This article is from the May/June 1996 AFRMA Rat & Mouse Tales news-magazine.
By Carmen Jane Booth, D.V.M.
Carmen Jane Booth, D.V.M.
I received a phone call regarding the benefit of spaying female rats as a means of preventing tumor development as rats age, as a result of an article published last year. The original article, “Effect of surgical removal of subcutaneous tumors on survival of rats,” was published in the Journal of the American Veterinary Medical Association, May 15, 1995, Volume 206, page 1575.
The study was undertaken to evaluate the practicality of surgical removal of mammary tumors and the effect on survival.
Ninety-six female Sprague-Dawley rats were divided into two groups; half were ovariectomized (ovaries removed) at 90 days of age and the other half underwent sham surgery (sexually intact). These two groups were further divided into four treatment groups of 12 rats, three of which were given different hormones (treated) and one was given saline (control) from days 91 to 250 for a different study involving osteoporosis (reduction in the amount of bone mass leading to fractures [common in postmenopausal women]). Post treatment rats were maintained for 380 days and any subcutaneous tumors that developed were removed surgically and examined histologically.
Necropsy (visual examination of the body after death) was attempted on rats that died or were found moribund and euthanized prior to the end of the study. At the end of the study (630 days of age) all rats were euthanized and only gross lesions in the mammary glands or pituitary gland were examined histologically (examination of tissues using a microscope).
Estrogens (female hormones) have been known for many years to be related to mammary tumors as mammals age. This is the reason that all veterinarians recommend spaying female dogs before their first heat. Estrogen has also been determined previously to increase the frequency of pituitary tumors. Estrogen increases the mitotic rate (rate of cell division) of pituitary cells especially those that secrete prolactin (another reproductive hormone). Sexually intact female rats have higher concentrations of prolactin.
One key point to keep in mind with the study is that they use small groups and euthanized the rats at 630 days (less than 2 years of age) so true life-span data is not available. Although more sexually intact rats had pituitary changes, the majority were clinically silent. Additionally, rodents can and do develop a wide variety of other types of tumors as they age that are completely unrelated to their sexual status that are equally as debilitating as pituitary tumors. I have read many different research papers on tumor incidence in aging rodents and cannot recommend the universal spaying of female rats to prolong life span as suggested by this particular study.