A middle-aged female dog finishes a normal heat cycle. Four to six weeks later she goes quiet. She drinks more water than usual, picks at her dinner, maybe vomits once. Her owner assumes she's tired, or that something disagreed with her. Two days after that she is in emergency surgery with a uterus the size of a football, full of pus, leaking bacterial toxins into her bloodstream.

That's pyometra, and it is one of the most common — and most lethal — conditions in unspayed female dogs. Cornell University's Riney Canine Health Center describes it plainly as a life-threatening infection requiring urgent veterinary care. What makes it so dangerous isn't that it's rare or exotic. It's that the early signs are vague enough to wait on, and waiting is exactly the wrong move.

What Pyometra Actually Is

Pyometra is a bacterial infection of the uterus with accumulation of pus inside the uterine cavity. It doesn't come out of nowhere — it's the endpoint of a process veterinarians call the cystic endometrial hyperplasia–pyometra complex.

Here's the sequence. After every heat cycle, whether or not a dog is bred, she enters diestrus, a phase lasting roughly two months during which progesterone dominates. Progesterone does several things at once: it thickens the uterine lining and stimulates its glands to secrete fluid, it keeps the cervix closed, it reduces the muscular contractions that would normally flush the uterus out, and it suppresses local immune defenses. Repeated over many cycles, this remodels the endometrium into a thickened, cyst-studded, fluid-filled tissue — cystic endometrial hyperplasia.

That environment is close to ideal for bacteria. Organisms from the normal vaginal flora travel up through the cervix, which is briefly open during heat, and find a warm, nutrient-rich, poorly defended space waiting for them. Escherichia coli is the culprit in roughly 70% of cases, and research has shown that progesterone actually increases the endometrium's affinity for E. coli adhesion — the hormone doesn't just fail to protect, it helps.

Who Gets It, and When

Pyometra is a disease of intact (unspayed) females, and risk climbs steeply with age and with the number of cycles a dog has been through. Frequently cited figures put lifetime risk at roughly one in four unspayed bitches by age 10, though estimates vary widely by population and breed. The mean age at diagnosis in most studies is around 7 years.

Breed matters more than many owners expect. Large-scale insurance and hospital data have identified dramatically elevated rates in certain breeds — Leonbergers, Irish Wolfhounds, Bernese Mountain Dogs, Great Danes, Saint Bernards, Rottweilers, and Golden Retrievers all appear repeatedly near the top of the list, with some breed-specific lifetime incidences reported near 70%.

Timing is the most useful clue for owners. Pyometra characteristically develops in the four to eight weeks following a heat cycle, when progesterone is at its peak. If your intact female was in heat last month and is now off her food, that connection deserves a phone call to your vet — not a wait-and-see week.

Open vs. Closed: The Distinction That Decides Everything

Pyometra comes in two forms, and the difference determines both how obvious it looks and how fast it kills.

Open pyometra means the cervix is still open, so infected material drains out. The hallmark is a foul-smelling vaginal discharge — creamy, yellow-green, or blood-tinged — which owners often find on bedding before they notice anything else. The dog is still sick, but the uterus is decompressing itself, which buys time and makes diagnosis easier.

Closed pyometra means the cervix is sealed. Nothing drains. Pus accumulates under pressure, the uterus distends, bacterial endotoxin floods the circulation, and the dog deteriorates quickly. There is no discharge to see, which is precisely why closed pyometra is missed until the dog is critically ill. Published research confirms that dogs with a closed cervix present with significantly more severe illness than those with open pyometra.

Sepsis is common either way: studies have found systemic inflammatory response or sepsis in roughly 60% of dogs presenting with pyometra.

Warning Signs to Take Seriously

The list below is deliberately mundane, because that's the problem — none of these individually screams "emergency."

Any of these in an unspayed female who was in heat within the last two months should be treated as a possible pyometra until proven otherwise.

How Vets Diagnose It

Diagnosis is usually quick. Bloodwork typically shows a markedly elevated white blood cell count, often with immature neutrophils, along with dehydration and sometimes evidence of kidney involvement. Abdominal ultrasound is the most reliable imaging tool — it shows a fluid-filled, enlarged uterus and helps distinguish pyometra from pregnancy or other causes of uterine distension. Radiographs may reveal a tubular soft-tissue mass in the caudal abdomen. A vaginal cytology and culture can identify the organism when discharge is present.

Treatment: Surgery Is the Standard

The definitive treatment is ovariohysterectomy — surgical removal of the infected uterus and the ovaries. It is the same procedure as a routine spay in principle, but not in practice: the patient is systemically ill, the uterus is fragile and can rupture, and stabilization with intravenous fluids and antibiotics comes first. It is emergency surgery performed on a compromised patient, which is one reason it costs several times what an elective spay does.

Prognosis with prompt surgery is good. Most dogs recover well and are discharged within a few days. Left untreated, pyometra progresses to septic shock, uterine rupture, peritonitis, and death.

Medical management — typically the progesterone-blocker aglepristone, often combined with a prostaglandin and antibiotics — exists as an alternative, but it is reserved for a narrow group: young, stable, valuable breeding females with open-cervix pyometra and no other uterine pathology. Reported short-term resolution is high (one study reported 92.3% clinical resolution with aglepristone), and average long-term success across studies is around 86%. The catch is recurrence: mean recurrence rates of roughly 29% are reported, with wide variation between studies, and one long-term Swedish follow-up found nearly half of medically treated dogs had another episode. Aglepristone is also not licensed in all countries, including the United States. For a dog who is not going to be bred, surgery is the better answer.

Prevention Is Nearly Absolute

Spaying removes the uterus and ovaries, and with them essentially all pyometra risk. This is the single strongest medical argument for spaying a female dog who will not be part of a planned breeding program, and it's worth weighing against the joint and cancer considerations that have made spay timing a more nuanced conversation in recent years — the right answer is a discussion with your vet about your specific dog's breed and size, not a blanket rule.

Two caveats. First, stump pyometra can occur in a spayed dog if a small remnant of uterine tissue is left behind and residual ovarian tissue is present to stimulate it. It's uncommon, but it means a spayed dog with purulent vaginal discharge still needs to be seen. Second, dogs given estrogen or progesterone-based hormone treatments are at increased risk, so those drugs should be used cautiously in intact females.

What About Cats?

Cats get pyometra too, but less often. Because queens are induced ovulators — they generally only ovulate after mating — they spend far less time under prolonged progesterone influence than dogs do, which is protective. When feline pyometra does occur, the presentation and treatment are broadly the same, and surgery is likewise the treatment of choice.

The Takeaway

Pyometra is common, it is deadly, and it is almost entirely preventable. If you have an intact female dog, put a note in your calendar at every heat cycle and stay alert for the following two months. Lethargy, reduced appetite, increased drinking, or any vaginal discharge in that window is a same-day veterinary visit — not a wait-until-Monday problem. The dogs who do badly with pyometra are, overwhelmingly, the ones who arrived late.