Should we recommend spaying cats for mammary tumour prevention? At what age?

Imagine this clinical scenario...

You are presented with a three-month-old female kitten for vaccination. The kitten’s owner wants to discuss options around spaying as they have read in a magazine that early spaying can protect female cats from mammary tumours. They want to know if this is true, and if so, at what age the procedure should be performed. You decide to consult the evidence.

The evidence

Four papers were identified that address the efficacy of spaying in the prevention of mammary tumours, and the most effective age to carry out said spaying. Three were case control studies (Dorn et al., 1968, Overley et al., 2005, Graf et al., 2016), and one was a cohort study (Hayes et al., 1981).

Dorn et al. (1968) studied 21 cats with malignant mammary tumours between 1963 and 1966. These were compared against a control group of 706 cats that was drawn from a probability sample survey of households in Alameda County, California, in 1965. Outcomes studied were; the risk of developing mammary tumours if spayed, annual incidence rates per 100,000 for spayed and entire cats, and the effect of sex on the risk of developing cancer.

Graf et al. (2016) studied 1501 cats with confirmed mammary tumours that were submitted to a medical registry that collects data on feline cancers between 1965 and 2008. These were compared against a control group of 21,715 cats that were present in the registry for non-mammary tumours. The outcome studied was; risk of developing a mammary tumour between spayed vs. entire cats.

Hayes et al. (1981) studied 110 female cats with mammary carcinoma that were treated between 1964 and 1978 in the USA. 52/110 cats were spayed, while the remaining 58/110 were intact. The outcome studied was; relative risk of developing mammary carcinoma in female cats comparing spayed to intact individuals.

Overley et al. (2005) studied 404 cats presented to a veterinary hospital in the USA between 2000 and 2001. 204 cats had diagnosed mammary carcinoma, and 200 were control cats. The outcomes studied were; the relationship between development of mammary carcinoma and spayed status, and the effect of spaying on risk of feline mammary carcinoma development.

Limitations of the evidence

The overall strength of evidence was weak, owing to a variety of limitations. Study design was one weakness – case-control and cohort studies are in the middle of the evidence pyramid, and therefore do not provide the strongest evidence.

Dorn et al. (1968) was limited by its age; some of the cases reported in this paper are more than 60 years old. Additionally, the sample size was small (21 cats), and no data was provided on the spaying status of these cats, all of which reduces this paper’s applicability to the topic at hand. There was also a lack of clarity on; whether the control group had tumours, how many cats were used as controls in statistical analysis, and what the diagnosis criteria for mammary tumours was.

Graf et al. (2016) collected data over many decades (1965–2008) which may have introduced biases related to changes in diagnostic criteria, record-keeping practices, and population demographics over time. This paper also only features cats from a cancer registry which introduces selection bias towards cats already diagnosed with tumours, rather than the general population. This paper also does not provide data on how many cats were spayed vs intact.

Hayes et al. (1981) was also susceptible to selection bias, as the only animals included in the study were cats with mammary carcinoma. There were also a disproportionate number of Siamese cats in the study, which limits the applicability of the findings to the general population. It is also unclear how cases were selected for the study.

Overley et al. (2005) failed to report the age for all cats, which means the findings aren’t applicable to the topic at hand. There was also a risk of bias thanks to some data being obtained from questionnaires sent to vets. This risk of bias was compounded by the fact that the control group had all been diagnosed with tumours in organs other than the mammary gland.

Summary of findings

Only one of the papers (Overley et al. 2005) studied provided enough data on the ages of cats studied to draw any conclusions around the impact a patient’s age has on the efficacy of spaying to prevent mammary tumours.

Overley et al. (2005) found that intact female cats were at increased risk of having mammary carcinoma when compared with spayed cats. They found that spaying decreased this risk by 91% if performed before 6 months of and an 86% reduction if performed before 12 months. However, they also found that spaying performed after two years of age increased the risk of mammary carcinoma development.

Dorn et al. (1968) found that intact female cats had an approximately seven times higher relative risk of mammary cancer than neutered females. Additionally, they found that the annual incidence rates for mammary tumour were 20.4 per 100,000 for spayed cats, and 31.8 per 100,000 for entire cats.

Graf et al. (2016) found that the odds of spayed female cats developing mammary tumours was lower than entire cats, indicating a reduced risk associated with spaying. Likewise, Hayes et al. (1981) found that the relative risk of mammary tumours in spayed females was significantly less than in intact females.

Conclusion

There is weak evidence to suggest that spaying female cats before they reach one year of age is effective at decreasing the risk of mammary tumour development. However, the overall strength of evidence is in need of improvement, and there is a clear need for prospective, well-designed cohort studies with controls to improve it.

Disclaimer

The application of evidence into practice should take into account multiple factors, not limited to: individual clinical expertise; patient’s circumstances and owner’s values; country, location or clinic where you work; the individual case in front of you; and the availability of therapies and resources.