(Study percentages were referenced from an article by L. M. Howe titled “Current perspectives on the optimal age to spay/castrate dogs and cats,” which was published in veterinary medicine research and reports.)
We have had some questions regarding benefits in waiting to spay and neuter (sterilise) animals. Spaying (female) and neutering/castrating (male) has been routine practice for decades in veterinary medicine to prevent undesirable behaviours, medical conditions, and diseases.
Recently, this practice has come under scrutiny and become a very controversial topic. There may not be a single optimal age to sterilise; it can depend on species, breed, body size, and breed-specific diseases. The age to sterilise certain breeds is becoming less clear, with many conflicting studies.
Obviously, one of the main and most important reasons for sterilising remains to prevent unwanted litters. Shelters are overflowing with animals and there are just not enough homes. We need to prevent unwanted pregnancies.
Another common benefit is to prevent reproductive disorders. These include pyometra and mammary neoplasia in female dogs and cats and prostatic hyperplasia and testicular cancer in dogs.
Mammary tumours are one of the most common types of cancer seen in intact (unspayed) female dogs. Fifty percent of these are malignant (and can grow and spread to other parts of the body). It is believed that if a female dog is spayed after the second heat or over two-and-a-half years of age, her risk of developing mammary cancer is 26%.
Pyometra is an infection of the uterus, which can be life-threatening. The uterus fills with pus, poisoning the body. It can also rupture or cause secondary complications resulting in organ failure. The risk of pyometra ranges from 25–66% in dogs over nine years old.
Benign prostatic hyperplasia can affect 75–80% on intact dogs by six years of age. Spayed female dogs are at greater risk for urethral sphincter mechanism incompetence, which can lead to urinary incontinence (leaking urine), especially when lying down or sleeping. The greatest risk is in dogs spayed early (three months) and larger breed dogs.
There are also some other breed-specific diseases related to timing of sterilisation. Rottweilers sterilised before twelve months of age have a risk of 28% for bone cancer; intact males and females have 8% chance. Male golden retrievers sterilised before twelve months of age have a risk of 10% for hip dysplasia; intact males have a 5% chance. Therefore, by waiting you have only reduced the risk of hip dysplasia by 5%.
The same is true for risk associated in cruciate rupture in golden retrievers; the risk is only reduced by 5%. There are also certain mast cell or lymphomas (types of cancers) that have a 2% higher rate in golden retrievers sterilised before 12 months. The risk is again minimal.
Labradors also only have a 4% reduction in risk of cruciate rupture, and hip and elbow dysplasia.
You should not use information related to a particular breed to make decisions in another breed.
In male dogs, sterilising before six months produces significant behavioural benefits. It decreases roaming by 90%, mounting behaviour by 70%, and aggression towards other males by 60%. Male cats castrated before five-and-a-half months decreases sexual behaviour, urine spraying, and aggression. There are numerous facts to consider when making the decision on when to sterilise.
Our personal view at this clinic remains to sterilise at six months of age.
Certain behavioural conditions can be prevented by castrating males. If done later, this has minimal effect as testosterone-driven behaviour becomes a learnt behaviour, even when that testosterone is reduced. Many animals are rehomed or put to sleep due to behavioural conditions, so our view is to prevent unwanted behaviour related to testosterone before it happens.
Life span in many dogs is increased by preventing life-threatening reproductive diseases. Those benefits taking precedence over the concern of other diseases that occur with less frequency in sterilised animals. In these studies, there are many factors that affect the outcome: diet, environment, body condition, exercise, etc.
The take-home message is that there are both benefits and risks involved in timing of sterilisation. The benefits heavily outweighing the risks for most patients.