I read that it's common for unspayed female cats to get pyometra. I thought maybe that's because people don't allow their cats to mate when in heat, which causes the illness. But if I allow my cat to mate, will she still be at the risk of developing pyometra? Why would chances of having it decrease or increase as a result of mating?
Yes, pyometras can happen after insemination, after copulation, after a heat cycle, after administrating estrogens and after parturition. It happens from progesterone lasting longer than usual in the uterus, it can even happen after a spay! There is always some sort of stump left from the uterus where the infection can make way.
It's more common in dogs than cats as cats need to be sexually stimulated in order to ovulate thus releasing progesterone, however she doesn't need stimulation to have a pyo.
Pyometras are most common in middle-age to senior pets, complications in hormone balance increase as they age. though it does on occasion happen in young cats and dogs.
Clinical signs of a pyometra are:
- Lethargy, non-responsive.
- Not want to eat or drink.
- White or dark bloody discharge from vulva.
- Swollen abdomen
Pyometras are an EMERGENCY and need to be corrected immediately - this is done by spaying the animal and being placed on antibiotics.
Here's a good article on the subject from the Merck Manual. Fee free to ask questions if any words confuse you, medical journals always use fancy words.