Given that I recently presented a couple of evidence-updates on the subject the health effects of neutering, the timing was excellent for the release last week of a new research study looking at the same issue.
Torres de la Riva G, Hart BL, Farver TB, Oberbauer AM, Messam LLM, et al. (2013) Neutering Dogs: Effects on Joint Disorders and Cancers in Golden Retrievers. PLoS ONE 8(2): e55937. doi:10.1371/journal.pone.0055937
This was a retrospective cohort study in which records were searched to identify Golden Retrievers who had been patients at the UC Davis veterinary school, were between 12 months and 8 years of age, and who could be classified as having been neutered “early” (before 12 months of age), “late” (after 12 months of age), or not at all. The occurrence of a number of diseases common in Golden Retrievers was then evaluated to see if it differed between dogs in these three categories. There are several reasons to be cautious in how we interpret the results of this study, but let’s start by looking at what those results were.
The authors looked at the occurrence of hip dysplasia (HD), elbow dysplasia (ED), cranial cruciate ligament rupture (CCL), and a number of cancers com including lymphosarcoma (LSA), hemangiosarcoma (HSA), mast cell tumor (MCT), osteosarcoma (OSA) and mammary cancer (MC). The figures below illustrate the relative occurrence of some of these diseases in males and females by neuter status (the diseases not shown did not occur often enough to be included in the analyses).
Figure 1. Percentages and number of cases over the total sample size for each neutering status group; intact and neutered early or late for male Golden Retrievers (1–8 years old) diagnosed with hip dysplasia (HD), cranial cruciate ligament tear (CCL), lymphosarcoma (LSA), hemangiosarcoma (HSA), and/or mast cell tumor (MCT) at the Veterinary Medical Teaching Hospital of the University of California, Davis, from 2000–2009.
For HD and LSA, the differences between early-neutered and intact or late-neutered groups were statistically significant (K-M), as were differences for CCL between intact and early-neutered groups.
Figure 2. Percentages and number of cases over the total sample size for each neutering status group; intact and neutered early or late for female Golden Retrievers (1–8 years old) diagnosed with hip dysplasia (HD), cranial cruciate ligament tear (CCL), lymphosarcoma (LSA), hemangiosarcoma (HSA), and/or mast cell tumor (MCT) at the Veterinary Medical Teaching Hospital of the University of California, Davis, from 2000–2009.
For CCL the difference between intact and early-neutered was statistically significant (K-M). For HSA, the differences between early and late-neutered and intact and late-neutered groups were statistically significant (RR), as were differences for MCT between early and late-neutered groups. A similar statistical comparison for late neutering and intact groups was not possible for MCT because there were 0 cases in the intact group.
doi:10.1371/journal.pone.0055937.g002
These results suggest that there is a complex and inconsistent pattern of associations between neuter status and these diseases. In general, there was a tendency for neutered animals to have higher rates of these diseases than intact animals and for early neutered animals to have higher rates than late neutered animals. However, there are many cases in which differences were not statistically significant or the difference was significant in one sex but not the other, and a few in which the direction of the difference was opposite that expected (for example with HAS occurring more often in late-neutered females than either intact or early neutered females).
The difficulty lies in knowing what these results mean in terms of predicting the risks for individual pets and making decisions about neutering in general. The natural tendency will be to look at these data and conclude that not neutering, or neutering after 12 months of age, is safer than the common current practice of neutering most animals before 12 months. However, this is not a conclusion we can reliably draw from these data.
To begin with, this is a group of dogs that are, in many ways, very different from most dogs neutered by veterinarians. Apart from only representing one breed, and a breed known to have higher rates of the diseases studied than most other breeds, the subjects were patients at a university veterinary hospital. In general, only the sickest animals with the most unusual or serious diseases are seen at universities. Most healthy animals or those with typical illness are seen at general veterinary practices. So it is uncertain if neutering will have the same effects as in this study on dogs of other breeds or those healthy enough to have never seen the inside of a university hospital.
The population also contained small numbers of intact and “late” neutered dogs. This makes it more likely that small, random differences in the health of individual dogs in the study group could alter the apparent effects of neutering on health. And the authors chose to limit the study population to dogs between 12 months and 8 years of age. Quite a few Golden Retrievers live past 8 years, and the rates of cancer generally go up dramatically with age. If the proportion of these diseases among intact and neutered animals over 8 years old are at all different from those in dogs under 8, then the direction and statistical significance of the differences between groups seen in this study could easily be altered.
And it’s important to consider the absolute risk and benefit numbers when making decisions about whether to neuter. If, in fact, neutering is strongly protective against a common cancer (as it is thought to be for mammary cancer) but also slightly increases the risk of a rare cancer, it may still make sense to neuter early depending on the unique situation of an individual patient.
Finally, two of the major diseases against which neutering is thought to be protective in females, uterine infection and mammary cancer, did not occur often enough to be included in the analysis. This may be because relatively few of the dogs were intact or because of the age cutoff. Obviously, if there really is a protective effect of neutering on these diseases that didn’t show up in this study, that might influence the interpretation of the results.
Overall, this is a useful piece of research adding to the information we have about the pros and cons of neutering and the possible role of age in the effect of neutering on disease. It should be over-interpreted as the final word by itself, but it should be incorporated into a broad analysis of all the available evidence.
As it stands, there is reason to believe early neutering has significant benefits in females, though as the recent systematic reviews pointed out the evidence behind this belief is not strong. There is also reason to be concerned about possible risks, though the evidence for this is also not robust yet. On balance, one can make a reasonable case on both sides, and the risk profile for individual dogs, as well as larger issues such as the problems associated with unwanted reproduction. Anyone who says there is an absolute and universal right answer concerning if and when to neuter female dogs is exaggerating by quite a bit.
For males, I believe the evidence of benefits from neutering, especially before 12 months of age, is not compelling, and I don’t see a strong reason to neuter earlier in the case of owned dogs with owners willing to commit to preventing roaming and unwanted reproduction and in the absence of intolerable interdog aggression. For large breeds in particular, delaying neutering of males seems reasonable, though we very much need better evidence to have confidence in such a recommendation. It is encouraging that more attention is now being paid to the complexities of neutering and the risks and benefits associated with it, and I am optimistic that this will lead us to more reliable guidelines in the future.
Anyone who says there is an absolute and universal right answer concerning if and when to neuter female dogs is exaggerating by quite a bit.>>>>
Sad to say government neuter programs never tell the client that. . If the dog has no medical or behavior problems that neuter will help I prefer not to produce a scar that shows everyone where unproven medical care has occurred .
Art Malernee dvm
Thank you for this article. Have spent the evening reading about this study on several blogs and your post is by far the most complete (in putting the data into context) and very understandable.
I do have a question that I forgot to ask above: have there been studies about behavioral “problems” and neutering? If so, briefly, what do they find? A lot of dogs are surrendered and ultimately die due to unwanted behavior, so in a sense, this is also a health issue (as well, of course, as training and education issues). Thank you!
and your post is by far the most complete (in putting the data into context) and very understandable.>>>>
The Harriet Hall of veterinary medicine.
Art
Awww, shucks! 🙂
Thanks for the kind words.
The literature of neutering and behavior is pretty limited and hard to interpret, but here is a brief discussion of the subject from my previous review of neutering effects:
Behavioral Benefits-
Behavioral problems are an important reason for relinquishment of pet dogs and cats by owners. [1,5,44] The most common problematic behaviors include aggression towards people or other animals, inappropriate elimination, and fearful behaviors.[45] To the extent that neutering increases or reduces the risks for these behaviors it can have an important impact on the relationship between pet and owner and ultimately on the pet’s survival.
The biological and environmental influences on animal behavior are complex and difficult to unravel. Specific behavior patterns are influenced by many environmental and individual factors which all interact, so epidemiological correlations are often unreliable in predicting the outcome of interventions in individual cases. However, there are some consistent patterns that emerge from studies on normal and problematic behaviors in dogs and cats which illustrate the potential behavioral benefits and risks associated with neutering.
Some studies have reported intact male dogs to be disproportionately involved in aggressive behavior. [46,47]. Others have reported marked reductions in aggression and other problem behaviors in male dogs as an effect of castration. In one study, roaming behavior decreased 90%, aggression between males decreased 62%, urine marking decreased 50%, and mounting decreased 80% following castration,[48] and several other studies have found similar results.[49,50,54] Some studies have also reported intact dogs to be more likely to bite humans than neutered animals.[52]
Castration also dramatically reduces fighting, urine spraying, and roaming in male cats.[52-54] One study has found intact cats to be more aggressive and less affectionate than neutered cats.[102]
1. Clancy EA, Rowan AN. Companion Animal Demographics in the United States: A Historical Perspective. In: Salem, DJ, Rowan, AN , editors. The State of the Animals II. Washington D.C., USA: Humane Society Press; 2003. p. 9-26.
5. Jessup DA. The welfare of feral cats and wildlife. Journal of the American veterinary Medical Association 2004;225(9):1377-83.
44. Scarlett JM, Salman MD, New JG, Kass PH. The role of veterinary practitioners in reducing dog and cat relinquishments and euthanasias. Journal of the American Veterinary Medical Association 2002;220(3):306-11.
45. Overall KL. Clinical behavioral medicine for small animals. St. Louis (MO), USA: Mosby; 1997.
46. Borchelt PL. Aggressive behavior of dogs kept as companion animals: classification and influence of sex, reproductive status, and breed. Applied Animal Ethology 1983;10:45-61.
47. Write JC, Nesselrote MS. Classification of behavioral problems in dogs: distributions of age, breed, sex, and reproductive status. Applied Animal Behavior Science 1987;19:169-78.
48. Hopkins SG, Schubert TA, Hart BL. Castration of adult ale dogs: effects on roaming, aggression urine spraying, and mounting. Journal of the American Veterinary Medical Association 1976;168:1108-10.
49. Maarschalkerweerd RJ, Endenburg N, Kirpensteijn J, Knol BW. Influence of orchiectomy on canine behaviour. Veterinary Record 1997;140(24):617-69.
50. Neilson JC, Eckstein RA, Hart BL. Effects of castration on problem behaviors in male dogs with reference to age and duration of behavior. Journal of the American Veterinary Medical Association 1997;211(2):180-82.
52. Hart BL, Cooper LC. Factors relating to urine spraying and fighting in prepubertally gonadectomized cats. Journal of the American Veterinary Medical Association 1984;184(10):1255-8.
54. Knol BW, Egberink-Alink ST. Treatment of problem behaviour in dogs and cats by castration and progestagen administration: a review. Veterinary Quarterly 1989;11(2):102-7.
102. Stubbs WP, Bloomberg MS, Scruggs SL, Shille VM, Lane TJ. Effects of prepubertal gonadectomy on physical and behavioral development in cats. Journal of the American Veterinary Medical Association 1996;209(11)1864-71.
Thank you for the link to your earlier summary. It is extremely helpful and comprehensive. A lot to think about; nothing yet to make me not neuter, as the balance seems to tilt somewhat in favor of neutering females for health reasons and males for behavioral reasons, but I am grateful that scientists are following all the consequences, positive and negative, and that you are providing full summaries.
I would like to see more conservative alternatives to spaying and neutering routinely offered by vets, e.g. a hysterectomy that leaves the ovaries intact on a bitch but removes the uterus, cervix, and fallopian tubes and hence stops bloody discharge during heat and prevents the risk of pyometra, and a vasectomy for a dog. Interestingly, the medical profession used to remove ovaries from humans routinely with hysterectomy, but now don’t and generally won’t remove ovaries unless there is a very good reason because of harmful hormonal effects. I had my first bitch spayed at 6 months old because I believed a lot of the hype at the time that it was much healthier and bitches in heat were a nightmare to take care of, and TBH I was travelling a lot at the time and thought it would be easier to manage, but I think in future I will try to find a vet who will do vasectomy/hysterectomy if I feel it’s necessary to eliminate the risk of breeding.
Of course, with these procedures dogs will still mate and exhibit sexual behaviour, so spay and neuter will always be preferred by some, and that should be their right.
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Good review with the right amount of skepticism. My vets are so pro-neutering it’s shocking and claim this article doesn’t exist, due to the financial implications of supporting neutering.
When I first read the study I missed the age cut off date, which I agree hides very pertinent data. I agree with the study separating the data though, as GRs are fairly likely to go from cancer at some point. Including that in the initial paper could dwarf the actual outcomes. More detailed analysis of long term effects, breaking down ages is needed. However that will require a larger sample size to stay significant.
My personal interpretation. Don’t neuter a large breed (particular one prone to bone problems) before it is physically mature. Then after, the pro/cons are more balanced. Anyone who thinks how can sexual organs impact bone strength? just has to look at the much greater evidence for young human castration.
I know this is very old but in case anyone else is reading through these comments, removing the uterus alone and leaving the ovaries does nothing to reduce the risk of pyometra. A pyometra will only develop if there is ovarian tissue present. It is for this reason that some vets will neuter a female by removing the ovaries and not carry out a full hysterectomy.
Were the uterus removed and ovaries left then something called a stump pyometra could develop which is just as serious as a “normal” pyometra.