In a recent post I discussed a paper that claimed to show that dogs lived longer if they kept their ovaries, despite a lot of evidence to the contrary. This article got an inordinate amount of attention in the secondary literature and the blogosphere, most of it quite uncritical. With this kind of observational study, it is easy to analyze the data in a way that will give a positive result, or even to design the study in such a way that a positive result is nearly guaranteed, as discussed here, and here.
In the most recent issue of JAVMA, there is a paper that provides an interesting counterpoint to
the earlier paper. This paper discussed dogs (and two cats) that had residual ovarian tissue left after spay surgeries. Most of these animals had ovarian tissue left in the normal anatomical position for ovaries to be, indicating that failure to identify and remove the entire ovary at the time of surgery is much more common than ectopic ovarian tissue is (I have seen one cat in 20 years that had one normal ovary and no identifiable grossly visible tissue on the other side, but had a couple of weak heat cycles after surgery, then nothing). So the vast majority of these cases can be blamed on careless surgical technique, not exceedingly rare anatomic anomalies. It was also interesting that surgical experience was not a factor, again indicating that complacency and sloppiness was to blame rather than inexperienced surgeons.
This paper had a limited number of cases (19 dogs and 2 cats), so I don't want to make too much of it, but it was interesting that this small paper supported other findings that removing ovaries can prevent certain types of cancer and infections in dogs. In addition to the expected signs of estrus (which are usually not something most pet owners want to deal with) there were five ovarian tumors, seven enlarged uterine remnants, one vaginal mass and two mammary masses. On histology and culture, there were five cases of ovarian cancer, eight cases of cystic endometrial hyperplasia, and one uterine stump pyometra. It is interesting to note that this rate of ovarian tumors is much higher than that reported for sexually intact female dogs (23.8% vs 6.8%). This may be an artifact of the small sample size, but it could indicate that leaving ovaries instead of removing them during a spay could significantly increase the risk of ovarian cancer in dogs. All of this supports other evidence that early removal of the ovaries and uterus can prevent several serious types of cancer in animals and can also prevent serious infections of the uterus (pyometra).
It will be interesting to see if some of the blogs and veterinary newsletters that responded so enthusiastically to the ovaries and longevity paper also give attention to this new paper in JAVMA. I suspect that they won't, as this paper is not really very surprising, but it does relate (and contradict) to the paper about rottweilers they were so excited about a month or two ago.