Spaying and tumor incidence in a pet rat population: a natural experiment


Most of the data on this web page about spaying and tumors in rats comes from studies on laboratory rats.  Controlled, randomized laboratory studies provide useful insight into rat health issues and the effects of test conditions.

However, it is also helpful to have data on pet rats themselves.  I recently received data on spaying and tumor development in pet rats (Joanne Bouchard pers. comm.).  I have run some descriptive and analytical statistics on the data and have permission to share the results here.


Ms. Bouchard runs takes in rescue rats in Canada and has provided me with the complete case histories of 33 female rats.  These rats were spayed between ages 1 and 21 months.  Their tumor histories were recorded and after they died necropsies were performed to determine the cause of death.

 These data come from a natural experiment. There are no unspayed female rats in this population so there is no natural control group.  However, some comparisons within the spayed group are possible.  I performed a Chi-square test on these and, due to the small sample size, applied Yates’ correction for continuity when warranted.  I used a web calculator to perform the Chi-square test (Preacher 2001).

Results and discussion

How many rats develop tumors after spaying?

Among the 33 rats that have died, 15.2% of the rats developed mammary tumors after being spayed (5 out of 33).

 Features of post-spay tumors

The rats in the deceased group that developed post-spay tumors tended to develop only one tumor (average: 1.2 tumors per rat; four rats developed one tumor, one rat developed two).

These post-spay tumors occurred relatively late in life (median age of post-spay tumor occurrence: 23 months; range 15-31 months), and remained small in size.

All five rats that developed post-spay tumors died of causes unrelated to their mammary tumors (mycoplasma (2), genital cancer (2), and multiple cysts in ears (1)).

None of the rats that are currently alive have developed post-spay tumors.

Age at spay surgery and post-spay tumors

The rats that were spayed at younger ages tended to be less likely to develop tumors later in life than those spayed later, but these differences were not statistically significant. However, the number of rats involved here is very small, so we may not yet conclude that age at spay has no effect.

Specifically, none of the eleven rats spayed at 2-9 months developed mammary tumors at any point during their lives.  Of the ten rats spayed between 10 and 15 months, three (30%) went on to develop one post-spay tumor each.  Of the twelve rats spayed between 18 and 21 months, two (16.7%) went on to develop mammary tumors (X2 = 3.701, p = 0.16).

Does having a spontaneous tumor before spay increase the chances of having a tumor post-spay?

Six of the 33 deceased rats had a tumor/spay surgery, meaning that they had already developed spontaneous tumors which were removed during the spay surgery.  These six tumor/spay rats were ages 12-21 months when they had their surgeries.  Of these rats, two (33.3%) went on to develop more tumors after the spay surgery.

Of the twelve rats that were 12 months or older when they were spayed and had not developed tumors prior to the spay surgery, two (16.7%) went on to develop tumors afterwards.

The impact of the pre-spay tumor was not significant.  The tumor/spay and the spay groups did not differ in their chances of developing a tumor later in life (Yates' X2 = 0.04, p = 0.84).

Age and cause of death of spayed rats

The 33 deceased rats died at an average age of 26.3 months (range 11-41 months).

Forty two percent of the rats died of mycoplasma (14 out of 33; 42.4%), while almost a quarter died of pituitary tumors (8 out of 33; 24.2%).  Two rats died of genital cancer (6.1%), and two of old age (6.1%). Twenty one percent (7 out of 33; 21.2%) died of other causes (ear cysts, internal hemorrhage, skin cancer, multiple strokes, malignant tumor on colon, growth wrapped around pancreas and omentum, a tooth abscess).

Pituitary tumors

Eight of the rats died of pituitary tumors.  Of these rats, three were spayed at 2 months, two at 8 months, one at 10 months, one at 18 months, and one at 21 months.


15.2% of spayed, deceased rats developed tumors after spaying.

The tumors that appeared did so relatively late in life (around 23 months) and remained small in size.  None of the rats that developed post-spay tumors died as a result of their mammary tumors.

There is a small tendency for a spay early in life to be more effective at preventing later mammary tumors than a spay later in life, but this tendency is not statistically significant.

Lastly, rats that have combined tumor/spay operations are no more likely to develop tumors later in life than rats that did not develop tumors before their spay surgery.


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