Pyometra or uterine infection

Deactivating this “time bomb” in time 💣🐶 

Pyometra is one of the main surgical emergencies we treat in veterinary medicine. It is a serious, progressive and, in many cases, potentially fatal uterine infection. The most alarming thing is that it usually progresses without obvious signs until the situation becomes critical. Many affected females arrive at the clinic already in shock, with fever, vomiting or a distended abdomen, and require emergency surgery to save their lives.

But what makes pyometra such a frustrating disease is that it is completely preventable. The solution is within reach of all guardians: early and scheduled sterilisation, which involves the removal of the uterus and ovaries, eliminates the possibility of this infection developing.

Knowing about this disease, its causes and its progression not only helps to detect it early, but also allows us to act responsibly before it appears. Committing to prevention is undoubtedly one of the most generous things we can do for the health and well-being of our furry companions.

What exactly is pyometra (and why does it occur ‘after the heat cycle’)?

Pyometra is a serious infection of the uterus that occurs in unspayed females. In most dogs, it typically occurs between 3 and 8 weeks after heat, when progesterone, a hormone that prepares the uterus for possible pregnancy, is predominant.

Why is it called “pyometra”? It comes from the Greek: pyo- (pus) and -metra (uterus). An ugly name, yes… but very honest. Sometimes a little etymological fact opens the door for the guardian to remember the concept. 

In a population study based on veterinary insurance data in Sweden, it was estimated that approximately 23–24% of intact female dogs developed pyometra before the age of 10, with significant variations depending on breed. In another case-control study matched by breed and age (87 pairs) on risk factors, a very powerful piece of information for guardians is highlighted: pyometra affects almost 25% of intact female dogs before the age of 10, and there is a strong predisposition by breed. Furthermore, when analysing interactions by breed, previous pregnancy appeared as a variable associated with pyometra. 

Here’s where it gets interesting (and why it’s so dangerous): progesterone causes several changes that, when combined, create a “perfect breeding ground” for bacteria to thrive:

• It promotes cystic endometrial hyperplasia (CEH): the endometrium (inner lining of the uterus) becomes more glandular and can secrete more fluids. In layman’s terms: the uterus becomes a “comfortable” place to accumulate fluid.
• It reduces uterine contractions (decreases the natural ‘cleaning’ of the uterus).
• It tends to keep the cervix (neck of the uterus) more closed, especially at certain times of the cycle.
• It modulates local immunity: the uterus is less able to defend itself against opportunistic bacteria.

Where do these bacteria come from? They often ascend from the vagina. The most common in female dogs is Escherichia coli, which in various studies appears to be responsible for a high percentage of cases (often more than half, reaching very high figures depending on the series and culture method). Some strains produce endotoxins (bacterial components that trigger systemic inflammation), and that is where things can get serious.

Open vs. closed pyometra: same disease, different “disguise”

Pyometra usually presents in two clinical forms:

Open pyometra

The cervix allows the contents to escape, and discharge may be seen from the vulva (sometimes purulent, sometimes bloody, often with a foul odour). It is “easier” to detect at home, but that does not mean it is mild.

Closed pyometra

The cervix remains closed and the pus is retained inside the uterus. Here, the abdomen may become distended and the infection may progress without obvious signs from the outside. This is the form that concerns us most because it can silently develop into serious complications.

Did you know 🤫: “Closed pyometra is the queen of camouflage.”

When the cervix is closed, there may be no visible discharge, so the owner does not see “anything unusual” until apathy, fever or vomiting appear. Useful phrase for box: “Just because there’s no discharge doesn’t mean it’s not there.” 

In both cases, we are talking about a veterinary emergency. The difference is that closed pyometra can “take you by surprise”.

Warning signs at home: what should NOT be normalised ⚠️

Pyometra does not always start with dramatic symptoms. In fact, owners often say, “She was acting strange, like she was off.” That phrase, in an intact female weeks after heat, is a very valuable clue.

The most common signs include:

• Lethargy and reduced interest in walking or playing.
• Loss of appetite; sometimes nausea or vomiting.
• Increased thirst and urine output: this is called polyuria-polydipsia (PU/PD).
• Simple explanation: some bacterial toxins and the inflammatory response can interfere with the kidney’s ability to concentrate urine, and the dog drinks more to compensate.
• Fever (not always present).
• Vulvar discharge (if open): not always “obvious pus”; sometimes it is a more discreet discharge.
• Distended abdomen (more typical when the uterus is greatly enlarged, especially if closed).
• Faster breathing, restlessness, weakness, pale mucous membranes: signs that may indicate a serious systemic condition.

Your vet advises 🩺: the “Heat + 1 month” rule

If your dog is not spayed and it has been 3–8 weeks since her heat, pay special attention to this triad: lethargy + decreased appetite + increased thirst/urination. With this combination, do not wait until tomorrow: an early check-up can change the prognosis.

With pyometra, time is of the essence. Better a silly phone call than a serious complication. If something smells fishy (literally or metaphorically), get it checked out.

How do we confirm it in the clinic? (diagnosis without mystery)

The diagnosis is usually based on three pillars:

1) Clinical examination

We assess hydration, temperature, abdominal pain, circulatory status, and signs of systemic inflammation.

2) Blood and urine tests

We often see leukocytosis (high white blood cell count) and, in severe cases, a “left shift” (more immature neutrophils, a sign of an intense response). There may be alterations in renal function (urea/creatinine) if there is dehydration, toxaemia, or secondary renal involvement. Changes in proteins (hyperglobulinaemia) and other alterations may also occur, depending on the case.

3) Imaging (ultrasound and sometimes X-ray)

Abdominal ultrasound is very useful: it allows us to see a dilated uterus with contents consistent with pus or inflammatory fluid. X-rays may show an enlarged uterus if it is very distended, but ultrasound gives us a more accurate and rapid confirmation.

Why it can be fatal: sepsis, shock and the “domino effect” 🧬

When the uterus becomes infected and filled with purulent material, the body enters a state of intense inflammation. In some cases, bacteria or their toxins enter the bloodstream and trigger SIRS (systemic inflammatory response syndrome) and sepsis.

In layman’s terms: the body “sounds the alarm,” and this can affect multiple organs:

• Kidneys (decreased perfusion, changes in urine concentration)
• Liver (changes in enzymes)
• Cardiovascular system (risk of shock in severe cases)
• Coagulation (alterations that increase surgical risk in unstable patients)

Pyometra does not kill solely “because of the uterus”: it kills because of the inflammatory storm it can unleash.

Treatment: why we almost always talk about surgery

Quick myth 🚫: “Antibiotics at home will fix it”

Pyometra is usually a problem of the uterine cavity with content that does not drain well (and with a hormonal component). Antibiotics can “mask” signs and delay the problem, but often do not resolve the source. Ideal box for educating without scaring: “It may improve on the outside while getting worse on the inside.” 

Is there a medical treatment?

Yes, but with very specific indications (e.g., selected breeding females, open pyometra, stable patient, no signs of sepsis, and very close monitoring). Therapy can be used to empty the uterus and block the effect of progesterone (depending on the protocol, with specific drugs). Even so, two points must be made clear:

1. It is not “the easy way out”: it requires check-ups, ultrasounds, and tests.
2. There may be recurrence: the underlying problem (uterine changes + hormonal dynamics) may repeat itself in future cycles.

In practice, when life is at stake, surgery is usually the safest option.

The best option (without a doubt): ovariohysterectomy (OHE)

Ovariectomy is the surgical removal of the ovaries and uterus. In pyometra, it is, in most cases, the treatment of choice because:

• It eliminates the source of infection.
• It cuts off the hormonal stimulus (ovaries) that promotes the condition.
• It reduces the risk of relapse (which can be high with medical treatments).

Before operating, we stabilise the patient: fluid therapy, analgesia, control of vomiting if necessary, appropriate antibiotics and monitoring. In complicated cases, this “before” is as important as the surgery itself.

In a retrospective series of 405 female dogs treated with ovariohysterectomy (OHE) in a non-specialised setting, survival at discharge was 97% (394/405). Three per cent had uterine rupture and 44% required hospitalisation for ≥2 nights.

In another series of 134 cats (queens) with pyometra treated surgically (OHE), survival at discharge was 100% (126/126; 95% CI 97–100%). In addition, useful clinical findings were reported: frequent dehydration (69%) and absolute neutrophilia (83% in those reported). 

Prevention that really works: scheduled sterilisation

Here comes the “anti-drama” part: pyometra is very effectively prevented by sterilisation.

Sterilisation is not just about “avoiding litters”. It is also about:

• avoiding emergency surgery,
• reducing the risk of uterine pathologies,
• and, depending on the timing of the sterilisation and the case, influencing the risk of mammary tumours (a topic with nuances and scientific debate, but with data in favour of a reduction if sterilisation is performed early).

When is the best time to spay?

The ideal answer depends on the dog: size, breed, age, development, family context, and clinical criteria. On a purely uterine level, scheduled sterilisation, with a stable and well-prepared patient, is more predictable than emergency surgery for pyometra (where we arrive with infection, dehydration, pain, and greater anaesthetic risk).

Many clinics recommend avoiding surgery during heat due to vascularisation issues, and look for a safer window in the cycle. The practical key is different: don’t wait to “see if it goes away” when there are suspicious signs, and don’t put off sterilisation for years if the goal is preventive.

“Okay, and if you’ve already operated… what do I do at home?” (practical post-operative care)

After surgery for pyometra, the goal is to recover calmly and without surprises. The essentials are usually:

• Real rest for the first few days: less activity = better healing.
• Elizabethan collar or bodysuit: preventing licking is half the battle.
• Medication exactly as prescribed (analgesia, antibiotics if necessary).
• Monitor the wound: a little mild inflammation may be normal; discharge, bad odour, opening or marked pain is not.
• Water available and small meals if there has been vomiting or loss of appetite.
• Veterinary check-up when indicated: remove stitches, general check-up, and confirm that everything is as it should be.

Sources and recommended reading (with full URLs)

Open access review on canine pyometra (pathophysiology, diagnosis and treatment): https://pmc.ncbi.nlm.nih.gov/articles/PMC10647846/

MSD / Merck Veterinary Manual – Cystic endometrial hyperplasia complex–pyometra in small animals: https://www.merckvetmanual.com/reproductive-system/diseases-of-the-female-reproductive-system/cystic-endometrial-hyperplasia-pyometra-complex-in-small-animals

PubMed (search for clinical and retrospective studies on surgical outcomes in canine pyometra):https://pubmed.ncbi.nlm.nih.gov/?term=canine+pyometra+outcome+ovariohysterectomy

PubMed (search focused on bacteriology and E. coli in canine pyometra):https://pubmed.ncbi.nlm.nih.gov/?term=canine+pyometra+Escherichia+coli

Cornell University – Informative veterinary information on mammary health (useful context if you link sterilisation + tumour prevention): https://www.vet.cornell.edu/departments-centers-and-institutes/cornell-canine-health-center/canine-health-information/mammary-cancer

Animal Medical Centre (AMC) – General information on sterilisation/castration (practical approach for guardians): https://www.amcny.org/pet_health_library/spaying-neutering-in-dogs/