The benefits and risks of neutering pets
For decades, veterinarians and animal associations have promoted the benefits of neutering to prevent unwanted litters. Every year, hundreds of thousands of unwanted dogs and cats, including puppies and kittens, are euthanised.
By having your dog or cat sterilised, you do your part to prevent the birth of unwanted puppies and kittens. Spaying and neutering prevent unwanted litters, help protect against some serious health problems, and may reduce many of the behavioural problems associated with the mating instinct. Removing a female dog or cat’s ovaries eliminates heat cycles and generally reduces the unwanted behaviours that may lead to owner frustration.
Removing the testes from male dogs and cats reduces the breeding instinct, making them less inclined to roam and more content to stay at home. Spaying an animal also has health benefits: from preventing uterine infections, prostatic hyperplasia, or breast, ovarian or testicular cancer which are fatal in about 50% of dogs, to avoiding unwanted behaviours such as aggressiveness, fleeing, digging, marking. But now there is evidence that sterilisation can have some negative impacts on health.
Overall, the data is complex and inconsistent, but a growing body of research seem to incriminate neutering as a cancer risk. However, this isn’t a clear cut relationship. Many factors need to be taken into consideration when looking at the relationship between interventions such as sterilisation and cancer. These can be the age when spayed, the breed, body size and sex.
All studies seem to be sure of one thing, the trends are not universal. Some cancers almost certainly occur less often following neutering, and others almost certainly occur more often. This link is likely to have a genetic component, manifested by the tendency of certain breeds to develop certain types of cancer. Different dog breeds also have different overall cancer risk.
As almost 1 in 3 dogs will develop testicular tumour in their lifetime. Castration, the removal of the testicles, evidently takes away that risk. For mammary tumours, the removal of the ovaries and the sexual hormones has a positive effect on tumour development, and the earlier, the better. Dogs spayed before their first, second or third heat cycle have a 0.5%, 8% and 26% chance respectively of developing mammary tumours. Tumour risk in the mammary glands in entire dogs is therefore higher than in neutered animals. Moreover, small breeds were at a higher risk of developing tumours of the mammary gland and the endocrine glands than large breeds. So hormones seem to clearly impact tumour development but not always in a bad way.
A recent publication took into account many factors and found that neutering appears to reduce the risk of adenocarcinomas, the most common type of cancer found, especially in females. Since over 60% of the adenomas/adenocarcinomas are found in the sexual organs, this primarily relates to breast cancer, as previously stated, which occurs much more often in intact females than in neutered females or males.
Other types of cancer, outside the genital organs either showed no difference in risk with neutering or appeared to be more common in neutered animals for one or both sexes. Females were at a lower risk for haemangioma/ haemangiosarcoma and squamous cell carcinoma compared with males, while they had a 33.7% higher risk for adenoma/adenocarcinoma overall. And unsurprisingly, age seemed to increase tumour risk.
In Golden Retrievers the dogs seem to be more at risk of developing tumours in various locations other than the sexual organs, but when the data was scrutinised, again disparities started showing up. Time of neutering seemed to have an important impact on cancer development as early neutering seems to have a more negative impact than a later operation. But this result cannot be extrapolated to other breeds of dogs.
Percentages and number of cases over the total sample size for each neutering status group; intact and neutered early or late for male/female Golden Retrievers (1–8 years old) diagnosed with hip dysplasia (HD), cranial cruciate ligament tear (CCL), lymphosarcoma (LSA), hemangiosarcoma (HSA), and/or mast cell tumor (MCT) at the Veterinary Medical Teaching Hospital of the University of California, Davis, from 2000–2009.
Interestingly, the effect of neutering status on tumour development seem to be partly dependent on the examination method, specifically on whether the animal was dead or alive at the time of diagnosis. Tumour incidence in post-mortem samples seem to be higher in neutered than in entire dogs, suggesting bias through investigation of mammary glands and testes in ex-vivo materials. This might explain why an important amount of studies seem to correlate neutering to cancer development. It is important to factor in collection method when examining studies.
So yes, some studies seem to point in the direction of neutering as having a cancer causing effect, but when scrutinised, these studies also mostly focus on very specific cases, and putting the study into context. Also the time of neutering should be considered, and is rarely informed. The best way to identify real trends is to compare data from different studies, often involving different populations (ages, breeds, geographic locations, etc.) and study methods (clinical patients, cancer registry studies, pathology lab studies, insurance company data, etc).
So for now, the advantages of neutering cats and dogs seems to outweigh the possible increase in the risk of the animal developing cancer.