Spay/neuter and the association with cancer in dogs: part three

“Reproduction is a risky affair.”

“Reproduction is a risky affair” is the attention-getting opening line in one of the studies I’ll review today (Hoffman, 2013).

But before we go through the new studies, let’s review my previous articles on this topic. They have been generating some controversy, and with good reason — this is a touchy, political subject!

In my first article, we discussed that sex hormones can promote some cancers (mammary and perianal adenomas), and that early spay/neuter surgeries effectively remove the sex hormones, and therefore can help prevent these cancers. We also talked about why a spay should be considered at time of mammary tumor removal, as dogs that are spayed within 2 years of the mammary tumor development had a survival advantage. Finally, we revealed the results of a recent systematic review of the published work on neutering and mammary tumors, which revealed the actual evidence that neutering reduces the risk of mammary tumors to be weak.

In my second article, I explored the idea that sex hormones may be PROTECTIVE again certain cancers, including very aggressive cancers such as osteosarcoma, bladder transitional cell carcinoma, prostate cancer, lymphoma, and heart tumors.

Of course, there are many factors that can influence cancer development in the body, as Dr. Dressler and I make very clear in our book The Dog Cancer Survival Guide. The reason I’m going into such detail in this series about the effects sex hormones do (and don’t) have on cancer development is because it’s important to understand each factor fully. There are some things we, as dog lovers, cannot control, and there are others that we can. It’s important to be fully informed as we fight the number one killer of dogs: cancer.

So let’s look at the latest study, one I’ve mentioned but haven’t yet fully discussed: the 2013 publication by Torres de la Riva et al called Neutering Dogs: Effects on Joint Disorders and Cancers in Golden Retrievers.

The Effects of Neutering Goldens on Cancer and Hip Dysplasia

The Torres de la Riva study looked at veterinary hospital records and reviewed those of 759 Golden Retrievers to see what health conditions are associated with spay/neuter surgeries.

Patients were classified as intact, neutered early (< 12 months old) or neutered late (>12 months old). Let’s look at the results:

• Hip dysplasia (HD)
  • Of early-neutered males, 10% were diagnosed with HD
  • This was two times (2X) the occurrence than intact males
• Cranial cruciate ligament tear (CCL)
  • NO cases of CCL in intact males or intact females
  • 5% early-neutered males
  • 8% early-neutered females
• Lymphoma (LSA)
  • Almost 10 % of early- neutered males diagnosed with LSA
    • This is three times (3X) more than intact males
  • No LSA cases in late-neutered males
  • No effect in females
• Mast cell tumor (MCT)
  • No cases of MCT in intact females
  • 6% late-neutered females
  • No effect in males
• Hemangiosarcoma (HSA)
  • 8 % late-neutered females
  • Four times (4X) times more than intact females and early-neutered females
  • No effect in males

According to this study, spay and neutering is associated with disease development in Golden Retrievers. Here’s a quote:

“For all five diseases analyzed in the present study, the disease rates in males and/or females were significantly increased when neutering was performed early and/or late. When a disease occurred in intact dogs, the occurrence was typically one-fourth to one-half that of early- and/or late-neutered dogs. When no intact dogs were diagnosed with a disease, such as with CCL in both sexes and MCT in females, the occurrence in early- and/or late-neutered dogs ranged between 4 and 8 percent of the sample.”

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Read more about all the things that put our dogs at risk in the book. 🙁

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While it is tempting to apply the results of this study to other breeds, this study was based on one breed from a single hospital database (UC Davis). We do not know if the effects of spay/neutering will be true for other breeds or all dogs. Perhaps different cancers and different joint diseases will be affected in other breeds or dogs. But since Goldens are one of the most popular American breeds and a common service dog, this is still important info even if it applies only to Goldens. (I see a lot of Golden in my Oncology Service at work.)

Still, to me, as a veterinary oncologist, this study highlights that we have much to learn about how spay/neuter affects cancer in dogs. The decision to spay or neuter and the timing of that surgery is much more complicated than we’ve thought it was.

This is especially enlightening when we realize that these surgeries are far less common in European countries than they are here. As Dr. Dressler and I remind ourselves and our clients in our book, there is more than one way to practice medicine.

I think it’s time to start having more discussions about spaying and neutering than we currently do. Especially when I look at the details provided by a second study that came out this year, titled Reproductive Capability Is Associated with Lifespan and Cause of Death in Companion Dogs (Hoffman, 2013). This study out if the University of Georgia and looked at a whopping 40,139 case from the Veterinary Medical Database (VMDB), and it reveals even more complications!

Spay/Neuter Affects How Long Dogs Live

In the Hoffman study, it was revealed that sterilization (spay/neuter) significantly affected survival of the 40,139 cases under review.

The average age of death was 7.9 years if intact, and 9.4 years if neutered.

So here it looks like sterilized dogs had an increased life expectancy (males 13.8%, females 26.3%).

Sterilized dogs were LESS likely to die of infectious disease and trauma, vascular disease, and degenerative disease.

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BUT sterilized dogs were MORE likely to die of cancer and immune-mediated disease.

Why are sterilized dogs LESS likely to die of infectious disease? One thought is that the female sex hormones progesterone and estrogen can be immunosuppressive (they can suppress the immune system). Does avoiding infection lead to longer lifespans?

Sterilization increased the risk of death due to cancer, but did not increase risk for all specific kinds of cancer.

In this study, spayed females were unlikely to develop mammary cancer.  But there was an increased risk to develop transitional cell carcinoma, osteosarcoma, lymphoma, and mast cell tumors. There was no effect for squamous cell carcinoma, melanoma, and prostate cancer.

Unlike the Davis study, this study did not look at age of spay/neuter. In addition, all these cases were ones referred to teaching hospitals, which may not be representative of the general dog population. This is highlighted by the shorter overall lifespan in these dogs than seen private practice.

Does socioeconomic status come into play? Owners that cannot afford spay/neuter may also lack resources to provide medical care for disease later in life. So are dogs owned by people who can afford to spay/netuer associated with better medical care, so they appear to live longer?

We now have new evidence that demonstrates that dogs who are spayed/neutered are at increased risk of dying of some cancers. And we see that at least in Goldens, the cancer rates in males and/or females were significantly increased when neutering was performed early and/or late.  We must continue to examine when dogs are dying and WHY.

Unlike in Europe, most dogs in the United States are spayed/neutered before one year of age, and often without much discussion on the part of the veterinarian and the dog owner. It is time to start having more discussions about the very real pros and cons of spay and neuter, and the timing of these surgeries.

Live longer, live well,

Dr Sue

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Other Articles in This Series

Spay/neuter and the association with cancer in dogs: part one

Spay/neuter and the association with cancer in dogs: part two