Orders to “Spay and Neuter Your Pets” have been a part of the American cultural landscape for decades, popping up everywhere from billboards to game shows. The idea that neutering is right for pets individually and collectively is well accepted. But recent research suggests surgical sterilization brings a complicated blend of long-term individual health risks as well as benefits.  

Two studies published this year within a 2-month period highlight the paradox. The first study, conducted by a team at the University of California, Davis, found a greater occurrence of hip dysplasia, cranial cruciate ligament tears and two types of cancer in sterilized golden retrievers compared with their intact counterparts. The research found the health risks generally were greater for dogs that were younger than 1 year when sterilized.  

The second study, conducted by researchers from the University of Georgia, Athens (UGA), found a strong association between sterilization and greater longevity.

The coincidental timing of these studies have fanned an already contentious discussion in veterinary medicine. How does pet sterilization relate to responsible pet care? What are the medical benefits? At what age, if ever, should dogs or cats be neutered? 

At the Veterinary Information Network (VIN), an online community for the profession, the UC Davis study prompted an ongoing message-board conversation that ranges from practitioner preferences on timing of neutering and ways to approach the topic with clients to critical analyses of the research. 

Dr. Shiri Hoshen, a veterinarian in New Mexico, commented wryly, “It’s not surprising that surgeries that can have some profound effects on dogs might be found to have … profound effects on dogs.”  

She elaborated: “Neutering can lead to dramatic changes in metabolism, muscle mass and hair coat — these are the three obvious things that I see with some regularity. A surgery that has such broad effects on an animal might have other effects.”  

In an interview, Hoshen observed that the issue of neutering is complex, sitting “at the crossroads of science, dogma and culture.”

The question of whether to neuter and if so, when, not only is medically important but strikes at social values, as well. Especially in the United States, where surgery to sterilize dogs and cats is considered integral to controlling pet overpopulation, the perception among veterinarians and the public alike is that responsible owners get their pets “fixed.”  

Illustrating how deeply feelings run on the issue, one practitioner spoke to the VIN News Service only on condition of anonymity for fear of jeopardizing her business relationship with local animal shelters.  

Referring to the standard rule of shelters that their charges be neutered before adoption, sometimes as young as 8 weeks old, the veterinarian said: “I predict that pediatric spay/neuter is going to become a conundrum for animal shelters and rescues. As more people become aware of the potential problems associated with pediatric gonadectomies (removal of reproductive organs), more people will be looking for their animals elsewhere.”  

Surgery to sterilize a female dog or cat, commonly referred to as spaying, typically involves removing the ovaries and uterus (“ovariohysterectomy” in the veterinary lexicon). In male dogs and cats, sterilization entails removing the testicles and associated glands. The procedure is called castration or, more commonly, neutering. However, the term “neutering” may refer to spaying, castration or both.  

Besides stopping pets from reproducing, neutering widely is considered helpful in eliminating or tempering undesirable behaviors such as roaming; spraying of urine or excessive marking of territory with urine; and aggression, especially in male animals. Owners of sterilized female pets needn’t worry about their animals going into heat, with the attendant bloody discharge, restlessness and attraction of suitors.  

The concept that neutering animals, especially before they reach sexual maturity, has implications for their health is not new. A 2007 review paper published in the Journal of the American Veterinary Medical Association cites more than 180 research papers dating back to the mid-20th century. Investigators have looked at the societal impacts of neutering, behavioral effects and effects on the incidence of a number of diseases, including cancers, endocrine disorders and bone problems.

Among the researchers to examine the medical effects of neutering on dogs is Dr. David J. Waters, a comparative oncologist at the Purdue University College of Veterinary Medicine, director of the Center for Exceptional Longevity Studies at the Gerald P. Murphy Cancer Foundation in Indiana and leader of a nationwide research study on aging and healthy longevity in pet dogs. 

In a 2009 study, Waters and colleagues studied the impact of sex and sterilization on longevity of Rottweilers. They found that intact female Rottweilers were more likely to achieve “exceptional longevity” than their male counterparts — just as women have a greater chance of living to age 100 than men. But Rottweilers whose ovaries were removed before age 4 lost that female survival advantage. Though the biological mechanism remains unknown, this study indicated a significant relationship between retaining ovaries and longer life spans.  

Despite the long history of pet neutering and the extensive body of research, much still remains to be learned about how removal of the gonads impacts animals in the long term.

For reasons that aren’t clear, the study published this February in the online peer-reviewed journal Public Library of Science One, “Neutering Dogs: Effects on Joint Disorders and Cancers in Golden Retrievers,” by a UC Davis team, captured wide attention among veterinarians and the public, meriting articles on a number of pet and veterinary medicine websites. 

Dr. Benjamin Hart, a professor emeritus in the Department of Anatomy, Physiology and Cell Biology at the UC Davis School of Veterinary Medicine, and colleagues analyzed the medical records of 759 golden retrievers seen at the UC Davis Veterinary Medical Teaching Hospital during a 10-year period.  

They discovered that neutering increased the chances of a dog developing certain bone disorders or cancers.  

The researchers analyzed not only whether a dog was intact or neutered, but the age at which a dog was neutered. The dogs were classified as “intact,” “neutered early” or “neutered late.” Early neuters were defined as those done before age 12 months; late neuters were done after age 12 months. The analysis found:

• Early-neutered males had double the incidence of hip dysplasia compared with intact males  
• Early-neutered males and females had 5 percent and 8 percent incidences, respectively, of cranial cruciate ligament tears or ruptures compared with no such cases in intact dogs  
• Early-neutered males had triple the incidence of lymphosarcoma compared with intact males  
• Late-neutered females had quadruple the incidence of hemangiosarcoma compared with intact and early-neutered females, and a 6 percent incidence of mast cell tumors, a cancer not found in intact females 

Lymphosarcoma is a cancer of white blood cells called lymphocytes. Hemangiosarcoma is a cancer of the cells lining the blood vessels. Why sterilization should be associated with these cancers is not known; one hypothesis is that reproductive hormones confer a protective effect. 

Commenting on the attention drawn by the study by Hart and team, Waters, the canine longevity researcher, said, “Dr. Hart is challenging folks to think differently. And I believe he is achieving his objective.”  

In an interview, Hart said, “Clearly there needs to be a shift in the discussion. There is a huge, powerful lobby that makes early spay/neuter politically correct.”  

He went on: “Neutering is the most frequently performed operation in veterinary medicine. It’s done early, yet we know this is the time when we are interrupting  the physiological role of gonadal hormones. As we learn the consequences, I think we owe it to our clients to give them that information and the dignity of the decision. Let’s put it on the table and let them decide.”  

Hart said he would like to see pet owners given a greater sense of autonomy in the matter. “In human medicine, we give people the dignity of a role in their own health-care decisions,” he said. “In some (other) areas of veterinary medicine, we give options.”  

Hart added that a number of prior studies confirm his group’s results. “Every disease where we found an effect of neutering has been shown in another publication to (be affected by neutering).”  

But in April, two months after the Hart team’s paper was published, another study appeared in the same scientific journal with what appears at first glance to be a contradictory message.  

In the study titled “Reproductive Capability is Associated with Lifespan and Cause of Death in Companion Dogs,” investigators at the University of Georgia, Athens, (UGA) examined the relationship between sterilization and life span in dogs. Their purpose was to explore the cost of reproduction on longevity of a species. The routine sterilization of North American dogs made them a good model for the subject. As the authors wrote, “By electing whether or not to sterilize their dogs, dog owners have inadvertently carried out a large-scale epidemiologic experiment on the consequences of effectively eliminating reproductive capability.”  

The team drew from a database of abstracted records on patients in veterinary medical teaching hospitals across the United States and Canada to examine the causes of death in more than 40,000 pet dogs that died in those hospitals between 1984 and 2004.  

The researchers found sterilization strongly associated with a longer life span and a decreased risk of death from infectious disease and trauma.

But here’s where things get complicated. The UGA researchers also found that, as in the Hart study, sterilization appeared to increase the risk of death from cancer. 

Looking at previous studies that had demonstrated a link between sterilization and certain types of cancer, the UGA researchers had questioned whether those results were confounded by age. Since cancer is a disease that develops over time, could the connection between cancer and sterilization be the result of neutered dogs living longer?

In their study, the UGA team controlled for the impact of age by dividing their subjects into nine age groups. Causes of death were examined for neutered and intact dogs in each group. Removing the effect of age, they found that, indeed, “sterilization increased risk of death due to neoplasia.”

Causes of death for each animal were grouped into categories such as cancer, trauma and infectious disease but the nature of the database the researchers used did not enable them to examine circumstances around each death.

Dr. Kate Creevy, an assistant professor of small animal medicine and co-author of the paper, described the database as a “fairly crude but large measure.” In other words, it was powerful in providing a large number of records, but lacked details. For instance, under the category of trauma, investigators might see that a patient was diagnosed with a femur fracture but have no information on how the bone break occurred.  

This means certain factors in the study are difficult to tease apart. Veterinarians discussing the study on a VIN message board wondered, for example, why intact animals would be more likely to die of infectious disease. Could it be because neutered animals tend to belong to owners who are more likely to safeguard their pets’ health and make sure they are vaccinated? Or is it that neutered animals are less likely to roam and therefore be exposed to pathogens?  

Creevy said she believes the findings can be taken at face value. “I am very satisfied that our data show that animals that are sterilized live longer,” she said. “While there is the possibility that that is because their owners care for them better, that (possibility) is less likely since all of the dogs were seen in teaching hospitals.”  

Considering the public-policy implications of studies on pet sterilization, Creevy said: “None of these findings exists in a vacuum. The decision to sterilize may remain somewhat independent of these findings. People (simply) may not want intact animals. I think there’s more than one topic there. In this country, the overwhelming pressure to argue for the value of sterilizing is the pet overpopulation problem – it is out of control, it is a catastrophe.”  

The spay/neuter discussion long has been framed in terms of pet population control. For some practitioners, reducing numbers of unwanted cats and dogs is a top priority.  

Dr. Brenda Griffin is one. An adjunct associate professor of shelter medicine at the University of Florida, Griffin said that in the 1980s, an estimated 17 million cats and dogs were killed annually in shelters in the United States. Now, between 4 million and 6 million enter shelters annually, and of these, approximately 3 million are euthanized.

She attributes the significant decrease in the shelter population to the success of spay/neuter programs in communities with intensive programs.  

In a joint interview, Griffin and Dr. Boyd Harrell, a veterinarian, business consultant and advocate of early neutering, expressed concerns with the size and design of the Hart study. “I can’t draw conclusions from this study because of the small sample size,” Griffin said, meaning that studying 759 dogs is not enough.

Moreover, she said, golden retrievers, the breed studied, are known to be predisposed to the medical conditions the researchers examined — hip displasia, cranial cruciate ligament tears, lymphosarcoma and hemangiosarcoma. “We must know there are a number of factors involved,” Griffin said.

Harrell pointed out that different groups will have different reasons for whether and when to neuter. “Animal control, shelter, private veterinarian, breeding operation … Each group looks at it from their own point of view. Without having a dialogue, all we get is groups that are separated.”  

On one point, all sides agree: Veterinarians and owners should discuss the pros and cons of sterilization. “I think there should always be a conversation between the owner and the veterinarian on the risk-(to)-benefit ratio,” Griffin said.  

Although veterinarians in North America commonly recommend neutering sometime between the ages of 6 months and 9 months, Griffin acknowledged, “We do not know the ideal age to spay and neuter.”  

In fact, she said, much remains unknown. “We have to weigh the risks and benefits in context. It’s difficult; we have to look at the spectrum of the quality of evidence,” she said. “The truth is that it takes years and years – multiple epidemiologic studies — to prove a causal relationship.”  

Regardless of the differences in their findings, the need for further investigation is one that unites the researchers. Creevy would like to see further study into the optimal age for neuter surgeries. She hopes for a solution that will “maximize the social benefit and minimize medical risks.”

Pointing to the paper by Waters, Creevy acknowledged that some evidence suggests “there is a longevity advantage to remaining intact for a period of time.”  

Hart believes timing is important. “Getting past that first year to 18 months in the male is critical…” he said “In the female, the picture is more complex because the cancers are increased in risk by both early and late spaying; spaying females between 16 months and 18 months of age could be the best time in the golden (retriever). Importantly, all of these complications from neutering will vary as a function of breed.” 

Hart said his research group is examining the effects of neutering on other breeds. Among the things he hopes to learn is the mechanism behind the effects of neutering on cancer development, which may help in preventing the cancers overall. “We need the breed-specific data – we have to be able to point to a physiologic mechanism,” he said. “Sex hormones play roles in growth across the board – not just in bone.”  

Hoshen, the private practitioner in New Mexico, summed up the situation in this VIN message board post: “There is always more research to be done, and it can always be done better. We are never going to have bulletproof answers to our complex (medical or other) questions... I present information to owners, and it is for them to decide. There is not a single right answer for everybody.”