As part of my ongoing surveillance of evidence concerning the benefits and risks of neutering, I have identified a new study on the subject.
Hart, B. L., Hart, L. A., Thigpen, A. P. and Willits, N. H. (2016). Neutering of German Shepherd Dogs: associated joint disorders, cancers and urinary incontinence. Veterinary Medicine and Scienc. doi: 10.1002/vms3.34
This study is a retrospective that looks through the records of the veterinary hospital at the University of California at Davis. It is a product of the same research group that has produced several other similar studies I have also reviewed. These earlier studies found some interesting differences between even closely related breeds, such as Labrador retrievers and golden retrievers, which suggests that generalizations applicable to all dogs regardless of breed are unlikely to be very reliable. This is always frustrating, since simply, universal rules are more appealing and easier to apply than nuanced, complicated risk/benefit calculations, but science often shows us that nature is more complex than we might wish. This study illustrates again the importance of breed in evaluating the potential effects of neutering.
This study includes some of the same limitations I have discussed when evaluating previous studies by this group. For example, except when evaluating mammary cancer, the authors did not consider diagnoses in dogs over eight years of age. Their rationale is that the effects of neutering might be swamped by other factors in older dogs, making them harder to detect. While this may be true, it is an assumption that has not been demonstrated to actually be true. And since overall health and disease occurrence, mortality, and longevity are the outcomes of real interest to pet owners, not just diseases that occur in the first eight years of life, using a cutoff like this can misrepresent the real importance of neutering. If, as some evidence suggests, neutered animals live longer overall than intact animals, for example, then neutering might still be the better choice even if one can show it causes more of some types of disease in the first eight years of life.
And a I have discussed in the context of their previous papers, and the authors themselves acknowledge, this study uses records pertaining to dogs seen at a university veterinary hospital. The people and patients who go to vet schools for treatment differ in a number of ways from those who don’t, and these differences may effect the diseases they get and the impact of neutering on health. This doesn’t invalidate the data or conclusions, of course, but it means we must be cautious in extrapolating these to first opinion practice populations.
The authors evaluated the impact of neutering before 6 months of age, between 6 and 11 months of age, between 1 and 2 years of age, and between 2 and 8 years of age on the occurrence of various disease compared with un-neutered dogs. Here is a brief summary of the findings:
- Neutering before 12 months of age (combining the < 6 months group and the 6-12 months group) was associated with a higher risk of cranial cruciate ligament ruptures in both neutered males and females compared with intact dogs. This is similar to the findings for golden retrievers, though no increase in risk was found for Labrador retrievers except in males neutered before 6 months of age.
- Neutering, regardless of age, was not associated with any difference in the risk of hip dysplasia or elbow dysplasia. In contrast, similar earlier studies did find an increase in hip dysplasia risk in male golden retrievers neutered before 12 months of age and an increase in risk for female Labrador retrievers neutered before 2 years, but no increase for female goldens or male labs. As always the devil is in the details.
- Neutering, regardless of age, was not associated with any increase in risk of any cancer. Some earlier studies have found inconsistent and complicated increases in the risk of some cancers that vary with age at neutering, sex, and breed. Broad generalizations about neutering and cancer risk seem difficult to support given this variability in the data.
- No statistically significant difference was found between neutered and intact females in the risk of mammary cancer. However, very few cases were found (only 14 out of 450 dogs), possibly due to the cutoff of 11 years of age used.
- Urinary incontinence was significantly more common in females neutered between 6 and 11 months of age than intact females. Differences for other neutering ages and intact females were not significant, but there were no cases of incontinence in intact females.
- All cases of pyometra were in intact females, of course. This only occurred in about 2.5% of the females, However, since this disease becomes more common with age and the dogs were only evaluated up to 8 years of age, this likely under-represents the real risk of this problem in intact females.
Overall, this study adds yet another small but useful bit of information for vets and dog owners to consider in making decisions about neutering. It emphasizes yet again that the impact of neutering differs significantly between breeds, and general rules cannot be reliably applied to all dogs.