Pyometra is a serious bacterial infection of the uterus that occurs in unspayed female dogs and, less commonly, cats. The uterus fills with pus, and without treatment the infection can become life-threatening. Nearly 25% of intact female dogs develop pyometra before age 10, making it one of the most common emergencies in unspayed pets. The good news: survival rates with surgery are high, around 97% when treated promptly.
How Pyometra Develops
The infection is driven by hormonal changes during a dog’s reproductive cycle. After each heat cycle, progesterone levels rise and cause the uterine lining to thicken in preparation for pregnancy. Over repeated cycles, especially without pregnancy, this thickening becomes exaggerated. The lining develops fluid-filled cysts, a condition called cystic endometrial hyperplasia, which creates a warm, nutrient-rich environment where bacteria thrive.
At the same time, progesterone suppresses the uterus’s ability to contract and expel material, and it dampens the local immune response. Bacteria, most often a type normally found in the dog’s intestinal tract, travel up through the cervix during or just after heat when it’s briefly open. Once inside, they colonize the compromised lining with little resistance. The uterus begins filling with pus, sometimes accumulating enough fluid to visibly distend the abdomen.
Pyometra typically shows up two to four months after a heat cycle. The risk increases with age because each cycle adds more damage to the uterine lining, but it can occur in younger dogs as well.
Open vs. Closed Pyometra
There are two forms, and the distinction matters because one is easier to spot and the other is more dangerous.
In open pyometra, the cervix remains partially open, allowing pus to drain out. The most obvious sign is vaginal discharge that ranges from thick and yellowish to bloody. It may appear on bedding, on the fur around the tail, or wherever the dog has been lying. Because the uterus can drain, pressure doesn’t build as quickly, which gives owners a visible warning sign.
In closed pyometra, the cervix seals shut, trapping all the infected fluid inside the uterus. With no discharge to alert you, the infection can progress silently. The uterus swells with pus, and the abdomen may become noticeably bloated. Closed pyometra is considered more dangerous because the infection builds pressure faster and the risk of the uterus rupturing increases.
Symptoms to Recognize
Beyond vaginal discharge (in open cases) or a swollen belly (in closed cases), dogs with pyometra commonly show a cluster of signs that reflect how sick they’re becoming:
- Loss of appetite, sometimes a complete refusal to eat
- Lethargy and depression, noticeably less energy or interest in activity
- Excessive thirst and urination, often one of the earliest changes owners notice
- Rapid heart rate and breathing
- Vomiting or fever in more advanced cases
The excessive drinking and urinating happens because toxins from the infection interfere with the kidneys’ ability to concentrate urine. Your dog compensates by drinking more. This symptom alone, appearing a few weeks to months after a heat cycle in an unspayed dog, warrants a veterinary visit.
Why It Becomes Life-Threatening
Pyometra isn’t just a localized infection. About 60% of affected dogs develop sepsis, a condition where the immune system’s response to the infection spirals out of control and begins damaging the body’s own organs. Blood pressure drops, blood flow to vital organs decreases, and organ function starts to deteriorate.
The kidneys are particularly vulnerable. The combination of bacterial toxins circulating in the bloodstream and reduced blood flow can cause kidney damage, sometimes showing up as elevated waste products in blood tests. If the infected uterus ruptures, pus spills into the abdominal cavity, causing peritonitis, a severe inflammation of the abdominal lining that dramatically worsens the prognosis. This is why speed matters: the longer the infection goes untreated, the more likely these complications become.
How Vets Confirm the Diagnosis
A vet will suspect pyometra based on the combination of symptoms and the dog’s reproductive history, particularly if she’s unspayed and had a heat cycle in the past few months. To confirm it, they typically use imaging.
Ultrasound is the most reliable tool. It can visualize fluid-filled uterine horns and distinguish a swollen uterus from other abdominal problems like intestinal blockages. X-rays can also reveal a dilated, fluid-dense structure in the abdomen, though ultrasound provides more detail. Bloodwork usually shows a dramatically elevated white blood cell count, reflecting the body’s fight against infection, along with possible signs of kidney stress or dehydration.
Surgery Is the Standard Treatment
The most effective treatment is an emergency spay, surgically removing the infected uterus and both ovaries. This eliminates the source of infection entirely. Before surgery, vets stabilize the dog with intravenous fluids and antibiotics to address dehydration, correct electrolyte imbalances, and start fighting the infection. This stabilization period can make a significant difference in how well the dog handles anesthesia and surgery.
Recovery from pyometra surgery is more involved than a routine spay. The tissues are inflamed and fragile, the dog is already sick, and the surgical site carries a higher risk of complications. Most dogs spend at least a day or two in the hospital on IV fluids and antibiotics. With proper care, though, the survival rate to hospital discharge is about 97%.
At home, recovery typically involves a course of oral antibiotics, restricted activity for 10 to 14 days, and monitoring the incision site. Most dogs start eating and acting more like themselves within a few days of surgery.
Medical Treatment Without Surgery
For dogs whose owners want to preserve breeding ability, there are non-surgical options, but they come with significant caveats. Medical treatment uses hormone-blocking drugs and medications that cause the uterus to contract and expel its contents. These protocols can be effective at reducing the infection, shrinking blood supply to the damaged uterine lining, and clearing inflammatory tissue.
The major limitation is recurrence. Because the uterus remains in place and the dog continues cycling, the conditions that caused pyometra in the first place will recur. Medical management is generally reserved for young, otherwise healthy dogs with open pyometra who are valuable for breeding, and even then, breeding is recommended at the very next cycle before the infection has a chance to return. It’s not considered appropriate for closed pyometra, where the risk of uterine rupture during drug-induced contractions is too high.
Prevention
Spaying is the only reliable way to prevent pyometra. Because the procedure removes the uterus entirely, there is no tissue left to become infected. In countries where routine spaying is common, pyometra rates are dramatically lower. In Sweden, where spaying is less culturally practiced, the nearly 25% lifetime incidence highlights just how common this condition is when the uterus remains intact.
If spaying isn’t an option, awareness of the timing is your best defense. Watch for any changes in behavior, appetite, thirst, or vaginal discharge in the two to four months following a heat cycle. Early detection, before sepsis or organ damage sets in, is the single biggest factor in a positive outcome.