Cancer

A documented outcome of spay/neuter is an increased incidence of certain types of cancer. It is consistent with the theory that spay/neuter results in a compromised immune system which is not able to effectively eliminate cancers at the cellular stage (otherwise known as cancer immunosurveillance). There are six types of cancer that have been researched with respect to spay/neuter:

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, hemangioma 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 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.
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.
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