Mares certainly get uterine infections–I’ve seen a ton of threads on the breeding forum about it. I don’t know about cats.
I’d suspect that long haired or low slung breeds are more susceptible to pyos, but that’s just a guess. I’d think the more hair or the lower to the ground the dog is, the greater chance of bacteria getting stuck in there. The study I posted previously did find different rates in different breeds, although I think you need access to the full article to view the data.
A quick google turns up this:
http://www.thedogplace.org/Reproduction/Estrus-Bitch-Wolf-Barber-DVM.asp
In other species (human, horse, cow sheep, goat, pig,and cat to name a few), progesterone levels drops after about two weeks of diestrus if the female does not become pregnant. Once progesterone levels are low, the “off” switch is released, uterine defenses gear up and the uterus can clear itself of any pathogens. In the bitch, however, uterine defense systems remain “off” for a much longer period of time (45 days or longer). When pathogens invade the uterus and are not removed during estrus, they are protected within the uterus and allowed to multiple for the entire diestral period. This explains why we see pyometra commonly in the bitch and rarely in the other domestic species. And this is why you should think of pyometra as not just an infection. It is also a hormonally induced disease.