"Our female dog was spayed when she was 4-5 months old. She is now 1-1/2 years old and bleeding; our male dog keeps trying to mate with her. The vet told my husband that sometimes this happens and that the way the dogs uterus is sometimes they miss an ovary and have to go back in. I cannot see that this is possible if they removed the uterus. Does anyone know if this is possible?"
In the dog and cat, the uterus is shaped like a very long "Y." The common stem (base of Y) is very short relative to the long "arms" or horns of the uterus. The horns are where the puppies and kittens are formed, attached, and grow during gestation.
The ovaries, while separate from the uterus, are attached via ligaments and blood vessels to both the uterine horns and body. The veterinarian must separate the ovaries from the attachments in the body by clamping and tying off (ligating) the blood vessels.
The uterus is then removed at the body (base of the "Y"), usually above the cervix so that the cervix remains in the body.
Each ovary is in a sac. The sac is often filled with fat - more so in older or overweight animals. Sometimes the ovarian tissue is diffuse, sometimes it is very small or not well formed at the time of the spay. It may also be ectopic, meaning it isn't where it should be in the body (a congenital problem).
In these abnormal situations, when the veterinarian clamps the tissue to ligate the blood vessels, very small bits of ovarian tissue may remain in the body after the surgery. This tissue can then grow and respond to chemical signals from the brain to produce the hormones that cause the heat cycle (estrus).
How then can the pet actually bleed if the uterus and ovaries have been removed? The lining tissue of the remaining vagina can swell and bleed in response to the hormones, simulating a heat cycle. Unless the uterus was not removed, your pet will not get pregnant.
Other possible situations that could simulate a heat cycle could be a vaginal or bladder infection, so your veterinarian will need to examine your pet to be sure.