As part of my ongoing coverage of the risks and benefits associated with neutering, I wanted to review a recent article on the subject, this one looking specifically at cancer risk and behavioral problems in Vizslas.
Zink, MC. Farhoodly, P. Elser, SE. Ruffini, LD. Gibbons, TA. Riegr, RH. Evaluation of the risk and age of onset of cancer and behavioral disorders in gonadectomized Vizslas. J. Amer Vet Med Assoc. 2014;244(3):309-319.
This study involved an analysis of data collected in 2008 through an online survey of owners of Vizslas in the U.S. and other countries (U.S. owners made up about 87% of the responses, with almost all the others coming from the UK, Australia, and Canada). Information was collected on about 2,500 dogs, and both cancer and behavioral problems were reported in about 25% of these.
The authors looked at the cancers and behavioral problems reported by owners as well as the age when individuals were neutered, if they were, the sex, and the age at which the medical problems examined were reported (though this last figure often had to be guessed at). The authors specifically excluded many conditions from the analysis, including some previously reported to be associated with neuter status. They did not, for example, consider orthopedic diseases because these were uncommon (~9% of the dogs). Oddly, they did not consider skin conditions either, though these were reported in about 20% of the dogs.
The reported results cover several cancers that are especially common in Vizslas, including Mast Cell Tumors (MCT), Hemangiosarcoma (HAS), and Lymphoma (LSA), as well as behavioral problems (noise phobias, separation anxiety, and various forms of aggression). The general results, broken down by age of neutering, are reported in the tables below.
The odds of MCT and LSA were higher for neutered than intact animals. The odds of HSA was higher for neutered females than for intact females, but there was no relationship between neutering and HSA risk for males. The odds of cancers other than these three were also higher for neutered than for intact animals. For all of these cancers, the odds were higher in those neutered after 12 months of age than in those neutered earlier.
For behavioral problems evaluated, the odds of having such a problem were higher in dogs neutered before 6 months of age than in intact dogs. There were no differences in the odds of behavior problems between intact dogs and those neutered after 6 months of age with the exception of storm phobia which was more common in neutered animals overall than in intact animals.
There was no difference in the age at death or the longevity of neutered dogs compared with intact dogs.
Limitations of the Study
The first potential limitation of this study is that the population of dogs included are potentially not representative of the general pet dog population. Only one breed was included, and only 60% were reportedly kept as “primarily a family pet,” with most of the others being used for show or hunting activities. About 23% were reported to have had offspring and almost half (43%) of the dogs were intact, whereas surveys suggest over 80% of the overall owned dog population is neutered. The average age at death was also reported to be 9 years, which seems quite young compared to many other breeds and mixed-breed dogs of similar size. Since there are genetic factors involved in many health conditions, and potentially developmental and environmental factors associated with how dogs are kept, caution must be used in extrapolating results from one population to another.
Another significant limitation of this study is the method of data collection. All data was collected by anonymous online questionnaire, with no attempt to verify the accuracy or validity of these data. Diagnoses of cancer and behavioral problems and assessment of age at neutering were based entirely on reports of owners, sometimes many years after the fact.
This raises a host of concerns. Owners may have reported diagnoses incorrectly, such as misidentifying cancers or reporting benign tumors as “cancer.” Owners may have been more likely to report cancer and/or neutering information if they believed there to be a relationship between the two or if they knew one purpose of the study was to examine such a relationship. Owners also identified cancer and skin conditions as top health concerns, suggesting a population of respondents particularly interested in these conditions, which might have affected their rate of reporting them.
There is also no way to identify what if any differences there were between people who participated in the survey and people who did not, or between the dogs owned by these different groups. It is likely that people who were aware of the survey and motivated to complete it differed in numerous ways from other Vizsla owners and owners of other kinds of dogs, and this again could affect the health conditions reported and the risk factors affecting them.
Another issue is that this study looked at potential risks posed by neutering, but it did not include assessment of most of the potential benefits of this procedure in this population. For females, for ecample, neutering is believed to be protective against mammary cancer (though the evidence is not as strong as commonly supposed), which in some populations is a very common and frequently malignant type of cancer. In this population, mammary cancer was reported in less than 1% of the females in the study, a rate dramatically less than in other populations studied. This suggests either that this population is a much lower risk of mammary cancer than others, in which case the protective effect of neutering might not be meaningful, or that the incidence of this disease was underreported.
Similarly, uterine infections (pyometra) are a common and serious disease in intact females, and these can be completely prevented by neutering. Yet the rate reported in this study was quite low. 22 cases were reported, which would be a rate of about 4% of the ~535 intact females included. Other studies have reported rates of 10-50% depending on age, so either this population has an unusually low rate of this disease, or the incidence was not accurately reported.
Health problems and cost or disruption for owners associated with estrus, reproductive behaviors, or actual reproduction were also not evaluated in this study.
Overall, the study found no difference in the longevity or overall mortality of neutered versus intact dogs. This is in contrast to other studies which suggest neutered animals may live longer on average than intact animals. More importantly, it calls into question the significance of the reported increase in cancer risk in neutered dogs. If neutered dogs are truly at significantly higher risk of often fatal diseases like HAS and LSA, one might expect intact animals to live longer as a results of being less likely to experience these diseases. And if the two groups have roughly the same life expectancy, perhaps there are benefits to neutering not reported here that counterbalance the risks discussed?
This study contributes useful new information to the ongoing process of evaluating the risks and benefits of neutering. It supports information from other studies, in Rottweilers and Golden Retrievers, that suggest neutering may increase the risk of some cancers, such as hemangiosarcoma and lymphosarcoma, in breeds predisposed to develop these diseases.
The study also has a number of significant limitations. The dogs in the study were all of one breed, and they differed in a number of ways from the general pet dog population, so findings in this group may not be applicable to other populations. The data was collected though anonymous questionnaires completed by owners, often years after the events being asked about, and there was no way to confirm the accuracy or validity of these reports. There is also a high risk that the people who chose to participate in the survey, and the dogs they own, are quite different from the general pet owning population and their pets, in their concerns, knowledge, and pet care practices.
The study did not examine many of the risks posed by being intact, which have to be considered in weighing the overall risks and benefits of neutering. Rates of pyometra and mammary cancer, common and serious medical problems prevented by neutering females, were far lower in this study than generally reported elsewhere, suggesting either that the study population was quite different from other dog populations or that the rates of these diseases were not accurately reported.
And it is unclear how significant the reported increase in the risk of cancers in neutered animals really is since there was no overall difference in the longevity of neutered and intact animals. If neutered animals are much more likely to get cancer, it is surprising that they tended on average to live just as long as intact dogs.
Overall, this study supports the current trend towards questioning the dogma of routine neutering for all dogs. The risks and benefits are likely to vary according to breed, age, and many other variables, and a one-size-fits-all approach is not ideal. Unfortunately, a great deal of additional research will need to be done for dog owners and veterinarians to have confidence in specific recommendations for individual dogs.