What is Pyometra in Cats & Dogs?

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Chronic uterine infections due to constantly fluctuating hormone levels, ovarian cysts, mammary and reproductive cancers, urine marking, and overpopulation are just some of the well-known repercussions of not spaying and neutering our cats and dogs. And there are numerous others.

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And one of the lesser known but most dramatic effects of keeping a dog or cat intact is pyometra, also commonly known as pyo. Pyometra is a serious, life-threatening medical emergency that is almost completely preventable through spaying, and that mostly occurs in older dogs and cats.

What is a uterine-stump pyometra?

With early spaying, the possibility of pyometra is almost eliminated. However, in the case of an incomplete ovariohysterectomy where a segment of the uterine body or horn becomes infected, a uterine-stump pyometra may occur. It's primarily due to ovarian tissue remaining in the animal or as a result of the use of progestational hormones.


What is pyometra in dogs and cats?

Pyometra is an infection in the uterus in female dogs and cats. With the exception of a uterine-stump pyometra occurring in a spayed dog or cat, spaying is the only way to prevent pyometra.

Not entirely understood, pyometra, based on the consensus among the veterinary community, develops due to bacteria gaining access to the uterus through the vagina. Once the infection sets in, the uterus fills with pus. Consequently, pyometra is a secondary infection that occurs as a result of hormonal changes in the female's reproductive tract.


In a nutshell, here's what happens in a pyometra: following estrus, progesterone, a hormone, stays elevated for up to two months, causing the uterus lining to thicken — basically in preparation for pregnancy. When a pregnancy does not occur for several consecutive estrus cycles, the still-thickening uterus develops cysts within the uterine tissues called cystic endometrial hyperplasia. This fluid-rich environment is ripe for the growth of bacteria.

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Due to the high levels of progesterone and thickened uterine walls, the muscles of the uterus cannot contract the way they should, thus are not able to expel the fluids yet allow access by bacteria. If the uterus is "normal," this thickening does not take place and bacteria from the vaginal vault is stopped in its tracks.


Normally, white blood cells protect against infection to allow sperm to safely enter the female's reproductive tract safely to induce pregnancy, but, in the incidence of pyometra, these bacteria-fighting cells cannot gain access to the uterus. And at the same time, the cervix, gateway to the uterus, is open during estrus (ready to receive sperm), allowing bacteria to gain easy access. It is the combination of these factors that lead to the dangerous infection known as pyometra.

Open versus closed pyometra and symptoms

In an open pyometra, which means the cervix remains open, pus drains from the uterus through the vagina to the outside. Telltale signs of an open pyometra include seeing an abnormal discharge on the hair under the tail of your cat or dog, or on bedding or furniture where your pet has recently laid. Also, watch for the following clinical signs:


  • Fever.
  • Lethargy, or lack of energy, sluggishness.
  • Increased water consumption.
  • Lack of appetite or anorexia.
  • Depression may be present.

Closed pyometra, characterized by a closed cervix, is a different story and a much more severe illness than an open pyometra because the pus cannot drain to the outside and could rupture, spilling the contents of the infection throughout the abdomen. Collecting inside the uterus, the pus causes the abdomen to distend and the bacteria releases toxins, which are then absorbed into the bloodstream. This is a rapidly disintegrating condition which needs immediate veterinary intervention. Watch for the following clinical signs:


  • Anorectic, which means lack of appetite to the point where your dog will not eat at all.
  • Extremely listless and obviously depressed.
  • Increased water consumption.
  • Vomiting or diarrhea may be concurrent.

What's happening in this severe form of pyometra is that toxins released by the bacteria impact the kidney's ability to retain fluid. With increased urine production, many dogs and cats drink water excessively to compensate.

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Diagnosis of pyometra

The following are red flags and used by your vet in the diagnosis of pyometra:


  • Vaginal discharge, even if it's slight (normally found in younger dogs).
  • Dogs with a history of drinking excessively and a recent estrus.
  • Painful, enlarged abdomen.
  • Lab work that indicates severe elevation of the white blood cell count.
  • Elevated globulins in the blood (proteins associated with the immune system).
  • Lab work indicates low specific gravity of the urine due to major bacterial infection.
  • X-rays of the abdomen indicate an enlarged uterus in the case of a closed pyometra. In open pyo, there will be less swelling.
  • Ultrasound results indicate thickened uterine walls, fluid accumulation, and increased size of the uterus.

Treatment of pyometra in cats and dogs

The standard treatment of pyometra in cats and dogs is to surgically remove the uterus in conjunction with a spay. However, it's not a routine, straight-forward spay when complicated by pyo. Be prepared to worry more, and to spend more than the average price of a normal, uncomplicated spay, sometimes, significantly so, depending on your veterinarian, the severity of the illness, and how far advanced it is. For example, an open pyometra is much less threatening than a closed pyometra, which in many cases can prove fatal.


Take heart if you have caught pyo on time — you and your dog or cat can enjoy many more wonderful years together.

Why spay your dog?

Spaying pet dogs and cats is wholly recommended by veterinarians for your pet's optimal health — physically, mentally, and emotionally — and for their overall welfare. And the stats show that most responsible pet parents are aware of the importance of spaying or neutering their pets; one of the most affordable, routine veterinary surgeries available.


Among the pros for spaying are a significant decrease in the risk of mammary tumors — the most common malignant tumors in female dogs if your dog is spayed before the age of 2.5 years. Spaying also greatly reduces the risk of perianal fistulas. Additionally, spaying completely removes the risk of uterine, cervical, and ovarian tumors.

And, the good news is that spaying your dog nearly eliminates the risk of life-threatening pyometra, which kills about 1% of intact female dogs in America, and overall, affects about 23% of female dogs.

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Why spay your cat?

As in dogs, spaying a cat provides a level of wellness, both physical and emotional, that is not shared by intact females. The enormous benefits of spaying a cat include both you and your cat's peace of mind — since a cat in estrus is in a highly disturbed state, which in turn, is distressing for you and the whole household.

Spaying also substantially decreases or eliminates the risk altogether of pyometra and mammary and uterine cancers and infections for your cat.

Always check with your veterinarian before changing your pet’s diet, medication, or physical activity routines. This information is not a substitute for a vet’s opinion.