Stuart Fitzgerald
Dr Stuart Fitzgerald (MVB MANZCVS, University College Dublin)
Backend of a Bulldog

Neutering, a term which can be applied for both male and female animals, is an essential part of responsible pet ownership. Apart from the few animals intended for breeding later in life, it is difficult to formulate a good argument against the procedure, for a number of very good reasons, which will be outlined below.

Neutering of male dogs, referred to as castration, reduces the incidence of several common hormone-related disorders, aids population control, and reduces roaming and aggressive behaviours. While full surgical castration, with the removal of the testes, is normally performed, there are alternatives, which will also be discussed.

Overview of Reproductive Anatomy in the Male

The primary function of the male reproductive tract is the development and delivery of sperm to pass on the individual’s genetic information. The external genitalia consist of the penis and the testicles, which are located in the scrotum between the hind limbs. Internally, sperm are transported from the testicles in the paired ductus deferens, which exit into a tubular structure called the urethra. It is through the urethra that the sperm are then released, along with other fluids, including a substantial amount of fluid from the prostate.

The prostate is a secretory gland that sits at the neck of the bladder, and which also drains into the urethra around the point where the ductus deferens enter. As well as surrounding the neck of the bladder, the prostate also sits in close contact with the rectum in the pelvic canal, a fact which will become relevant later in our discussion.

As well as producing sperm, the testicles are also the main source of testosterone production in the body. This is a particular type of hormone, called an androgen, which results in typical male physical and behavioural patterns, encouraging lean muscle mass development, sexual behaviour, and potentially, aggressive and dominance-type behaviours.

Medical Reasons for Castration

Dog having an ultra sound at the vet clinic


As part of the normal development of a male dog, the testicles originate close to the kidneys during the foetal stage, before beginning a slow process of descent into the scrotum. This process is controlled by a muscular structure called the gubernaculum. This journey does not always proceed as expected, and one or both testicles may become ‘stuck’ along the route. This is commonly seen in Maltese and Greyhounds, amongst other breeds, and the condition is termed cryptorchidism. The entrapped testicle(s) is most commonly located in the inguinal canal, the passage between the abdomen and the outer abdominal wall, although it can potentially be found anywhere along the route described.

Male puppies that do not have both testicles within the scrotal sac by 12 weeks of age may be diagnosed with the condition, although full descent can (rarely) take a further 6 months in some toy breeds. Although the retained testicle may be much smaller than one located in the scrotum, it will remain hormonally active, producing at least as much testosterone as a normal testicle. In addition, such retained testicles are at far higher risk of developing tumours later in life (see below). Castration is therefore considered mandatory for affected dogs. Cryptorchid dogs with one normal testicle are likely to be fertile, and will pass the condition to a large proportion of their male offspring.


Testicular tumours are common in unneutered male dogs, with an increased prevalence being recognised in certain breeds, for example, Boxers, German Shepherds, and Maltese. Several different types of testicular tumours are seen, which may be benign or malignant. Although a vigilant owner or veterinary surgeon may pick up on such growths at an early stage, many become quite large, and can produce other symptoms, depending on which cell type the tumour originates from. All testicular tumours are more common in older dogs, typically over 10 years of age. The two most common forms of testicular tumour are discussed below.

Sertoli Cell Tumours

In health, Sertoli cells are responsible for nurturing the growing sperm cells. Part of the mechanism through which they do this is by secreting oestrogen, one of the female sex hormones, in very low quantities. Tumours derived from Sertoli cells can produce far more oestrogen, potentially resulting in feminisation syndrome, where the dog may lose hair, adopt a female position for urinating, and become attractive to other male dogs. Other signs, such as anaemia, may also be seen. The disordered hormone status can also result in changes to the cells lining the prostate, a process known as squamous metaplasia, causing loss of blood in the urine or ejaculate. Around one in seven Sertoli cell tumours are malignant, having the potential to spread to sites beyond the testicle. They are particularly common in cryptorchid dogs.

Leydig/Interstitial Cell Tumours

While Sertoli cell tumours are often very large at the time of discovery, Leydig cell tumours average less than one inch in diameter. These are typically benign, and may be incidental findings on a veterinary exam. However, they too can cause an increase in circulating female hormones, and can therefore cause some of the signs seen for Sertoli cell tumours, although they are usually less dramatic. Prostate changes, which may lead to prostatitis and infection, are the most common complications.

Prostate Disorders


The size and health of the prostate is determined by a healthy balance of sex hormones. As discussed, changes in the levels of these hormones, as seen with testicular tumours, can lead to internal changes which render the prostate susceptible to infection and the development of cystic, fluid-filled cavities. The prostate is also a very sensitive organ, and such changes usually result in a considerable amount of discomfort. To an owner, this may manifest simply as changes in behaviour, and more than one older dog suffering from prostatitis has been described by the owner as simply becoming a ‘grumpy old man’. A thorough veterinary examination, including palpation of the prostate, will usually determine the source of the problem, although analysis of urine and an abdominal ultrasound exam may sometimes be necessary.

Benign Prostatic Hyperplasia

Even in the absence of changes in hormone levels, the prostate tends to enlarge and become troublesome with advancing age. Prolonged stimulation by testosterone can cause progressive enlargement of the organ, with or without accompanying inflammatory or cystic changes. This process is termed benign prostatic hyperplasia (BPH), and it manifests as either blood loss from the urethra or difficulties in passing faeces. Stools produced by dogs with BPH may also be noticeably flattened, as the faecal material must squeeze past the enlarged organ in the pelvic canal. BPH is treated by removing the source of the testosterone, usually by castration.

Prostatic Tumours

Although not significantly impacted by neutering, I am including a brief description of prostatic tumours for completeness. Growths arising in the prostate can be quite aggressive, causing a lot of tissue destruction within the organ, and often severe pain. In the early stages, signs may mimic those of BPH or prostatic inflammation, and differentiation of these disorders requires ultrasound and tissue examinations. The prognosis for prostatic tumours is usually poor, with these cancers having a tendency to spread to the bones of the lower spine.


A condition which is better prevented than treated is perineal herniation. This is an uncommon problem, which is seen exclusively in older unneutered male dogs. The chronic influence of testosterone on the muscles of the perineum (the area between the tail base and scrotum) and the prostate, which enlarges with age, results in a rupture between the perineal muscles. These muscles are responsible for retaining the pelvic organs, such as the prostate, rectum, and bladder, in their normal position.

The rectum is usually the first organ to herniate, whereby it typically slides under the skin of the perineum to develop a sharp bend. This bend creates difficulty for the dog in passing faeces, causing him to strain harder, increasing pressure on the hernia, and further enlarging it. With progressive damage, other organs may also herniate, and if left for a long period, what seems to the owner to be a simple case of constipation can actually present a major surgical challenge.

Repair of a perineal hernia can be challenging, although no two are the same. Considerable skill is required for the surgeon to replace the pelvic organs and repair the hole constituting the hernia without causing further damage, to large nerves in the area in particular. In addition to performing the repair, castration is necessary to prevent recurrence.

Behavioural Reasons for Castration

By removing the source of testosterone, we can prevent or treat of behaviours which may be seen as inappropriate in certain settings. For example, while it may be perfectly normal and natural for a male dog to attempt to mount and mate with a female dog in heat, the same behaviour can be entirely inappropriate in a family with children, who the dog attempts to mount. These sexual behaviours, as well as roaming in search of in-heat females, can be greatly reduced or eliminated by neutering. However, a certain learned component also exists, and it is far less likely that castration will eliminate a behaviour that an eight-year-old dog has spent a lifetime learning.

Aggression is the other type of behaviour that may be amenable to neutering, though not in every case. The input of a behaviourist is always valuable in aggression cases, in order to identify the trigger for the behaviour, and to suggest remedies for it. However, it is safe to generalise that dominance and territorial-type aggressions are more amenable to treatment with a combination of neutering and behavioural coaching.

It is important to realise that these behaviours, as well as most of the medical conditions outlined above, are far less likely to occur in the first place with early neutering.

When to Neuter

Far from being set in stone, the question of the optimal time for neutering is a constant topic of debate in the veterinary community. As a general rule, most male dogs can be neutered from around 6 months of age. Factors which may be considered in choosing the time of neutering include the dog’s environment and his behaviour toward other dogs and people, general health, and breed. Large and giant breed dogs will normally be left until around one year of age before neutering to avoid any unintended interference with their normal development, although most of the available evidence suggests that this is unlikely to be an issue.

Castration Technique

Neutering Operation

Castration involves the surgical removal of both testicles. The procedure is usually very quick, but requires general anaesthesia. When under anaesthetic, the dog is laid on his back, and the hair around and in front of the scrotum is carefully shaved and scrubbed with an antiseptic preparation. The surgeon makes an incision just in front of the scrotum, and advances the testicles one at a time until they protrude through the wound.

A quick incision onto the surface of the testicle itself allows it be pulled through the incision, where its blood vessels and ductus deferens are clamped and tied. At this point, the testicle can be excised, and the tied vessels allowed to retract within the body. When both testicles have been removed, the skin and subcutaneous tissues are closed, usually with several sutures, before the dog is woken from anaesthesia.

Pain relief is administered to all surgical patients as a matter of routine, and most castrated dogs recover smoothly, without overt pain or distress. Smaller or more sensitive dogs may need to take a supply of painkillers home when they are discharged from hospital, something which usually happens later on the day of surgery. Castration is not a technically demanding procedure, and complications are very rare. I would estimate, based on years of experience, that over 90% are pain free and wanting to be fully active within 48 hours of the procedure. This can present a challenge, as rest is important in the postoperative period to allow the wound to heal, so owners must be prepared to keep their dog somewhat confined for around the first week.

Alternatives to Castration

Vasectomy presents an alternative surgical technique which may be used to prevent a male dog from breeding. However, it simply involves removing a portion of the ductus deferens, rather than the testicles. As such, the dog’s hormone levels remain unchanged, and all of the medical and behavioural problems discussed above remain a concern. For this reason, it is very rarely practised.

In recent years, an injectable implant containing deslorelin, a disruptor hormone, has been made available. This implant is placed under the dog’s skin, and slowly releases its active ingredient to cut down on the production of testosterone and other hormones over its active period of 6–12 months. Thus, it achieves many of the aims of surgical castration; however, care must be taken not to forget to repeat administration before the effects wear off. Additionally, it is likely that some dogs receiving the therapy will still go on to develop testicular tumours.

Adverse Effects of Castration

There are two main concerns which should be discussed when arranging castration: weight gain and coat changes. Neutering of both males and females reduces their energy requirements, meaning that for a given amount of food, a neutered dog is more likely to become overweight than an unneutered dog. However, this has led to a general misconception that all neutered dogs are destined to become obese: this is simply not true. Dogs should be fed slightly less, or fed a diet formulated for neutered animals. This should be begun immediately after surgery, and regular weigh-ins with your veterinarian or veterinary nurse are advisable for the first few months. With a little care and attention, weight gain need not become an issue.

The hair quality in some breeds can be affected by neutering. For example, spaniels tend to develop a more ‘woolly’ appearance in the months following neutering. This is a cosmetic problem, rather than requiring any particular treatment, and regular brushing and a high quality diet will go some way to ameliorating it.