A surprising question…….
Since the 1970’s vets have advocated neutering for prevention of cancer in dogs and cats. There is evidence for a reduction in breast cancer in neutered female dogs(1) and cats (2). Surgical neutering also eliminates the risk of testicular or ovarian cancer.
However, in recent years research has suggested that some types of cancer are seen more often in neutered dogs. Although these studies have made headlines, we must look more deeply to see if they answer this question.
What research has been done?
In 2013, vets at the University of California examined the records of 769 Golden Retrievers (3). Their aim was to establish if there was a link between neutering and cancer. They concluded that neutering increased the risk of 3 types of cancer that occur in the dog (haemangiosarcoma, lymphoma and mast cell tumours). The results were complicated by gender and the timing of neutering. For example, early neutering (before 12 months) increased the risk of lymphoma more in male dogs than in female dogs. Whereas haemangiosarcoma occurred more often in late neutered females than in early neutered females, entire or neutered male dogs.
These results were then compared with a study in Labrador Retrievers(4). The authors’ found that neutering affected cancer risk far less in Labradors.
An internet study surveying Vizla owners (5) suggested that neutered dogs had a higher incidence of mast cell tumours, lymphoma and haemangiosarcoma (females only).
Is there anything more recent?
A multi-breed study was carried out in 2020. This study (6) was designed to aid decision making around neutering and time of neutering. An increase in the risk of developing cancer was only seen in 2 small breeds, the Cocker spaniel and the Boston terrier. The risk was only significant for the male Cocker spaniel, neutered before 1 year of age and the female Boston terrier, neutered before 2 years of age. The study found an increased risk of cancer in Golden Retrievers, Labrador Retrievers and German Shepherd dogs when they were neutered. The authors’ conclusion was that there was an elevated cancer risk in relatively few of the 35 breeds studied.
How might neutering lead to cancer?
These complicated and conflicting results reflect how difficult cancer research can be. The development and treatment of cancer is affected by many factors: genetic, age, veterinary factors, environmental factors, sex hormones and more. Few of the studies found a simple cause-effect relationship. It is probable that most cancer development is multifactorial. Looking at some of the complexities can help us apply these studies to the health of our own dog.
Genetic factors
Golden Retrievers have higher rates of disease caused by cancer than many other breeds (3). Some breeds are more susceptible to certain cancers. For example. Rottweilers have a high incidence of bone cancer (osteosarcoma)(7). A similar study showed that early neutering may add to their increased risk. A breed susceptibility to a particular type or types of cancer suggest that there must be strong genetic component involved. This means that we cannot apply these studies to other breeds.
Age
Cancer is seen more often in older dogs. Neutered dogs are known to live longer than entire dogs. (8,9). A Finnish study (9) showed that entire dogs were more likely to die from trauma and infectious disease and neutered dogs more likely to die from cancer or immune mediated disease. Therefore, it is possible that neutered dogs are more likely to live long enough to develop cancer. Although they may develop more cancer, it does not shorten their lives compared to entire dogs.
Veterinary factors
The studies mentioned above occurred in veterinary hospitals or on the internet. So, we know that the dogs involved belonged to committed owners who are interested in and invest in the health of their dogs. It is probable that more of these dogs are neutered because of current veterinary advice. These studies are more likely to include higher numbers of neutered dogs which can bias the results. In the Golden Retriever study there were no cases of mast cell tumour in entire female dogs. This may be because neutering affects this cancer or because no entire dogs were referred to the hospital. Veterinary research often relies on a small number of cases so conclusions can be compromised.
The case load at a veterinary hospital depends upon cases being referred by vets in practice. Vets choose the hospitals for many reasons. Some cases would be sent to different hospitals if there is a team with considerable expertise. Therefore, some diseases may not be seen as often in an individual hospital.
The Vizla study was very large, which is helpful for reducing bias but none of the results were verified by vets. Histories were provided by owners who may not have all the medical records available.
Environmental factors
There are indications that the environment affects cancer development in other species. For example, smoking and lung cancer in humans, the oncornovirus causing breast cancer in mice. Further research is necessary to determine if environmental factors play a role in cancer in small animals.
Sex hormones
Some tumours are more prevalent in male dogs and others in female dogs, this would suggest sex hormones can play a part in cancer development.
The studies above suggest that sex hormones may have a protective effect in some cancers. This has also been studied in canine prostate cancer as neutered dogs have an increased risk of disease (10). It is thought that a lack of testosterone may favour progression of this tumour. Neutering and time of neutering appear to affect the incidence of cancer.
So what does this actually mean?
In conclusion, more studies are necessary to research the causes of cancer. Most of the relevant studies have been carried out in large or giant breed dogs. Currently there is enough evidence to suggest that large breed female dogs should be neutered between the second and third season and male dogs when they reach skeletal maturity (11).
The decision whether and when to neuter should be made on an individual basis taking into account: breed, prevalence of disease in the breed, age, sex and behavioural factors. There are risks in deciding not to neuter such as pyometra (womb infection), diabetes and breast cancer in the bitch. In the male dog castration prevents testicular cancer but may also stop them wandering and being involved in accidents. Assessing the risks involved and discussing them with your vet will help you make the right decision for you and your dog.
References and further reading:
2. Overley R, Sohofer F, Goldschmidt MH, Sherer D, Sorenmo KU. Association between ovariohysterectomy and feline mammary carcinoma. JVIM. 2008
11. Goh CSS. Age of neutering in large and giant breed dogs – a matter of opinion. Clinicians Brief
Discussion