Muffy is a 10 year old, intact female, black lab. She has never had puppies and presents today for an ovariohysterectomy (spay). She has been generally healthy although her heat cycles have been a little irregular in the past year or so. The last 2 cycles were only about 3 months apart, the last about 1 month ago. On physical examination she has severe tartar, is slightly overweight, and a skin tag on her abdomen. The exam is otherwise considered normal. The preanesthetic labs are normal. You expect her to be a routine spay, although given the irregular heat cycles there is concern about ovarian problems.
In surgery you find this:
While not alarming, the uterus is slightly bigger than you expected and the ovaries appear to be cystic. There is also a cystic structure within the uterus at the level of the cervix. Surgery continues routinely and she makes a normal recovery. After surgery you examine the ovaries and the inside of the uterus. The ovaries have many large cystic structures present. The uterus has some mucus present and the walls are very cystic and hypertrophied.
The reproductive system in the dog has some unique features. While most animals have predictable cycling, the dog does not. It only cycles about twice per year, but unpredictably and ranging from 1 to 3 times per year. Dogs do not slough the lining of their uterus. The lining just keeps building up and up. Every time a dog goes into heat, her uterus and ovaries behave as though she is pregnant for about 45 days. The ovaries maintain the pregnancy for this long. When the ovarian activity regresses, we will often see her behave as though had puppies. This can take the form of nesting, behavioral changes, and even lactation.
The important aspect of this for Muffy and many other older female dogs is the accumulation of the uterine lining. This lining ages and its immune system does not work as well. The uterus is not quite a sterile place (the cervix is open during the heat cycle), so bacteria are present. If the immune function is compromised enough, infection can start and cause a variety of problems like a pyometra (the uterus fills with pus, essentially a huge abscess).
Additionally, older females commonly have some ovarian problems causing persistent hormonal stimulation of the uterine lining. A relatively high percentage will go on to develop pyometra. The solution for this is to spay the female dogs after they are done reproducing.
Muffy's history and the findings during her ovariohysterectomy suggest strongly that she was moving towards a pyometra. It was good to spay her now while she is still healthy. Taking a critical patient to surgery carries many risks.