I have written extensively about the scientific evidence concerning the benefits and risks of neutering. Overall, the data is complex, and significant effects of neutering on specific health risks are rarely definitively demonstrated. One of the most controversial issues, the influence of neutering on cancer risk, illustrates this. Some cancers are more common in neutered animals, some are more common in intact animals, and the effect of neutering on cancer risk varies with sex, breed, age at neutering, and possibly other factors. When looking at the issue in cats, we have the added challenge of far less research data than is available for dogs. However, one new study has added a bit more information to help evaluate the subject.
Graf R, et al. Swiss Feline Cancer Registry 1965-2008: the Influence of Sex, Breed and Age on Tumour Types and Tumour Locations, Journal of Comparative Pathology (2016).
There are a number of limitations to this paper that have to be considered. Laboratories in Switzerland that analyze tumors contributed data to a central registry. The study reports an analysis of data collected in this registry over a very long period of time. This often creates problems since definitions of disease and the behavior of animal owners and veterinarians, in terms of relevant issues like how likely to diagnose and treat cancer, submit samples to laboratories, and so on, change over time. The data collected at the beginning of the time period analyzed may or may not be truly comparable to the data collected later.
Also, use of the registry is voluntary, so it is not clear how representative this data is of the overall cat population in Switzerland, much less anywhere else. And exactly how the data is collected and processed is not reported in the paper, and many important terms are not well defined. For example, it is not clear how cats are classified as neutered or intact, and there is no discussion of the age at which neutered cats are neutered, which has been an important factor in other studies. So evaluating potential sources of bias and error effectively is impossible, and therefore this is the sort of data that have to be taken with some skepticism.
Nevertheless, given the paucity of data on the effect of neutering on feline cancer risk, this paper is a useful addition. The portion of the report that relates to neuter status is contained in the following two tables:
There are several interesting patterns that can be discerned in these tables. The first is that there is a pretty consistently higher cancer risk in neutered cats compared with intact cats. This ranges from about 25-50% higher for most cancer types, though there are some for which there is no apparent difference in risk (fibrosarcomas for males and adenomas/adenocarcinomas for females).
Looking at specific tumor locations, the general pattern is for neutered cats to have a higher risk of tumors in some locations (skin, gastrointestinal tract, cardiorespiratory system, and oral cavity) and a lower risk for mammary tumors (in both males and females, though females are at much higher risk overall for mammary tumors than males, whether intact or neutered). If these differences are consistent in other study populations, it might help shed more light on the specific mechanisms by which sex hormones could be protective against some cancers while neutral or even a risk factor with respect to others.
While there are many limitations and caveats to these data, they do suggest a pretty consistent mild-moderate increase in cancer risk associated with neutering in cats. As always, this has to be balanced against the other health risks and benefits associated with neutering, as well as other important issues, such as the ethical and environmental impact of cat reproduction. This paper emphasizes the complexity of biology and the multifaceted potential effects, both good and bad, of any intervention which significantly effects the physiology of our animal companions and patients.