What Is Pyometra in Dogs? (Causes, Types and Risks)

Pyometra is a bacterial infection of the uterus in which pus collects inside the womb, and it happens almost exclusively in unspayed female dogs, usually in the weeks following a heat cycle. If your intact female dog is suddenly flat, off her food, drinking and urinating more than usual, vomiting, or passing a discharge from her vulva, call your veterinarian now or go straight to an emergency clinic. Pyometra does not resolve on its own, and the gap between a dog who looks a little off and a dog in septic shock can be very short.

The word itself is literal: pyo for pus, metra for uterus. The uterus fills with infected fluid, the wall stretches, bacteria and the toxins they release leak into the bloodstream, and the kidneys, liver and circulation all come under strain. That is why this is classed as a surgical emergency rather than an infection that can be watched for a few days. If you are reading this with a dog in front of you who seems unwell, stop here and phone a clinic first.

What causes the infection, and how dogs get it

Pyometra is a hormone driven disease with a bacterial ending. It is not caused by a dirty environment, a missed bath, a mating, or anything an owner did wrong.

Every time a female dog comes into season and ovulates, progesterone rises and stays high for weeks, whether or not she is pregnant. Progesterone does several things to the uterus at once. It thickens the lining so an embryo could implant. It encourages the uterine glands to secrete fluid. It relaxes the muscular wall so the uterus contracts less. And it dampens the local immune response inside the uterus, because a developing pregnancy must not be rejected as foreign tissue.

Repeat that cycle year after year with no pregnancy, and the lining can develop cystic endometrial hyperplasia, a thickened, cystic, fluid rich surface. That tissue is an unusually good place for bacteria to live.

During a heat the cervix opens. Bacteria that normally sit harmlessly in the vagina, lower urinary tract and skin can travel upward into the uterus at that point. Escherichia coli, a bacterium that lives in the dog's own gut, is commonly identified in these cases. In a healthy non cycling uterus those bacteria are cleared. In a progesterone primed, cystic uterus with fluid pooling in it, they multiply instead, and pus builds behind a cervix that is closing again.

So the honest answer to how did my dog get pyometra is this: she got it from her own normal reproductive hormones, acting on a uterus that has cycled many times, plus her own ordinary bacteria. Certain hormone medications, including oestrogen given after an unwanted mating and progestin based drugs used to suppress or delay seasons, can also increase risk by exaggerating the same hormonal picture. If your dog has ever received one of these, mention it to your vet.

Open, closed and stump: the forms pyometra takes

There are two main types, defined by whether the cervix is open enough to let the infected material drain. The distinction matters because it changes what you will see at home, not how serious the disease is.

Feature	Open pyometra	Closed pyometra
Cervix	Partly open, so uterine contents can drain out	Sealed, so pus is trapped inside
What you see	Bloody, yellow, brown or creamy discharge from the vulva, often with a strong smell; dog may lick the area constantly	Usually no discharge at all
Typical presentation	Illness plus an obvious visible clue	Vague illness: lethargy, poor appetite, thirst, vomiting, sometimes a swollen belly
Why it is dangerous	Still a systemic infection that can progress to sepsis	Pressure builds with nowhere to go, raising the risk of rupture and peritonitis; often diagnosed later
Owner takeaway	Easier to spot, still an emergency	Easier to miss, and frequently the sicker dog by the time it is found

Closed pyometra is the one that catches people out. There is no discharge to find, so the only signals are a dog who is tired, drinking a lot, eating little, maybe vomiting, maybe with a tense or enlarged abdomen. Owners often describe it as their dog just not being herself.

There is a third, uncommon form. Stump pyometra occurs when a small amount of uterine tissue remains after a spay and is exposed to hormones, usually because a fragment of ovarian tissue was left behind or, rarely, because of hormone medication. That remnant tissue can become infected in the same way. It is unusual, but it is the reason a spayed dog with discharge, thirst and lethargy should still be examined rather than ruled out on history alone.

Which dogs are most at risk

The single biggest risk factor is being an intact adult female. Beyond that, the pattern veterinary teams see most often includes:

• Middle aged and older intact females, because risk accumulates with the number of heat cycles a dog has been through
• Dogs who have never had a litter, or who have not had one recently
• Dogs who have received oestrogen or progestin containing drugs
• Dogs with a history of irregular seasons, prolonged bleeding or recurrent false pregnancies
• Any intact female in the weeks after a season, which is the classic window

Is it common? Pyometra is one of the more frequently seen emergencies in unspayed middle aged and older female dogs, familiar enough that most general practices recognise the picture quickly. Published reports describe breed differences in risk, and research suggests the condition is more frequent in some breeds than others, but the practical message does not change with breed: if she is intact and she has had seasons, she can develop it.

Spaying before these changes develop removes the uterus and ovaries and therefore removes the disease from the picture almost entirely. The best timing for an individual dog depends on breed, size, orthopaedic risk and lifestyle, so talk to your vet about when, rather than treating it as a fixed rule.

Warning signs that mean a vet visit today

Call a clinic if an unspayed female dog shows any of the following, particularly within two months of a season:

• Marked increase in drinking and urinating
• Loss of appetite or refusing food entirely
• Lethargy, weakness, reluctance to move or collapse
• Vomiting or diarrhoea
• Vaginal discharge of any colour, especially if it smells
• A distended, tense or painful abdomen
• Panting, shaking, or a temperature that seems high or unusually low
• Pale gums, a fast heart rate, or a dog who is difficult to rouse

None of these are specific to pyometra on their own, which is exactly why they need a professional eye. Increased thirst and urination, for example, can reflect the effect of bacterial toxins on the kidneys. A dog who is still bright but drinking more than usual after a heat still warrants a same day call. For a broader picture of what counts as an emergency in dogs generally, read the emergency signs guide linked below.

How vets diagnose and treat it

Diagnosis usually combines history, physical examination and imaging. A vet will ask when the last season was, palpate the abdomen carefully, and in most cases use ultrasound or radiographs to look for an enlarged, fluid filled uterus. Bloodwork assesses white cell counts, kidney values, hydration and signs of systemic infection, and urine testing is common. Vaginal cytology and culture may be used when there is discharge.

The standard of care is surgical removal of the infected uterus and ovaries, an ovariohysterectomy performed as an emergency rather than as a routine spay. It is a more demanding procedure than an elective spay because the uterus is fragile and the patient is often unstable. Intravenous fluids, antibiotics and pain relief support the dog before, during and after surgery, and some dogs need a period of hospitalisation.

Medical management, using drugs that block progesterone and encourage the uterus to empty, is sometimes considered for stable dogs with open pyometra who are genuinely valuable for breeding. It is not a softer option. It requires close monitoring, it is not suitable for closed cases or for dogs who are already systemically ill, and recurrence at a later season is possible. Results vary from dog to dog, and the decision belongs with the veterinarian who has examined the patient and seen the bloodwork.

What prevention actually looks like

There is no supplement, diet, hygiene routine or home remedy that prevents or manages pyometra. The only reliable prevention is removal of the reproductive organs, which is why spaying is discussed so consistently in this context.

For owners keeping a dog intact by choice, prevention becomes vigilance: know the date of every season, watch closely for the following two months, weigh her regularly, and act on subtle changes in thirst, appetite and energy rather than waiting to see. Keep a record of any hormone medication she has received. And raise the question directly at her next check up, because the right plan for an intact female changes as she ages.

Key Takeaways

• Pyometra is a pus filled bacterial infection of the uterus, seen almost only in unspayed females, typically in the weeks after a heat cycle.
• The cause is hormonal rather than hygienic: repeated progesterone exposure changes the uterine lining and lowers local defences, allowing the dog's own bacteria to take hold.
• Open pyometra drains visible discharge; closed pyometra traps pus inside and is often found later and in a sicker dog.
• Risk rises with age and with the number of seasons a dog has had, and hormone medications can increase it further.
• Stump pyometra can affect a spayed dog when uterine and ovarian tissue remain, so spayed dogs with these signs still need examining.
• Emergency surgery to remove the uterus and ovaries, with fluids and antibiotics, is the standard treatment.
• Any suspected case is a same day call, not a wait and see situation.

Your veterinarian owns the diagnosis and the treatment plan here, and with pyometra that ownership is absolute: this is a condition diagnosed with imaging and bloodwork and resolved in an operating theatre. Nothing given at home substitutes for that, and nothing should delay it. If you are unsure whether what you are seeing is serious, make the call anyway, because an unnecessary phone call costs far less than a missed one.

Related reading: Dog Emergency Signs.