Medical Corner - The Great Spay Debate

Mary Ann Isaksen
From the May/June 1998 Rat & Mouse Gazette

Recently, a "Rat Fan Club" member told The Rat Report editor, Debbie Ducommun, that she was upset after reading my portion of the article "To Spay Or Not To Spay", co-written by Debbie Ducommun and myself, which appeared in the November/December 1995 issue of Rat & Mouse Gazette. She was angry because she felt that my opinion in the article was based on emotion instead of 'scientific fact'. My opinion, at that time, was based on a lack of facts, the opinion of my vet, and certainly, emotion. It was requested that another article be written on this subject.


Debbieís portion of the article was entitled "Spaying Prevents Tumors!". In it, she wrote about a scientific study done to evaluate the effect of various hormones on osteoporosis in spayed and unspayed rats. During the study, they recorded the number of mammary and pituitary tumors found in each group. The rats in the study were all killed at 630 days (21 months of age). The conclusion was that spaying is an effective means of preventing tumors.

Half of the Sprague-Dawley laboratory rats used in the study were spayed at the age of three months and were divided into four groups. Three of these groups were given different types of hormones until they were six months old. The following are tumor incidence statistics from the study.

  • 24 of 49 unspayed rats developed mammary tumors.
  • 2 of 47 spayed rats developed mammary tumors.
  • 4 of the unspayed rats tumors were malignant.
  • 20 of the unspayed rats died spontaneously before the end of the study.
  • 5 of the spayed rats died spontaneously before the end of the study.
  • 9 of the rats who died spontaneously were unavailable for autospy (reason not disclosed in study).
  • 5 of the unspayed rats who died spontaneously who were autopsied had pituitary tumors.
  • 22 of the unspayed rats killed and necropsied at the end of the study had pituitary tumors.
  • 2 of the spayed rats killed and necropsied at the end of the study had pituitary tumors.

From these statistics, her conclusion was that spaying extends life.


My portion of the article was entitled simply "The Other Side Of The Story". I agreed that spayed rats do get fewer tumors, but pointed out the risks involved with surgery, and detailed a few problems I saw with the study (which I will go into detail about later in this article).

I also discussed the demise of two of my own rats who I felt made a good case study for proof that spaying may not actually extend life. Both rats began wasting away at the age of two years (which is commonly what we see happen with our elderly pets), although one of the rats had been spayed at five months of age and the other was unspayed. The spayed female did remain tumor free, but lived only one month longer (2 years 4-1/2 months) than the unspayed female, who had been bleeding vaginally on and off for over a year and also had a small, benign mammary tumor.

My conclusion was that, although spaying does reduce tumor incidence, it is not a guarantee that it will extend the life of your pet.

HARMONY, unspayed, lived to be 2 years 3-1/2 months old.

Photo by Mary Ann Isaksen


The fact that so many of these rats died spontaneously before even reaching 21 months of age causes me concern. I have had a lot of rats over the years and there have been very few female rats in my care die before the age of two years. This leads me to believe there was something else wrong with the rats in this study.

The rats used in this study were Sprague-Dawley albino lab rats. According to the book The Biology and Medicine of Rabbits and Rodents, this particular strain of laboratory rat has a high incidence of spontaneous mammary tumors, probably much more prevalent than in our pet rats.

Also, the hormones administered in this study did increase the number of tumors these rats developed, so how can we be sure they didnít influence the study otherwise?


In a later article, Debbie reviewed two other studies done with different strains of lab rats (Wistar, Brown Norway, and Sprague-Dawley) that showed that spaying did extend the life span of the rats in the study, although thereís no way of knowing how long or if all spayed rats will follow that example. The average age of the rats in my rattery has always been between 2 and 2-1/2 years of age, and my spayed rat still only lived to be 2 years 4-1/2 months of age. So, yes, I believe that spaying reduces the incidence of tumors in female rats and may extend the life of the rat, but do I recommend it or tell people they should have it done? No. I discuss it as an available option and let them make up their own mind.

I donít feel comfortable telling someone to spend $60 to $150 for a procedure that may or may not extend the life of their pet, not to mention the fact that itís an invasive surgical procedure (goes into the body cavity), that most veterinarians probably have not performed, and may not be qualified to perform. In addition, with any surgical procedure using general anethesia, there is a risk that the patient may not survive. Itís not common for a qualified vet to lose a healthy rat during a surgical procedure, but it certainly happens.

Finding a qualified vet to perform this procedure is probably the scariest part, in my opinion. I have heard far too many horror stories from people about botched surgeries and just plain bad veterinary care when it comes to rats. The following is a quote from the book Ferrets, Rabbits, and Rodents - Clinical Medicine and Surgery. "An area of concern is the unfamiliarity of clinical veterinarians with rodent biology. Although a great deal of information has been accumulated on wild and laboratory rodents, very little of this information pertains to pet rodents."

I, personally, feel more comfortable leaving my females intact and having tumors removed if they appear. In my experience, and according to the book Ferrets, Rabbits, and Rodents - Clinical Medicine and Surgery, most mammary tumors (and even uterine tumors) in rats are benign (the opposite is true for mice). The surgery to remove benign mammary tumors is much less invasive than spaying as the tumor normally lies just under the skin. No incision into the body cavity is necessary for these simple excisions.

I have never lost a rat during a simple tumor removal operation, but if I did, I would feel better knowing that the rat in question lived a good life up until that point (I would still be very sad). If I had recommended that a rat be spayed at a young age and the rat died in surgery, I would feel terrible that a life had been wasted. Therefore, I donít push the issue on people and try very hard to discuss all the possibilities when asked about this subject.


Scientific fact is the only thing considered in laboratories, but it should not be the only consideration while dealing with our pets. Just because a vet says that something can be done, doesnít mean that something may not go wrong and affect the outcome - negatively. Weíre not dealing with laboratory rats here, we are talking about our beloved pets. How can emotions not play a huge role in our decisions regarding their lives, and why shouldnít they? However, emotions must not be the only factor in your decision. My point is that, in every situation, all aspects must be weighed to reach the best decision for each individual.


If I could say with 100% certainty that spaying a rat would completely stop that rat from ever getting a mammary or pituitary tumor, and it would definitely extend the ratís life, then, without hesitation I would tell everyone I know to spay their rats, but I canít say that. I canít even say that a certain rat will or will not get a mammary or pituitary tumor.

Debbie has kept track of the tumors her female rats have had - 44% have had mammary tumors and 16% pituitary tumors. Those are pretty bad odds according to her, but remembering that those 44% probably could have had their mammary tumors easily removed, those odds sound pretty good to me (although I believe my percentage is even less, with a higher number of rats). If only 44% of my rats ended up with mammary tumors, my vet bills would be less than if I had spayed each one from the start, and I would feel better knowing that I didnít risk their life in a possible unneccesary surgical procedure. However, pituitary tumors can NOT be removed and will ultimately kill your rat, so if youíre worried about your rat being one of the small percentage of rats who get them, then you should consider having the rat spayed. Spaying REDUCES the incidence of mammary and pituitary tumors - greatly, but it is NOT a guarantee that a rat wonít get them.

I encourage everyone to look at all sides of an issue before making a decision regarding the life of your pet. I know this article probably hasnít made it any easier for people to make up their own minds, but at least you are aware of the possibilities. If you do decide to spay your rat, seek out a veterinarian knowledgable and experienced with this procedure in rats.