Pyometra is a serious bacterial infection of the uterus that occurs in unspayed female cats. Left untreated, it can be fatal, with mortality rates around 6% even among cats that receive surgical treatment. The infection develops when hormonal changes after a heat cycle create conditions inside the uterus that allow bacteria to thrive, and the body can’t clear the infection on its own.
How Pyometra Develops
The process starts with progesterone, the hormone that rises after a cat ovulates. Each time a cat goes through a heat cycle, estrogen primes the uterine lining, then progesterone takes over. Over repeated cycles, progesterone causes the glands lining the uterus to thicken, enlarge, and fill with fluid. This thickened, fluid-filled state is called cystic endometrial hyperplasia, and it sets the stage for infection.
Progesterone does more than just change the uterine lining. It suppresses the local immune response inside the uterus, stimulates gland secretions that create an ideal environment for bacteria, causes the cervix to close (trapping whatever is inside), and reduces the muscular contractions that would normally help the uterus clear debris. Bacteria from the vagina, most commonly E. coli, migrate upward into this warm, nutrient-rich, immune-suppressed environment and multiply rapidly. The result is a uterus filling with pus.
Open vs. Closed Pyometra
There are two forms of pyometra, and the distinction matters because it affects how sick the cat gets and how quickly you’ll notice something is wrong.
In open pyometra, the cervix remains partially open, allowing pus to drain out of the body. You’ll typically notice a foul-smelling, cream-colored or bloody vaginal discharge. Because the infection is draining rather than building up pressure, cats with open pyometra tend to be less critically ill and are usually brought to the vet sooner simply because the discharge is hard to miss.
In closed pyometra, the cervix seals shut, trapping all the infected material inside the uterus. There’s no visible discharge, which makes it harder to recognize. The toxins from the bacteria absorb into the bloodstream, and the cat becomes significantly sicker. Closed pyometra also carries a much higher risk of the uterus rupturing. In one study of 120 cats treated surgically, 4% experienced uterine rupture, and cats with closed pyometra had roughly 17 times greater odds of rupture compared to those with the open form.
Signs to Watch For
The symptoms depend on whether the pyometra is open or closed, but most cats show some combination of the following:
- Vaginal discharge (cream-colored or bloody, present only in open pyometra)
- Lethargy and loss of appetite, often the earliest and most commonly reported signs
- Increased thirst and urination, caused by toxins affecting kidney function
- Swollen or painful abdomen
- Vomiting
- Fever
- Pale gums, weakness, or collapse in advanced cases
Pyometra typically develops within a few weeks after a heat cycle. If your unspayed cat seems unusually tired, is drinking more water than normal, or has any vaginal discharge, those signs together should prompt an urgent vet visit. Cats with closed pyometra can deteriorate quickly because there’s no external sign until systemic illness sets in.
How Vets Diagnose It
A vet will suspect pyometra based on the cat’s history (unspayed, recent heat cycle) and symptoms. The key diagnostic tool is ultrasound, which reveals a uterus distended with fluid. In a normal cat, the uterus is too small to see clearly on imaging. In pyometra, the uterine horns can swell dramatically. One published case showed a uterus expanded to 2.7 cm in diameter, and both horns could be traced all the way up to the kidneys.
Ultrasound can confirm that the uterus is enlarged and fluid-filled, but it can’t always distinguish pus from other types of fluid buildup. The combination of imaging findings, bloodwork showing signs of infection (elevated or abnormally low white blood cell counts, anemia, dehydration), and clinical symptoms gives the vet enough information to move forward with treatment.
Surgery Is the Standard Treatment
The primary treatment is surgical removal of the uterus and ovaries, essentially the same procedure as a spay but performed on an infected, fragile organ. This is curative: once the uterus is gone, pyometra cannot recur.
Before surgery, most cats need stabilization with IV fluids, pain medication, and antibiotics to address dehydration and infection. This pre-surgical support can make a significant difference in outcome, especially for cats that are already critically ill. The surgery itself is more complex than a routine spay because the uterus is enlarged, inflamed, and at risk of leaking or rupturing during handling.
Recovery typically takes about two weeks. Your cat will likely come home with pain medication and either a cone or a recovery suit to prevent licking at the incision. External stitches are uncommon for this procedure, but if they’re used, they come out at the two-week follow-up. During recovery, check the incision daily for redness, discharge, or excessive bruising. Antibiotics are often continued for several weeks after discharge.
Medical Treatment as an Alternative
For cats with breeding value, there is a non-surgical option: hormone injections that cause the uterus to contract and expel the infected material. This approach only works with open pyometra, where the cervix allows drainage. Using it on a closed pyometra is dangerous because the contractions have nowhere to push the pus, increasing the risk of rupture.
In a study of 21 cats treated this way, 95% resumed normal heat cycles and 85% went on to produce healthy litters. Those are encouraging numbers, but medical management requires close veterinary monitoring over several days, carries more side effects, and doesn’t prevent the condition from returning after future heat cycles. For the vast majority of cat owners, surgery is the safer and more definitive choice.
Why Spaying Prevents Pyometra Entirely
Pyometra cannot develop without a uterus and the hormonal cycling that drives it. Spaying removes both. Any unspayed female cat is at risk, and the risk increases with age as the uterine lining accumulates changes from repeated heat cycles. Cats that have never been bred are not protected; they cycle hormonally whether or not they mate, and each cycle adds to the progressive thickening that makes infection more likely.
For cats not intended for breeding, spaying before the first or second heat cycle eliminates pyometra risk completely, along with reducing the risk of mammary tumors. For cats already diagnosed with pyometra, the surgical cure is the same procedure, just performed under emergency conditions with a sicker patient and a higher complication rate. Early spaying is, by a wide margin, the easier path.