Cancer

HAHD_Adult_Death_Causes

According to the Journal of the American Veterinary Medical Association (JAVMA), adult dogs die overwhelmingly of cancer. As the graph on the right (from the actual study reviewed in the JAVMA157) shows, cancer deaths in adult dogs occur at more than three times the rate of the next most frequent cause of death, i.e., trauma.

The American Animal Hospital Association (AAHA) advises the public the six most common cancers in dogs are lymphoma, hemangiosarcoma, mast cell tumor, osteosarcoma, melanoma and mammary cancer. Research from many sources shows lymphoma, hemangiosarcoma, mast cell tumor and osteosarcoma occur at significantly higher rates in spayed or neutered dogs.4,13,14,25,60,61,66,86,87,109,151,153,155,156  The science shows that spay/neuter acts as an endocrine disruptor and as such, is a major contributory agent in the development of cancer in dogs. (See our discussion of endocrine disruptors on the Hormones and the Endocrine System page of this website.)

There are many theories regarding how humans and dogs develop cancer. Most likely it’s a combination of factors which lead to the development of cancer. Three factors often considered are:

  1. genetic predisposition
  2. immune suppression
  3. exposure to environmental triggers

Some environmental triggers are known carcinogens. However, many environmental triggers are endocrine disruptors. Certainly, when one has more than one of these risk factors in play, the chances for the development of cancer are much greater.

In dogs that have been spayed or neutered, the sex hormones are disrupted. This is the exact method of action whereby endocrine disruptors in the environment (e.g., DDT, dioxins, PCBs, BPA) can cause immune suppression and the development of cancer in dogs. Therefore, once you have spayed/neutered your dog, you have essentially exposed them to the greatest endocrine disruptor of all, i.e., spay/neuter. This is not only an explanation for the increased rates of deadly cancer among spayed or neutered dogs, it is a critically important fact that the veterinary community has either overlooked or fails to acknowledge.

If a specific breed, e.g., Golden Retrievers, actually does have a genetic predisposition to cancer, then spay/neuter will greatly increase the number of Golden Retrievers diagnosed with cancer. This is because you now have two risk factors in play, (possibly three if the dog is also exposed to endocrine disrupting chemicals in its food, water, bedding or where the dog plays). We will discuss this in greater detail later on this page.

The following table will briefly review the six cancers whose relationship to spay/neuter has been researched (including the four most prevalent, deadly cancers).

Cancer Type
Description
Osteosarcoma
(bone cancer)

Research has shown a correlation between spay/neuter and the occurrence of osteosarcoma (bone cancer) in dogs. Most primary bone tumors in dogs are malignant, and approximately 85% are osteosarcomas. Osteosarcomas are highly aggressive tumors, characterized by local invasion/destruction and distant metastasis (spread to other organs). Osteosarcoma commonly affects the appendicular skeleton (limbs) of large to giant breed dogs, but can also occur in the axial skeleton (skull, ribs, vertebrae, pelvis), which is a more common primary site in smaller dogs. Spayed and neutered dogs are twice as likely 13 to develop osteosarcoma.
Hemangiosarcoma (HSA)
(found in the heart and spleen)

In an age-matched case controlled study 24, spayed females were found to have a 2.2 times risk of splenic HSA compared to intact females. A retrospective study of cardiac HSA risk 25 factors found a more than 5 times greater risk in spayed female dogs compared to intact female dogs and a 1.6 times higher risk in neutered male dogs compared to intact male dogs. The authors suggest a protective effect of sex hormones against HSA, especially in females.
Lymphosarcoma (LSA)
(Lymphoma)
LSA is the third most common cancer diagnosed in dogs. It is a cancer of lymphocytes (a type of blood cell) and lymphoid tissues. Lymphoid tissue is normally present in many places in the body including lymph nodes, spleen, liver, gastrointestinal tract and bone marrow. Early neutered males were diagnosed with LSA 3 times more 60 often than intact males. Neutering females 61 through 8 years of age increases the risk of acquiring at least one of three cancers (lymphosarcoma, hemangiosarcoma and mast cell tumor) to a level 3–4 times that of leaving the female dog intact.
Mast Cell Tumor (MCT)

Mast cells are cells that occur in the skin and other tissues, like the intestines and respiratory tract. They are also an integral part of the immune system. Although it is not clearly understood, studies indicate that estrogen and progesterone may influence mast cell tumors.

In 2011 a study 87 found spayed females had a 4 times greater risk of MCT development than intact females. Spayed females in some specific breeds had even higher risks.

2013 UCD study 60 found no cases of MCT in intact females, but 6% in late spayed females. Spaying females through 8 years of age increases the risk of acquiring at least one of three cancers 61 (lymphosarcoma, hemangioma and mast cell tumor) to a level 3–4 times that of leaving the female dog intact.

Mammary Cancer (MC)
The most frequently mentioned advantage 99 of early spaying of female dogs is protection against mammary (breast) cancer. However, a recent meta-analysis 72 of published studies on spaying females and MC found that the evidence linking spaying to a reduced risk of MC is weak, and not a sound basis for firm recommendations. We suggest dog guardians approach breast cancer as we do in women, i.e., with regular screenings by vets and periodic manual breast exams by dog guardians.
Prostate Cancer (PC)
Unlike humans, in dogs prostate cancer is potentiated by the removal of testosterone. An extensive study in 2002 5 found that prostate cancer occurred in neutered males 4 times as frequently as in intact males.

GENETIC PREDISPOSITION

Some breeds, e.g., Golden Retrievers,  have a highly increased incidence of cancer. This leads researchers to theorize Golden Retrievers have a genetic predisposition to develop cancer. In fact, over 60% of Golden Retrievers die of cancer. Because of the very large percentage of cancer deaths among Golden Retrievers, in 2012 the Morris Animal Foundation (MAF) launched the Golden Retriever Lifetime Study (GRLS).  The cancers to be studied are the same four cancers shown in studies to be increased by spay/neuter:

  1. osteosarcoma
  2. hemangiosarcoma
  3. lymphoma
  4. mast cell tumor

These four types of cancer cause approximately 80 percent of the cancer deaths in Golden Retriever dogs. The GRLS will look for genetic links to cancer as well as evaluate environmental factors such as diet, water source, exposure to pesticides, etc. Participating dogs can be intact, spayed or neutered when entering the study. There is no restriction should a guardian decide to spay or neuter their dog at any time within the study.

We applaud the desire to study canine cancer in the hopes of reducing its incidence, but it is hard to understand the reasoning behind the design of the GRLS study. The four cancers under investigation in the GRLS are the very same cancers already known to be dramatically increased by spay/neuter.4,13,14,25,60,61,66,86,87,109,151,153,155,156  By including spayed/neutered dogs in the GRLS it seriously complicates the ability to draw conclusions about what exactly influenced the development of cancer. Was it an aberrant gene/gene sequence, spay/neuter as an endocrine disruptor or a different environmental endocrine disruptor? This dilemma is acknowledged by the National Institute of Environmental Health Sciences as they state, “The difficulty of assessing public health effects is increased by the fact that people are typically exposed to multiple endocrine disruptors simultaneously.” Had the GRLS restricted its participants to intact dogs only, you could more effectively calculate the influence of genetics and environmental endocrine disruptors upon the risk for cancer in Golden Retrievers.

We find it hard to understand why guardians of Golden Retrievers are not advised to avoid spay/neuter because of the elevated cancer rate. If the current studies are valid, this action alone would produce a significant decrease in cancer incidence in Golden Retrievers.

SEX HORMONES AS A PROTECTOR AGAINST CANCER

To further emphasize the need to follow the research, we see that female dogs of all breeds have 3-4 times the risk of developing lymphoma, hemangiosarcoma, or mast cell tumor if they have been spayed. In some breeds, the risk is even greater.24,25,60,61,66,87,109  The research indicates there is an inherent protective effect of estrogens against development of these cancers in intact dogs, but the exact mechanism is as yet unknown.155,156

With respect to prostate cancer in male dogs, there appears to be an inherent protective effect of testosterone. Neutered dogs have a 4 times greater incidence of prostate cancer.5 There are also some indications of a protective effect of testosterone with respect to hemangiosarcoma.25

PREDISPOSITION BY BREED CHARACTERISTIC

Some dogs by virtue of their breed or breed characteristics have a predisposition to develop a particular type of cancer. In fact, dogs over 80 pounds have been shown to be at least sixty times more likely to develop an osteosarcoma than dogs weighing less than 75 pounds. This would include large breeds such as Labradors, Golden Retrievers, German Shepherds, Rottweilers, Dobermans, Weimeraners, Greyhounds and Boxers. Other breeds commonly affected include Great Danes, Saint Bernards, Great Pyrenees, Newfoundlands, Bernese Mountain Dogs, and Irish Wolfhounds.152,153,154  It has been well documented that spayed/neutered dogs are significantly more likely to develop osteosarcoma than dogs that are sexually intact.

Our point is that if a dog, by virtue of its breed/breed characteristics, has a predisposition to develop a certain type of cancer (in this case osteosarcoma) and if spay or neuter has been shown to raise the incidence of this specific cancer, then you should be looking to find a method of sterilization other than spay or neuter in order to minimize the risk of cancer.

CONCLUSIONS

The evidence is very strong that spay or neuter is responsible for an increased incidence of cancer in dogs. Recognizing spay and neuter as endocrine disruptors of the highest magnitude serves to provide an explanation for the observed facts. The veterinary community is behind the curve here, as they are not making any tangible recommendations to address prevention of cancer in the canine population. It appears their investment in spay/neuter has trumped their common sense. We can only suggest those who are currently guardians of spayed or neutered pets think seriously about hormone testing and hormone balancing if hormone levels are abnormal. It is possible restoring hormones to physiologic levels will help prevent your dog from being another victim of cancer. Certainly if you are getting a new dog, and want to prevent your dog from breeding, you can opt for a hormone-sparing method of sterilization. You will enjoy life with a healthier and happier dog if you do so.

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