Pyometra is an infection of the uterus, and one of the most important diseases to know about if you live with an unspayed female dog or cat. It can come on suddenly, and left untreated it can be deadly.
An intact (unspayed) female who is off her food, unusually quiet, or drinking more in the weeks after a heat needs to see a veterinarian the same day. That holds even if you have not seen any discharge and even if she does not have a fever.
This infographic is from Veterinary Facts, a page created to share interesting and useful information about pets and how to look after them.
What is pyometra in dogs and cats?
Pyometra means a uterus filled with pus. It affects intact females, whether or not they have ever had a litter. Rarely, it also develops in a spayed female when some ovarian tissue was left behind after surgery (more on that under prevention).
In dogs it is not a rare disease. In Swedish insurance data from 1995-1996, an estimated 23-24% of insured female dogs would have had pyometra by 10 years of age, roughly 1 in 4. Risk varied widely by breed, from 10% to 54%, with rough Collies, Rottweilers, Cavalier King Charles Spaniels, Golden Retrievers, Bernese Mountain Dogs and English Cocker Spaniels among the higher-risk breeds.
Veterinarians describe pyometra as open or closed, depending on the cervix, the narrow opening between the uterus and the vagina. With an open cervix, pus can drain out through the vulva. With a closed cervix, it stays trapped inside the uterus.
What causes pyometra?
Pyometra is the result of hormonal and structural changes in the lining of the uterus, combined with bacteria. The hormone progesterone plays the key role.
After every heat, whether she was bred or not, a female enters the progesterone phase of her cycle, called diestrus. The cervix, which was open during her heat, begins to close, and the uterine lining should settle back to normal. In some females the lining instead responds abnormally to progesterone: it thickens and develops fluid-filled cysts. This is cystic endometrial hyperplasia (CEH). CEH usually comes before pyometra, although not every case of CEH becomes one.
Progesterone also turns the uterus into an easy place for bacteria to grow. It:
- stimulates the uterine glands to secrete fluid
- suppresses the local immune response
- closes the cervix, so the uterus cannot drain
- reduces the contractions of the uterine muscle
Bacteria moving up from the vagina find this environment ideal. The most common is Escherichia coli (E. coli), which in dogs typically comes from her own normal bacterial flora, not from an outside source.
Because pyometra starts during diestrus, signs usually show up in the weeks after a heat, and it is generally diagnosed within about 4 months of one. If your dog seems unwell, think back to when she was last in heat. Our guide to the common behaviors of a female dog in heat can help you place that date.
Pyometra can follow any heat, her first or her tenth. In dogs, though, it becomes more common with age, and most cases are middle-aged or older females.
Symptoms of pyometra
Most signs are the same whether the cervix is open or closed. Owners may notice:
- Poor appetite or not eating
- Lethargy and depression
- Drinking more and urinating more
- Vomiting, and sometimes diarrhea
- Vaginal discharge that looks like pus or is bloody, or discharge on the tail, back end or bedding (open pyometra)
- A swollen belly (more typical of closed pyometra)
- Abdominal pain
- Pale gums
- Fever, in some dogs
Open pyometra. Some dogs have no clinical signs other than the vaginal discharge, so a dog can look fairly well and still have pyometra.
Closed pyometra. With no discharge, the most obvious clue is missing and the disease is easy to miss. Pus builds up with nowhere to go, and these dogs tend to be sicker. In a Swedish university hospital study of 111 dogs (2005-2012), dogs with a closed cervix were more severely ill on arrival, and sepsis was diagnosed in 77% of them compared with 51% of dogs with an open cervix.
Do not wait for a fever. In a study of 356 dogs treated at a Swedish university hospital in 2006-2007, only about a third had one. Early signs can be vague, so many dogs are already quite sick by the time they are seen.
When to call your vet: any of the signs above in an intact female, especially in the weeks after a heat, is a reason to call your veterinarian the same day. A normal temperature or the absence of discharge does not rule pyometra out.
How vets diagnose pyometra
Veterinarians consider pyometra in any intact female dog that is sick. Your veterinarian will ask about her last heat, any breeding and any hormone treatment, then examine her, including her abdomen and vulva. Tests usually include:
- Bloodwork. A complete blood count (CBC) often shows a high white blood cell count, though it can also be low, and chemistry tests can show dehydration and raised kidney values. For the veterinary team: in the Swedish study of 356 dogs, a low white blood cell count (leukopenia) was associated with a much higher risk of peritonitis.
- Urinalysis, to help assess the kidneys and her overall condition.
- X-rays (radiographs). These can show an enlarged uterus and are often very suggestive, but they are not definitive.
- Abdominal ultrasound. This shows a thickened uterine wall and a distended, fluid-filled uterus, and it confirms that she is not pregnant.
- Vaginal cytology, a swab of the discharge examined under the microscope, when discharge is present.
How is pyometra treated?
Pyometra is a medical emergency. Many dogs arrive seriously ill, so stabilization comes first: the veterinary team gives intravenous fluids and broad-spectrum antibiotics to get her ready for surgery. Sepsis, the body's overwhelming response to infection, is common. In the Swedish study of 111 dogs, it was diagnosed in 59% of them.
Once she is stable, the treatment of choice is an emergency spay (ovariohysterectomy), the surgery that removes the ovaries and uterus. It is the safest and most effective treatment, and it removes both the infection and any risk of it coming back. After surgery, she stays in the hospital so the team can watch for complications. When she goes home, your veterinarian will send any medication she needs and explain the recheck plan.
The main complications are peritonitis (inflammation and infection of the abdominal lining), which can happen if the uterus ruptures or leaks pus into the abdomen, and sepsis. In the study of 356 dogs, peritonitis was found in 40 of the 315 dogs treated with surgery.
The outlook after surgery is generally good, and better still when the dog is diagnosed early. The American College of Veterinary Surgeons puts survival with surgery as high as 80-100% when contamination of the abdomen is avoided and the infection responds to treatment. In the same study of 356 dogs, 4 of the 315 surgically treated dogs (1%) died after surgery. Those odds depend on getting the infection under control in time, which is why the same-day visit matters.
Can pyometra be treated without surgery?
Sometimes, in carefully selected dogs. The usual reason is a valuable breeding female whose owner wants to keep her fertile. A medical approach combines hormonal drugs (prostaglandins, dopamine agonists, or progesterone blockers) with antibiotics, under close veterinary monitoring. According to the Merck Veterinary Manual, progesterone receptor blockers are not currently available in the United States. Medical treatment can take several days to work and can cause side effects such as panting and vomiting.
This option is only for dogs that are stable and not systemically ill, and success should be seen as temporary: pyometra can come back after medical treatment, so spaying is recommended once her breeding career is over. Your veterinarian decides whether a dog is a candidate and chooses the drugs and doses.
Antibiotics alone are not a treatment for pyometra. Without effective treatment, the infection can lead to sepsis, rupture of the uterus with peritonitis, and death.
Pyometra in cats
Cats get pyometra too, through a similar mechanism. A cat mostly ovulates when she mates but can also ovulate on her own, and if she does not become pregnant, her body keeps producing progesterone for about 40 days. Signs are similar to those in dogs but usually less obvious, and drinking and urinating more is much less common in cats. Ultrasound is the most useful test. In Swedish insured cats from 1999 to 2006, the case fatality rate was 5.7%. Our article on pyometra in cats covers the feline side in detail.
Preventing pyometra
Spaying prevents pyometra in practice, because it removes the uterus and ovaries. The ACVS notes that most dogs and cats spayed early in life will not develop it. Our post on why you should spay or neuter your pet covers the wider benefits.
Whether and when to spay is a conversation to have with your veterinarian, based on your dog's breed, size, health and plans.
For the veterinary team: stump pyometra (infection of the remaining uterine stump) can occur after an incomplete spay, when ovarian tissue is still present or the animal has received progestational hormones. Because CEH is driven by progesterone and may be aggravated by estrogens, ask about hormone treatment in any unwell female, spayed or not.
If you keep an intact female for breeding, watch her closely after every heat and call your veterinarian at the first sign she is not herself.
Sources
- American College of Veterinary Surgeons: Pyometra
- Merck Veterinary Manual: Cystic Endometrial Hyperplasia-Pyometra Complex in Small Animals
- Veterinary Clinics of North America: Pyometra in Small Animals (Hagman, 2018)
- Veterinary Clinics of North America: Pyometra in Small Animals 2.0 (Hagman, 2022)
- Veterinary Clinics of North America: Pyometra in Small Animals 3.0 (Hagman, 2023)
- Journal of Veterinary Internal Medicine: Breed risk of pyometra in insured dogs in Sweden (Egenvall et al., 2001)
- BMC Veterinary Research: Outcome of pyometra in female dogs and predictors of peritonitis and prolonged postoperative hospitalization (Jitpean et al., 2014)
- BMC Veterinary Research: Closed cervix is associated with more severe illness in dogs with pyometra (Jitpean et al., 2017)
- Veterinary Microbiology: Escherichia coli strains isolated from the uterus and urinary bladder of bitches suffering from pyometra (Hagman and Kühn, 2002)
- Journal of Feline Medicine and Surgery: Pyometra in the queen: To spay or not to spay? (Hollinshead and Krekeler, 2016)
- Theriogenology: Incidence of pyometra in Swedish insured cats (Hagman et al., 2014)
This article is for general information and is not a substitute for advice from your veterinarian.
