Benefits and Risks of Neutering, An Evidence Update: Effects of Neutering on Longevity and Cause of Death in Dogs

Another new study has recently been published addressing the complex issue of the risks and benefits of neutering dogs. This report certainly will not answer all the outstanding questions or quiet the debate about when and if to neuter dogs, but it does add some useful information.

Hoffman JM, Creevy KE, Promislow DEL (2013) Reproductive Capability Is Associated with Lifespan and Cause of Death in Companion Dogs. PLoS ONE 8(4): e61082. doi:10.1371/journal.pone.0061082

The Study
The study was a retrospective analysis of records from the Veterinary Medical Database (VMDB). This is a collection of medical records from veterinary school teaching hospitals going back to 1964. The authors pulled all records in this database for dogs seen at participating hopsitals between 1984 and 2004, a total of over 80,000 records. They excluded individuals with missing information about relevant variables (such as neuter status, age at death, and so on). The final analyses involved over 40,000 records. Slightly over half of the subjects were neutered, and slight less than half were intact at the time of death (56.4% and 43.6% respectively).

Results
Life Span
Overall, males lived about 14% longer when neutered, and females lived about 26% longer when neutered. From the Kaplan-Meier plots presented, it appears differences were less after about 12 years of age. This is interesting in light of the reports of another study, on Rottweilers, that suggested exceptional longevity was more likely in animals not neutered or neutered later in life. It may be that those individuals who achieve exceptional longevity are different from those who do not in ways that outweigh any effect of neutering on longevity, and thus they may not be an appropriate population in which to study the effect of neutering on life span.

Cause of Death
The table below shows the odds ratios for sterilized males and females compared to intact dogs for specific categories of causes of death. 

Causes of Death for Neutered Dogs

Sterilized dogs were significantly less likely to die from infection, trauma, degenerative disease and vascular disease. They were significantly more likely to die from immune-mediated disease and cancer. Most of these differences held for both males and females and when the effect of age was accounted for, meaning that sterilized and intact dogs did not die from different causes simply because they died at different ages.

Specific cancers were examined if they occurred in more than 1% of the total population. Sterilized dogs were at greater risk for death from all of these eight types of cancer except mammary cancer, supporting the notion that neutering in females is protective against this disease.

It is important to remember that these are relative differences in risk. Other studies have shown that the incidence of mammary cancer may be much higher than of the others, such as lymphoma and osteosarcoma, and that breed plays a role. So if neutering is protective against a very common cancer and increases the risk of a very rare cancer, overall it may still be safer to be neutered than not. That is why overall life expectancy information is relevant as well. It makes sense to perform an intervention that raises the risk of a rare disease if most patients still live longer because they are protected against a common disease.

Several specific infectious diseases occurred with sufficient frequency to be analyzed individually. Sterilized dogs were at lower risk of death due to four of these five disease. However, these results are a bit more problematic than those concerning cancer risk. Unlike most cancers, the parasitic and infectious diseases listed are almost all completely preventable with proper vaccination or other preventative veterinary care. This raises the question of whether differences between sterilized and intact animals in these causes of death may be confounded by differences in the level of care they receive, in the socioeconomic status of their owners, in the time and place in which they lived, and many other variables not specifically addressed in this study.  

It is interesting that the same patterns in differences between cause of death in neutered and intact dogs were seen across many different breeds, each with their own specific risk pattern for particular diseases. Cancer, for example, appeared to be a more common cause of death in neutered compared to intact animals even in breeds with relatively low rates of cancer. This strengthens the idea that neutering is directly associated with the risk of such disease and also emphasizes the importance of considering the absolute, rather than relative risk, and the overall risk picture in any individual patient.

Limitations
There are a number of significant imitations to this study. As already mentioned, there are many potential confounding variables not addressed, including the time and place where subjects lived, the overall level of veterinary care they received, the socioeconomic status of their owners, the purpose for which the dogs were owned (work, companionship, breeding, etc), and many others.

In particular, the higher risk of preventable causes of death in sterilized dogs might be simply a reflection of less aggressive preventative veterinary care in these patients. However, studies in humans and in mice have suggested that infectious disease risk is lower in neutered individuals in the absence of differences in environment, resources, or preventative healthcare, so there may be a true biological relationship at work as well.

The authors also acknowledge that lack of information about the age at neutering and previous reproductive activity is a significant limitation. And it is understood that patients at university veterinary hospitals are often representative of a very different population than those seen in general practice, so the applicability of these results to the general dog population may be problematic.

Nevertheless, this study has a number of strengths and cannot simply be dismissed. The size of the population and the ability to correct cause of death for age and breed are particularly important in identifying real general associations between neutering and health that can be further investigated. Comparisons between this population and dogs in Europe, where neutering is uncommon, would be particularly useful in establishing the specific effects of neutering on disease risk for both males and females across a range of breeds, sizes, and lifestyles.

As I have often said, I believe it is a good thing we are questioning the established dogma about universal early neutering, which is based on pretty limited evidence, I think it would be unfortunate, however, to replace it with another dogma on the basis of, as yet, no better evidence. I suspect a siple rule that can easily be applied in all cases (always neuter, never neuter, neuter before/after Age X, etc) is never going to be reliable because physiology is simply more complex and nuanced than that. Recommendations about neutering any specific individual will have to take into account the overall picture of risks and benefits based on age, sex, breed, size, life circumstances, and other variables. As always, we must try to tailor our recommendations to the specific needs of individual patients using the best available evidence to inform, but not blindly dictate, what we recommend.

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16 Responses to Benefits and Risks of Neutering, An Evidence Update: Effects of Neutering on Longevity and Cause of Death in Dogs

  1. Rolf Wagels says:

    Hey
    thanks for this great blog. I am a vet in Germany and wondering what makes you think that neutering is uncommon in Europe? It is certainly not uncommon where I live. My wife is small animal surgeon, and the most common thing is neutering cats and dogs.
    Again, appreciate your work for EBM!
    Rolf

  2. skeptvet says:

    Thanks for the kind words!

    Well, of course Europe is a large and varied collection of individual countries, so I am probably over generalizing. I know neutering is less common in the UK compared with the U.S., and I know it is even less common in the Scandinavian countries (I believe even illegal, except for medical reasons, in Norway). But I don’t have any country-specific or overall EU data on the rates of neutering throughout Europe. If you are aware of specific numbers for particular countries, I would very much appreciate hearing them.

  3. Art says:

    Here is a unofficial translation of anti neuter laws. if you live in Europe and do not want blood spots on the bed sheets twice a year or a male dog that likes to hump you can have a deslorelin shot updated when you go to the vet for a check up. The shots are also given to chemically castrate pigs premarket rather than surgically castrate them to avoid the taste called taint in the meat and it is reported more cost effective than surgical castration of Pigs. In Europe owners can get a shot to spay or neuter their pet that can last up to thee years so the shot takes the place of surgical procedures but in the USA we are not quite there yet.

    The Animal Welfare Act:

    §9 Medical and surgical treatment must be implemented justifiably with regard to animal welfare in keeping with the animal’s functional abilities and quality of life.

    No surgical treatment or removal of body parts on animals can occur without a warrantable reason with regard to the animal’s health. However, the warranted marking of animals in animal husbandry is permissible. Removal of horns or neutering/gelding/spaying is permissible when necessary with regard to animal welfare or other special reasons.

    Necessary anaesthetization or pain relief shall be administered for painful surgical treatment.

    In addition in the explanation to the new act from 2010 it is stipulated that the animal owner’s wishes should be considered during evaluations.

    (Source: Lovdata.no, ScienceNordic’s unofficial translation)

    Statistics:

    It’s legal to neuter and spay dogs in Sweden and Denmark.

    The Swedish Veterinary Association informs that less than seven percent of dogs in Sweden are neutered or spayed.

    Denmark and Norway have no statistics in this regard, but according to the Norwegian Veterinary Association the shares in the Scandinavian countries are fairly equal.

    By comparison over 90 percent of dogs in the USA are neutered or spayed.

    Opinion of kennel clubs:

    We have contacted the kennel clubs of Norway, Sweden and Denmark.

    None of the kennel clubs support a wide-scale neutering such as in the USA, but would rather have the procedure remain the exception to the rule here in Scandinavia.

  4. skeptvet says:

    Thanks for the info, Art.

  5. Art says:

    The size of the population and the ability to correct cause of death for age and breed are particularly important in identifying real general associations between neutering and health that can be further investigated. >>>>>

    they haven’t controlled for confounding factors.
    if we factored out cofounding factors between neutering and level of care i suspect we would see that neutering decreases lifespan. At least neuter up to 4 years of age. The prospective randomized human studies show the longer you keep your ovaries the longer you can be expected to live. No randomized prospective human castration studies yet.:( There is a rodent study showing if you take the ovaries out and do a ovary transplant on half the ones getting ovary transplants live longer.

    Art

  6. skeptvet says:

    Well, there are potential confounders not accounted for, as there always are. However, we can’t simply guess as to the effect of these. We have to recognize the limitations of the data, but we can’t simply ignore it because we believe other variables would change the results.

    And the lab animal data, as well as the observational human research, are conflicting. Some studies show neutering lengthens life, others do not (there are references in the original paper). So again, there isn’t a definitive case to be either way, and we have to be careful about cherry picking studies that are consistent with our intuitions.

  7. Art says:

    So again, there isn’t a definitive case to be either way >>> opinion on both sides take on a almost religious fever.

    Reminds me of the circumcision debate.
    Art

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  11. Ron Bedford says:

    ORs cannot be negative. Unless this is some sort of notation I’m not familiar w/. It almost seems as though the OR values stated should be added to 1 to get traditional ORs.

  12. skeptvet says:

    Excellent point. The details of the analytic methods used are discussed in the paper, so you could check there. I suspect these are actually relative risks rather than odds ratios, but it isn’t clear to me from the methods section of the paper. If you figure out what they’ve done here, let us know!

  13. skeptvet says:

    I had a few more stats-savvy folks look at the article, and the consensus is that the numbers reported are log OR rather than OR. This is what was done in Figure 1, which was labeled properly. Still not sure why they reported LOR since OR is easier to interpret, but the underlying stats appear valid. Thanks again for pointing this out.

  14. Teresa Tanasi says:

    I am a lay person who lives in a rural area in a state with some of the lousiest pet owners I have ever seen. I am aware in many European nations they have what I would deem physician assistants here. I am fairly certain they can spay. The issue in the U.S. is there are some really lousy pet owners that are the reason 5500 are pts here a DAY. Backyard breeders and accidental breeding accounts for this. I buried another DUMPED dog in my area last week. I am wondering is the F.D.A. permitting this anti fertility shot here? I would be most willing to do it, as I know other good pet owners would BUT alas, I rescued a mini chihauhau (countless chis and Pitt bills die monthly in Arizona because of back yard breeders) and because he is so tiny, he would have been adopted as a stud machine by back yard breeders in this area( no registration, no problem, keep them till you are tired of them and then drive to a rural area and dump them) I wish this could work in the U.S. as I believe much of Europe has more civilized treatment of animals ( no cropping or docking for years) Alas, in some areas too many irresponsible jerks. Great article! Thank you all for sharing.

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