TO SPAY . . .

by Debbie Ducommun
From the Nov/Dec 1995 Rat & Mouse Gazette

I'm excited to report to you on a study published in the May issue of the Journal of the Veterinary Medical Association. The study was actually done to study the effect of various hormones on osteoporosis in spayed and unspayed rats, but they also recorded the number of mammary and pituitary tumors observed in each group. Unfortunately, all the rats were killed at 630 days of age, which is less than 2 years, but even so it's very clear that spaying rats is an effective means of preventing tumors.

The strain of rats used was Sprague-Dawley, a standard albino lab rat. (This is the type I have in my lab.) There were 96 rats in the study, and half of them were spayed at 3 months of age. The spayed and unspayed rats were each divided into 4 groups, 3 of which received different types of hormones (estradiol, etidronate, risedronate). The hormones were given until the rats were about 6 months old and then discontinued. The hormones didn't influence tumor incidence. At autopsy, one of the supposedly spayed rats was found to still have an ovary, so she was reclassified as unspayed, making 49 unspayed and 47 spayed rats.

Whenever subcutaneous tumors were detected on any of the rats they were surgically removed and examined by a pathologist. Mammary tumors developed in 24 of the 49 unspayed rats. In contrast, only 2 of the 47 spayed rats developed mammary tumors! That means that almost half of the unspayed rats developed mammary tumors, while only 4% of the spayed rats did so. Eight rats had multiple tumors removed and 4 underwent a second surgery to remove recurring tumors, although the article failed to report whether these rats were spayed or not (they were probably unspayed rats).


In the unspayed group, 20 of the mammary tumors removed were fibroadenomas (benign) but four rats developed adenocarcinomas (breast cancer) and one of these died from it. Three of the rats with the adenocarcinomas also had fibroadenomas. None of the spayed rats developed breast cancer. Of the other tumors that were found in the unspayed rats, one was a lipoma (benign), and one rat developed both an fibroadenoma and a pilomatrixoma (both benign). One rat developed a hemangiosarcoma (cancer of the blood vessels) and she did not survive her surgery.

In the spayed rats, both mammary tumors were fibroadenomas. One other rat developed a fibrosarcoma (cancer) which was partially removed by surgery twice without complications, but the rat died during the third surgery. There was no evidence of metastasis in this rat.


Another significant finding was that the spayed rats were more likely to survive to the end of the study. Twenty of the unspayed rats died spontaneously before the end of the study, while only 5 of the spayed rats died. Unfortunately, 9 of the rats who died spontaneously were unavailable for autopsy (the article didn't say why) but of those that were autopsied, 5 of the unspayed, but none of the spayed rats had pituitary tumors.

These 5 rats showed neurological signs caused by their pituitary tumors prior to their death, including head tilt, circling, inability to drink from a water bottle, and lethargy. Since these symptoms are similar to those caused by an inner ear infection, it may be that some pet rats with these symptoms have pituitary tumors instead and are misdiagnosed as having an inner ear infection.

During the autopsies that were performed on all the rats killed at the end of the study it was found that 22 of the unspayed rats and 2 of the spayed rats had pituitary tumors that had not yet caused symptoms. Altogether, 27 of 41 unspayed rats had pituitary tumors, while only 2 of 46 spayed rats did. In addition, 5 other unspayed rats had changes in their pituitary glands that precede tumor formation. So, actually, 78% of the unspayed rats that were autopsied had diseased pituitary glands! Only 4% of the spayed rats had pituitary tumors.


The reason that spaying reduces the incidence of both mammary and pituitary tumors is related to hormone levels. Estrogen stimulates the growth and activity of pituitary cells when not counteracted by progesterone. As female rats grow older and reach menopause their progesterone levels drop very low, which allows the estrogen to act more strongly on the pituitary gland. The ovaries are the primary source of estrogen, so spaying dramatically reduces estrogen levels. The pituitary gland produces prolactin, a hormone that stimulates the mammary glands. It is not known if estrogen causes mammary tumors directly or by over stimulating the pituitary gland. Males have low estrogen levels and also have a naturally low incidence of both these tumors.

At whatever age spaying is done, it will reduce estrogen levels. So, although it is probably best to spay rats at a young age-between 3 and 6 months-it is probably also beneficial to have older rats spayed as well. However, the older the rat, the more likely that tumors, especially a pituitary tumor, will already be forming. Probably, spaying should be done before the rat is a year old to be assured of preventing tumor formation.

If you have a rat that already has a mammary tumor and you decide to have it surgically removed, you should also consider having her spayed at the same time to help prevent future tumors. Other benefits of spaying that shouldn't be overlooked are the elimination of uterine infections and cancer of the uterus and ovaries. Spaying is one of the best ways to ensure your female rats live as long as possible.


by Mary Ann Isaksen
From the Nov/Dec 1995 Rat & Mouse Gazette

I agree that spayed female rats probably do tend to get fewer mammary tumors than unspayed rats, but it is a major surgical procedure, and, in my opinion the risk outweighs the possible benefits. Any time any animal is put under using a general anesthesia there is a risk of losing that animal during surgery.

The previous article does not offer any real proof that spaying extends life span. I also have difficulty in believing that the hormones administered over the three month period during this study did not influence tumor incidence. With twenty out of the forty-nine unspayed rats dying spontaneously before the end of the study, when the rats who lived to the end of the study were killed at only 630 days of age (1 year 9 months) that leads me to suspect there was more involved here, since most rats normally live to be at least two years old.

Also, the strain of rats used in this study was Sprague-Dawley, a standard albino lab rat. According to the book The Biology and Medicine of Rabbits and Rodents, written by John E. Harkness and Joseph E. Wagner, this particular strain of laboratory rats has a high incidence of spontaneous mammary tumors. That would account for why so many of the unspayed rats in this study grew mammary tumors at such a young age. Although mammary tumors are quite common among our pet population, they are not as prevalent as was shown in this study.


A few months ago, I lost two of my females who I feel would have made a good case study in this matter. Maxie, the older of the two, had been spayed at about five months of age. At two years, she began wasting away which is commonly what we see happen in our elderly rats. However, she was tumor free.

Harmony, about one month younger than Maxie, was unspayed and had been bleeding vaginally periodically for over a year when she died at age 2 years 3 1/2 months. She obviously had some sort of female disorder-possibly cancer or genital mycoplasma. She also had a mammary tumor about one inch in diameter that was loose under the skin indicating it was probably benign. At two years, she too, began wasting away in the same manner as Maxie.

In my eyes there really was no difference between them as far as quality of life was concerned. Maxie lived to be 2 years 4 1/2 months old compared to Harmony at 2 years 3 1/2 months. They were also cagemates and died only five days apart, with Maxie being the first to go.

By observing this particular case, and not having any other real data to look at, it is my opinion that spaying, although it may stop tumor growth, does not extend life expectancy. Is the risk of surgery then worth it? Studies in this area on large numbers of animals need to be done to convince me that spaying is worth the risk. Another thing to consider-mammary tumors are most often benign (not cancerous), are easily removed if caught early, therefore making it a minor surgical procedure, and not all females will get them at any point in their life. This opinion is also shared by my vet who has over fifteen years of experience treating rats.

If you do decide to spay your pet, please be sure to find a veterinarian who has experience with this procedure in rats.