on. Likewise neutering could increase the incidence of cranial cruciate disease in larger dogs, especially those neutered before 6 months of age, probably because of a negative effect on the shape of the still-developing stifle joint. A recent study of common disease in Golden Retrievers compared the incidence of these diseases in dogs neutered before 12 months old with dogs neutered after 12 months old. The incidence of hip dysplasia and cranial cruciate disease was higher in early-neutered males and females. Osteosarcoma is a very serious cancer of the long bones in predominantly larger dogs with an incidence rate of as high as 6%; one study found an incidence of 12% in Rottweilers. It appears that neutering increases the risk of developing osteosarcoma in large breeds of dogs and the primary factor is neutering before sexual maturity. Thereafter, the negative affect of neutering on osteosarcoma risk decreases the longer a dog remains in tact, although there is some doubt in the interpretation of this data.

Although considered to be an entirely preventable disease irrespective of cause some studies have shown an overall lowering of metabolic rate in neutered dogs, but other studies have found no difference. However, some neutered dogs do seem to change their feeding habits and consistently consume more calories than their un-neutered counterparts. Linked to increased weight of course is an increased risk in orthopaedic diseases, such as cranial cruciate disease and hip dysplasia, and other metabolic diseases like diabetes. Any direct link between neutering and risk of developing diabetes in dogs is not known, although there is a clear link in female cats in particular arising as a result of the abolition of some protective regulatory effects of the female sex hormones on insulin.

Part 2 will follow shortly

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