Pyometra in Cats: The Infection That Hides Behind a Clean Cat

Pyometra is an infection of the uterus that fills it with pus, and in cats it is a quiet disease until it is suddenly not. The queen who had a heat a few weeks ago goes off her food, drinks more than usual, sleeps more than usual, and looks, to a loving eye, a little under the weather. The one visible sign, a discharge, is often gone before anyone sees it, because she has groomed it away. Every primary source we read files pyometra under the same heading: a medical emergency. This page explains why, with the numbers behind each claim.

What is actually happening inside her

The mechanism is hormonal before it is bacterial. After a heat, VCA explains, progesterone stays elevated for several weeks, thickening the uterine lining in preparation for a pregnancy. When pregnancy does not follow, cycle after cycle, that lining keeps thickening until cysts form, a condition called cystic endometrial hyperplasia. The thickened, cystic lining secretes fluid, "creating an ideal environment where bacteria can grow."

Progesterone does three more unhelpful things at once, as the Merck Veterinary Manual lists them: it suppresses the immune response, it closes the cervix, and it dampens the contractions that would normally expel fluid. Bacteria that live harmlessly in the vagina climb into a uterus that has, for hormonal reasons, stopped defending itself.

The usual culprit is Escherichia coli. In the review by Ragnvi Hagman, the Swedish reproduction specialist whose work anchors most of this page, E. coli is found in about 71% of feline pyometras that grow anything in culture, with streptococci at about 19% and staphylococci at 8%; roughly one culture in five grows nothing at all. A separate 2021 Brazilian study that cultured 30 pyometra cases, 23 dogs and 7 cats together, found E. coli alone in 57.7% and linked it to the most severe uterine damage.

How common it is, and at what age

The best population figure comes from Sweden, where pet insurance covers a large share of cats and claims can be counted. Hagman and colleagues analyzed insured cats from 1999 to 2006 and found a mean incidence of about 17 cats per 10,000 cat-years at risk, a median age at diagnosis of 4 years, a marked rise after age 7, and a case fatality rate of 5.7%, slightly higher than the 3 to 4% reported in dogs. Living in town or in the country made no difference.

Ages disagree across the literature, and the disagreement is explained by who was counted. The Swedish insured population gives a median of 4 years. Hagman's 2018 review, summarized by the EveryCat Health Foundation, gives a mean of 5.6 years and adds that about 2.2% of queens are diagnosed before the age of 13. A 2016 review by Hollinshead and Krekeler, working from a different case mix, puts the average at 7.6 years with a range of 1 to 20. A median and a mean are different statistics, and an insured national population is not a referral case series; we publish all three rather than pick one.

The United States has one large case series rather than an incidence rate: 183 cats treated at the Animal Medical Center in New York between 1979 and 1984. We found no population figure for the UK, Canada or Australia; the largest UK study of common feline disorders, an 18,249-cat VetCompass paper, does not list pyometra in its reported table, which says nothing either way about its frequency there.

The breeds that carry more risk

The Swedish data also measured breed. One breed stands far apart, and seven more sit above the population average. Figures are cats per 10,000 cat-years at risk, from the Hagman 2014 abstract, which gives the group of seven as "over 60" without breaking them out.

International Cat Care's own Sphynx breed page cites this study by name. Two cautions travel with the table: these are Swedish insured cats, not a US or UK population, and a breed figure describes risk across a breed, never the odds for one cat.

When it strikes: the weeks after a heat

Three primary sources give three windows, and the difference is the type of document. VCA's consumer guide says pyometra "usually occurs two to eight weeks after the last estrus." The Hollinshead and Krekeler review says clinical signs "most commonly occur within 4 weeks of the onset of oestrus," most often in the diestrus phase that follows it, and that this applies to queens that were mated, that were induced to ovulate, or that ovulated on their own. Hagman's 2018 review, written for clinicians deciding what to suspect, casts the widest net: the disease should be considered in any intact queen presenting with illness "particularly if within 4 months after estrus."

The practical reading is the clinician's: a recent heat makes pyometra likely, and a heat four months ago does not rule it out. What a heat looks like, and how often it comes back, is on our cat in heat page.

The signs, and why cats hide them

Cats show pyometra less clearly than dogs, and that is a measured finding rather than an impression. The New York series of 183 cats concluded that signs and examination findings were "similar, but not as conspicuous as those reported in the dog," and the 2016 review adds that queens "often go undiagnosed as there may be few or only very mild clinical signs," with the classic bloody discharge going unnoticed. VCA gives the everyday reason: cats are so concerned with cleanliness that the discharge "is usually cleaned up before it can be seen."

What the sources describe, across open and closed forms:

• Lethargy and a poor appetite, the two most common signs in EveryCat's summary of the literature.
• Drinking and urinating more than usual. VCA explains that bacterial toxins affect the kidney's ability to retain water, so the cat drinks to compensate; this happens in both forms.
• Vomiting or diarrhea, in some cats.
• A discharge from the vulva, pus-like and often bloody, only when the cervix is open, and often already groomed away. Any blood from a cat's vulva is a vet visit, never a "period": cats do not have periods.
• A swollen abdomen, when the cervix is closed and the uterus fills. This is the form that can be mistaken for pregnancy, and the form that becomes dangerous fastest.
• Fever, which "may or may not be present," in both VCA's and EveryCat's wording.

Open versus closed

The cervix decides how the disease presents, and the closed form is the one every source warns about hardest.

The absence of a discharge is not reassurance. It is often the more dangerous form.

How vets confirm it

Pyometra is confirmed at the clinic, not at home. VCA describes the usual picture: a severe rise in white blood cells, often raised globulins, and very dilute urine because of the toxins' effect on the kidneys. X-rays often show the enlarged uterus when the cervix is closed; in the New York series, radiographs revealed a large uterus in 138 of 169 cats. With an open cervix the uterus may be barely enlarged, and the review literature is unanimous that abdominal ultrasound is the best tool, both to see the uterus and to tell a pus-filled one from a normal pregnancy. One test that does not help: culturing the discharge, because the same bacteria live in healthy cats' vaginas.

Treatment: surgery, and what it achieves

The treatment of choice, in every source, is an ovariohysterectomy: the surgical removal of the ovaries and uterus, the same operation as a routine spay performed on a sick animal with an infected organ. Merck calls the surgery curative. Cats are stabilized first with intravenous fluids, and antibiotics continue after surgery for a period the vet sets (the manuals we read give different typical durations, so we do not print one).

The outcome figures line up across independent sources. The ACVS puts survival "as high as 80-100% if abdominal contamination is avoided and the systemic infection is treated." EveryCat, citing Hagman, gives a mortality of 3 to 20% in uncomplicated cases. The New York series reported that signs resolved in 168 of 183 cats and that 15 (8%) died or were euthanized, with post-operative complications in about 20% that generally resolved within two weeks. Most uncomplicated patients go home within one to two days.

The figure that argues for speed is a different one. EveryCat reports that queens whose uterus ruptures before surgery face a mortality rate of 57%. The gap between an 8% mortality in a treated series and a 57% mortality after rupture is the whole case for not waiting.

Medical treatment, for the breeding queen who is not very ill

For a breeding queen with an open cervix who is otherwise stable, a medical route exists, and it is worth knowing how thin its evidence base is, because the percentages travel the web without their sample sizes.

• Prostaglandins (cloprostenol and relatives) lower progesterone, open the cervix and make the uterus contract to expel pus. VCA lists the side effects (restlessness, panting, vomiting, salivation, abdominal pain, starting about fifteen minutes after the injection and lasting hours), warns that there is no improvement for about 48 hours, and that contractions can rupture a closed uterus.
• The prostaglandin evidence is one 2014 Argentinian study of five open-cervix queens: all recovered, all cycled normally again, and two later delivered litters.
• Aglepristone, a drug that blocks progesterone receptors, has two small feline studies. A Turkish study of ten queens in 2009 reported nine responding, with no recurrence over two years and no side effects. An Italian study of five queens in 2022 found the uterus back to normal on ultrasound ten days after starting treatment, and three of the four cats later mated had an uneventful pregnancy.

EveryCat's summary of this literature: medical therapy of open-cervix pyometra resolves illness in about 95% of cats, returns about 60% to fertility, and recurs in 0 to 14%; Merck's pet-owner manual puts fertility after medical treatment at around 70%. Those figures rest on a few dozen cats in total. Severely ill cats, and closed-cervix cases, are surgical patients.

What it costs, honestly

No veterinary clinic in any of our four markets publishes a pyometra fee, and we did not find one; what circulates online are editorial estimates. Two US pet-health sites give ranges of roughly $750 to $2,000 and $1,000 to $6,000, and the higher one adds that a true figure "is impossible to estimate" because cases vary so much. Treat those as the shape of the bill, not as a quote, and ask for an estimate at admission. We found no UK, Canadian or Australian figure to set beside them, and no insurer's own page stating whether pyometra is covered; how policies treat an emergency like this is the subject of our cat insurance guide.

Prevention: the spay, and its one small exception

A spayed cat has no uterus to infect, which is why every welfare charity we read lists pyometra among the reasons to neuter: Cats Protection names it beside ovarian, uterine and mammary cancers as a risk of leaving a female entire, and International Cat Care says the same. The full argument, with the age question by market, is on our neutering guide.

The exception is rare and worth a sentence. If ovarian tissue is left behind at surgery, a condition called ovarian remnant syndrome, the cat can keep cycling and can develop "stump pyometra" in the remaining tissue. A 2012 case series described three queens showing heat behavior after their spay: two had stump pyometra, and one died two days after the corrective surgery. A 2025 Bologna study of 93 ovarian-remnant cases, 23 of them cats, lists stump pyometra among the consequences of leaving the condition untreated but gives no rate for cats.

Hormonal heat suppression carries its own risk: the 2016 review notes that queens given high doses of progestin drugs such as megestrol acetate can develop the same cystic lining and pyometra. A spay reduces the risk to close to zero; "impossible" is a word the case reports do not allow.

What to take away

• Pyometra is a pus-filled infection of the uterus driven by progesterone after a heat; in Swedish insured cats it struck about 17 cats per 10,000 cat-years, with a median age of 4 and a case fatality of 5.7% (Hagman et al. 2014).
• The Sphynx carried an incidence of 433 per 10,000 cat-years in that study, 25 times the average; Siberian, Ocicat, Korat, Siamese, Ragdoll, Maine Coon and Bengal all sat above 60.
• The window after a heat is stated three ways by three primary sources: two to eight weeks (VCA), within four weeks of the onset of heat (Hollinshead and Krekeler 2016), and up to four months (Hagman 2018); a recent heat plus illness is the pattern to act on.
• Cats show pyometra less conspicuously than dogs (183-cat New York series, 1987), and the telltale discharge is usually groomed away; a closed cervix produces no discharge at all and is the faster, more dangerous form.
• Surgery to remove the ovaries and uterus is the standard treatment, with survival reported between about 80 and 97% across four sources, against a 57% mortality once the uterus has ruptured.
• Medical treatment with prostaglandins or aglepristone exists for stable breeding queens with an open cervix, and its 95% resolution and 60 to 70% fertility figures rest on studies of five to ten cats each.