Spaying dogs and cats: how much should I take out?

Spaying of dogs and cats is such a common and important procedure that it is the only surgery you are guaranteed to get to do all by yourself (twice!) at my veterinary school. Traditionally, vets have taken out the whole package (the uterus and both ovaries). Leave the ovaries in, and the animal still goes into heat, even if she can’t get pregnant, and you are liable to have an irritated owner on your hands. But is there a good reason to take the uterus out, or can you leave it in? More and more veterinarians are starting to think that less is more.

The commentary “Ovariohysterectomy versus ovariectomy for elective sterilization of female dogs and cats: is removal of the uterus necessary?” provides an overview of the current arguments for and against ovariectomy (removing only the ovaries, abbreviated OVE) versus the more traditional ovariohysterectomy (removing the uterus and the ovaries, abbreviated OVH).

The reasons to leave the uterus in are pretty obvious. You can make a smaller incision if you are only taking out the ovaries, and smaller incisions are obviously preferable where possible. While you’re at it, you can center your incision over the ovaries instead of having to center it further towards the animal’s tail so as to get the uterus as well. The ovaries can be difficult to fully visualize, as they can be tucked deep into the abdomen; placing the incision further towards the animal’s head makes it easier to see what you’re doing, so you can be sure to get the whole thing and not leave little bits of ovary behind. If you leave little bits behind, the animal can still go through heat cycles. This happens more often than you might think.

Finally, removing fewer organs leaves fewer chances for the surgeon to make a mistake. Mistakes do happen, especially with less experienced surgeons. Specifically, a surgeon could ligate (tie a suture around) something that should not be ligated, like a ureter. (Tie a suture around a ureter and the animal is going to have significant problems with one kidney, to say the least.) Alternatively, a surgeon could fail to sufficiently ligate something that needed that ligation to stop bleeding, resulting in hemorrhage into the abdomen. These complications would theoretically be somewhat less common with OVE than with OVH, because, with fewer organs to tie off before removal in OVE, fewer ligations are required.

Unfortunately, research has not yet been done to assess the frequency of such complications with OVE, so the benefit is just theoretical. Moreover, we have no evidence that the smaller incision in OVE makes any difference to the animal’s pain levels. Post-spay animals do not appear to require less pain medication after OVE compared to OVH.

Another consideration in choosing OVH over OVE is pyometra, a disease most commonly found in unspayed animals. As an animal ages, its uterus becomes less able to fight off bacterial invaders, and infection of the uterus can be a big (life-threatening) deal. To avoid the problem, remove the uterus.

However, although the causes of pyometra are not fully understood, we do know that it doesn’t happen unless progesterone levels are elevated, as happens during the estrous cycle. And animals without ovaries don’t get elevated progesterone levels unless we give them progesterone, something we don’t generally do to dogs and cats. This means that animals who have only their ovaries removed won’t get pyometra, even though the uterus (the infected organ in this disease) is left behind, because they won’t be going through heat cycles which result in elevated progesterone levels.

In practice, veterinarians do sometimes see animals who had only their ovaries removed get pyometra — but only if little bits of ovary were mistakenly left behind (ovarian remnant syndrome). So if you remove the ovaries properly, the animal will not be at risk for pyometra. And, as discussed earlier, it’s theoretically easier to remove the ovaries properly if you center your incision over them and leave the uterus in place.

The authors conclude by arguing that OVE is the preferable procedure, due to the theoretically reduced complication rate. Personally, I really like the less is more approach to surgery; if you can leave it in, I think you should. But I do wish we had some more solid evidence in support of OVE. Time for some clinical studies comparing the two procedures!

DeTora, M., & McCarthy, R. (2011). Ovariohysterectomy versus ovariectomy for elective sterilization of female dogs and cats: is removal of the uterus necessary? Journal of the American Veterinary Medical Association, 239 (11), 1409-1412 DOI: 10.2460/javma.239.11.1409