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Determining the Best Age at Which to Spay or Neuter a Dog: An Evidenced-Based Analysis

When Should You Get Your Dog Spayed or Neutered?


This article is courtesy of AKC Canine Health Foundation.


By Margaret Root-Kustritz, DVM, PhD

University of Minnesota


In many parts of the world, due to cultural or economic prohibitions, bitches and dogs are not spayed or castrated unless they have reproductive tract disease. However, in the United States, virtually all bitches and dogs are rendered sterile by surgery at some point in their life. This better allows for reproduction control in animals no longer capable of or not considered desirable for breeding, and eliminates behaviors and physical changes related to presence of reproductive hormones that dog owners find objectionable. The surgeries most commonly performed are ovariohysterectomy (removal of the uterus and both ovaries), commonly called spaying, and castration (removal of both testes and the associated epididymes). Castration is commonly also called neutering, although that term most correctly can be used for surgery of either gender. Collectively, these surgeries can be referred to as gonadectomy, removal of the gonads or reproductive organs.


Removal of the ovaries eliminates secretion of the hormones estrogen and progesterone. Removal of the testes eliminates secretion of the hormone testosterone. Elimination of these hormones obviously leads to decreases in behaviors and physical changes associated with their secretion, such as heat behavior, swelling of the vulva, and estrous bleeding in bitches, and mounting and roaming in dogs. However, reproductive hormones have effects on other tissues in the body and removal of those hormones may inadvertently impact those systems negatively. Other, less obvious, hormone changes also occur after gonadectomy, including persistent elevation in hormones that control the secretion of estrogen, progesterone, and testosterone. Whether these other hormone changes affect other systems positively or negatively often is unclear.


This paper is a review of what has been demonstrated in the veterinary literature regarding effect of gonadectomy on the animal as a whole. This discussion does not address the societal problem of pet overpopulation. The author feels that animals with no owner or guardian should be spayed or castrated before adoption into a new home as one of many initiatives necessary to decrease the number of dogs euthanized in the United States annually. This discussion instead refers to dogs with responsible owners or guardians who maintain dogs as household pets, do not allow the animals to roam free, and provide the animals with regular veterinary care.


Evidence in this context is defined as credible information from peer-reviewed research. Studies involving more dogs are more valuable than reports of single cases. Multiple studies documenting a given phenomenon are more valuable than single papers. Incidence in this context is reported as a percent; this is the number of affected animals out of a random sample of 100. In veterinary medicine, any condition with an incidence greater than 1% is considered common. Readers are encouraged to carefully read all manuscripts of interest and to ask their veterinarian for clarification if needed. This paper is condensed from a more detailed, extensively referenced manuscript that may be available through your veterinarian (Root Kustritz MV. Determining the optimal age for gonadectomy of dogs and cats. Journal of the American Veterinary Medical Association 2007;231(11):1665-1675).


Why do we perform spay or castration at 6 months of age?


Most veterinarians in the United States recommend bitches and dogs be spayed or castrated between 6 and 9 months of age. This is not based in science; no one has performed a large-scale study in which bitches and dogs underwent gonadectomy at various ages and were tracked throughout life to determine what abnormalities developed relative to age at gonadectomy. It is thought that the current age recommendation arose after the World War II, when increasing affluence of American families first permitted them to treat animals as household pets and were, therefore, more interested in controlling manifestations of reproductive hormone secretion and very interested in making sure the animal survived surgery. Anesthetic and surgical techniques available at that time necessitated the animal be at least 6 months of age.


With current anesthetic agents, anesthetic monitoring equipment, and surgical techniques, it has been demonstrated in multiple studies that bitches and dogs can safely undergo gonadectomy when as young as 6 to 8 weeks of age. Surgical complication rate does not vary between groups undergoing surgery when very young compared to those undergoing surgery at the more traditional age, with overall postoperative complication rate reported as 6.1%. The vast majority of these post surgical complications are transient and do not require veterinary care.


Effects of gonadectomy on behavior


Behaviors that are most likely to be affected by gonadectomy are those that are sexually dimorphic (seen primarily in one gender). Examples of sexually dimorphic behaviors include flagging in bitches, and mounting and urine marking in dogs. Incidence of sexually dimorphic behaviors decreases after gonadectomy in bitches and dogs, with the decrease in incidence not correlated with length of time the animal has shown the behavior prior to gonadectomy.


Those behaviors that are not sexually dimorphic, including most forms of aggression, are not decreased in incidence by gonadectomy. One behavioral consequence of spaying that has been documented in several studies is an increase in reactivity towards humans with unfamiliar dogs and increased aggression toward family members. This may be hormonally related; there may also be a breed predisposition.


There is no evidence documenting a decline in trainability of working female or male dogs after spay or castration. One study documented an increase in development of senile behaviors after gonadectomy in male dogs. However, that study had very few dogs in the intact male group and other studies, looking directly at changes in brain tissue, are not supportive of that finding.


Comments  4

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  • Disputing a statement
    10/27/2013 11:04am

    I simply HAVE to dispute a statement made in this article. Where in the world did you find data to support saying "However, in the United States, virtually all bitches and dogs are rendered sterile by surgery at some point in their life."??? That's preposterous!

  • 02/25/2014 08:05pm

    Excellent observation. Here is the South, MOST dogs and cats are NOT EVER surgically altered; there are too many antiquated cultural beliefs and mistrust of real science and medicine as well as the fact a large or majority of pets live outdoors and the owners are not recognizing many or the behaviors, and some have a close identification with their pet's sexuality and their own. Regardless of the reason, I have no earthly idea where the author came up with the idea that most or virtually all pets in the USA are sterilized at some point. Were that the case, we would not be killing over 4 million adoptable pets each year for lack of available homes, and dog bites, HBC cases, dog on dog trauma, cat bites abscesses, FIV, and countless other daily avoidable negative interactions would be drastically reduced. And how many small and medium dogs can wait till 6 months of age to spay and STILL "beat" the first heat cycle?? Large breeds, maybe so, but most smaller ones will start a first estrus cycle at 5-6 months, and many veterinarians (and owners) hesitate to spay a female while she is in active estrus, anyway. This makes me wonder about the veracity of the rest of the article.

  • 02/27/2014 09:57am

    Pat, when I received notice of your comment, it reminded me of this post. Since it was reprinted from another source, I figured the author likely never saw our comments and felt I just HAD to follow up. I tracked the doctor down at the U of MN and emailed her. She replied:
    "Thank you for bringing this to my attention. Research has been done since this was written showing variation in spay and castration rates across the country. It is less than I stated and more than was stated by those responding.
    Dr. Peggy Root"
    How's THAT for clearing things up statistically? hahaha

  • Where is evidence?
    10/01/2014 01:18pm

    Where is the evidence that spaying a dog reduces her risk of cancer? Everyone throws numbers around but no one provides a reference study. It's almost like an urban legend that gets perpetuated. I found this metastudy that looked at all other related studies and it says that when you remove the studies that had a bias, there is little if any correlation:


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