Pyometra is a serious bacterial infection of the uterus that affects intact (unspayed) female dogs, typically developing one to two months after a heat cycle. Up to 20% of intact female dogs develop pyometra before the age of 10, making it one of the most common reproductive emergencies in veterinary medicine. Left untreated, it can lead to sepsis, peritonitis, and death. With prompt surgical treatment, survival rates are high, ranging from 92% to 99%.
How Pyometra Develops
The infection is driven by progesterone, the hormone that dominates the phase after a dog’s heat cycle. Each time a dog goes through a heat cycle, estrogen primes the uterine lining to become more responsive to progesterone. Progesterone then causes the glands in the uterine wall to thicken, enlarge, and fill with fluid. This thickening is called cystic endometrial hyperplasia, and it’s the foundation pyometra builds on.
Progesterone does several things that set the stage for infection. It suppresses the local immune response in the uterus, stimulates glandular secretions that create a warm and nutrient-rich environment for bacteria, causes the cervix to close (trapping fluid inside), and reduces the muscular contractions that would normally help clear the uterus. Bacteria from the vagina, most commonly E. coli, take advantage of these conditions and colonize the uterus. The result is a uterus filled with pus that can become massively distended.
Open vs. Closed Pyometra
Pyometra comes in two forms depending on whether the cervix stays open or closes completely. In open pyometra, the cervix remains partially open and pus drains from the vulva. You may notice a foul-smelling, bloody, or cream-colored discharge. Dogs with open pyometra are often still visibly ill, but the drainage provides some relief because infected material isn’t fully trapped inside.
Closed pyometra is more dangerous. The cervix seals shut, so all the pus and bacteria accumulate inside the uterus with no way out. The uterus swells dramatically, and toxins from the bacteria absorb into the bloodstream more readily. Dogs with closed pyometra tend to deteriorate faster and face a higher risk of the uterus rupturing, which spills infected material into the abdomen and causes life-threatening peritonitis.
Symptoms to Recognize
Pyometra typically appears four to eight weeks after a heat cycle. The signs can come on gradually or seem to appear overnight. Common symptoms include:
- Vaginal discharge: ranging from bloody to yellow-green or white pus (only present in open pyometra)
- Lethargy: your dog seems unusually tired or uninterested in activity
- Loss of appetite: refusing food or eating much less than normal
- Increased thirst and urination: toxins released by the bacteria interfere with the kidneys’ ability to concentrate urine, so dogs drink and urinate far more than usual
- Vomiting
- Swollen or firm abdomen: especially in closed pyometra where the uterus is distended
Some dogs show only one or two of these signs early on, which can make pyometra easy to miss. A dog that’s drinking excessively and seems sluggish a few weeks after her heat cycle warrants a veterinary visit, even without visible discharge.
How It’s Diagnosed
Veterinarians typically suspect pyometra based on a dog’s history (intact female, recent heat cycle) combined with the symptoms above. Ultrasound is the most reliable noninvasive tool, allowing the vet to visualize the fluid-filled uterus directly. The endometrial glands can dilate to 0.3 to 2.0 centimeters in diameter when affected, and ultrasound can also help assess uterine wall thickness, which matters for gauging rupture risk.
Bloodwork usually shows significant changes. White blood cell counts in dogs with pyometra range widely, from 2,500 to nearly 200,000 cells per cubic millimeter, and 70% to 90% of cases show an elevated proportion of immature white blood cells, a sign the body is fighting a serious infection. Abdominal X-rays can reveal an enlarged, fluid-dense uterus, though ultrasound provides more detail.
Surgical Treatment
The standard treatment is an emergency spay, surgically removing the uterus and ovaries. This eliminates the source of infection entirely and prevents recurrence. For dogs that aren’t intended for breeding, surgery is the clear recommendation. Mortality rates for surgical treatment are low, between 1% and 8%, though the risk increases if the dog is already septic, if the uterus has ruptured, or if treatment was delayed.
Surgery for pyometra is more complex and higher risk than a routine spay. The uterus is fragile, swollen, and heavy with infected fluid. Veterinarians take care to remove it without spillage into the abdomen. Dogs typically receive intravenous fluids and antibiotics before, during, and after surgery. Most dogs improve noticeably within 24 to 48 hours after the infected uterus is removed, though full recovery takes one to two weeks.
Medical Treatment for Breeding Dogs
For dogs with high breeding value, medical management is sometimes attempted. This involves hormonal medications that cause the cervix to open and the uterus to contract, expelling the infected material, combined with antibiotics to fight the bacterial infection. Medical treatment is only considered for open pyometra cases where the cervix is already partially open and the dog is stable enough to tolerate the process.
Medical treatment has significant limitations. It doesn’t address the underlying uterine changes that made the dog susceptible in the first place, so pyometra commonly recurs after subsequent heat cycles. Dogs treated medically should be bred on their next cycle if possible, then spayed. This approach also carries its own risks, including incomplete drainage and worsening of the infection, and it requires close veterinary monitoring over several days.
Stump Pyometra in Spayed Dogs
In rare cases, pyometra can develop even in dogs that have been spayed. This happens when a small piece of uterine tissue or ovarian tissue is left behind during the spay surgery. If residual ovarian tissue continues producing progesterone, and residual uterine tissue is present, bacteria from the vagina can colonize that remaining stump, creating what’s called stump pyometra.
Diagnosing stump pyometra requires a high index of suspicion because most owners and veterinarians assume a spayed dog is protected. Ultrasound can identify the remaining tissue, and vaginal cytology can reveal whether functional ovarian tissue is still producing hormones. Treatment involves a second surgery to remove the residual uterine and ovarian tissue.
Prevention
Spaying is the only reliable way to prevent pyometra. Because the condition is progesterone-driven and worsens with repeated heat cycles, every cycle an intact dog goes through increases her cumulative risk. Dogs spayed before their first heat cycle have essentially zero risk. For dogs kept intact for breeding purposes, awareness of the typical timing (one to two months post-heat) and early recognition of symptoms are the best defenses against a life-threatening outcome.