This is another in my series of evidence updates on the risks and benefits of neutering in dogs and cats. I will be updating the evidence and conclusions of my original 2010 review based on a series of systematic reviews being produced by a research group in the UK. The first such update concerned the effect of neutering and age at neutering on urinary incontinence in female dogs. This update concerns one of the most important reasons to considering neutering female dogs: mammary cancer.
Beauvais W, Cardwell JM, Brodbelt DC. The effect of neutering on the risk of mammary tumours in dogs–a systematic review.J Small Anim Pract. 2012 Jun;53(6):314-22.
In my 2010 review, I found evidence suggesting that mammary cancer is very common in intact female dogs, that it is malignant about half the time, and that neutering dramatically reduces the risk of this disease. I also found evidence suggesting that neutering is most effective in preventing mammary cancer if performed before the first heat, and that the benefit disappears after the third estrus cycle. This is a major reason why I still recommend neutering young female dogs.
The UK group found only 4 research reports that addressed the question of the impact of neutering on mammary cancer risk and that met the minimum quality criteria for review. One of these also addressed the issue of whether age at neutering was associated with mammary cancer risk. They reported the following results:
- All studies had significant methodological flaws, and none provided information that could be confidently generalized to the general dog population.
- One study found a strong protective effect of neutering. Another found some protective effects but these were inconsistent. The remaining two studies found no association between neutering and mammary cancer risk.
- The one study that evaluate age at neutering found a significant decrease in the protective benefits of neutering with each of the first three heats, and no benefit after the third heat.
Once again, the general conclusions are consistent with the current consensus, that neutering young female dogs likely has a meaningful beneficial effect on reducing the risk of mammary cancer later in life. However, the existing evidence is extremely weak, and this conclusion has to be viewed as tentative. While current evidence does not justify a change in the common practice of recommending neutering to prevent mammary cancer, we cannot view our current conclusion with any great confidence, and we desperately need more and better quality research to determine if the current evidence is correct or if a significant change in our recommendations is called for.
As always, the decision to neuter must be based on the entirety of the known risks and benefits, with a clear understanding of the uncertainty involved, as well as on the unique circumstances of each individual. The value of this review is not so much in answering our questions about the subject, which it cannot do, but in making it clear what the strengths and weaknesses are in the evidence behind our current recommendations and in guiding us in the development of better evidence that will permit more confident conclusions.