CONDITION
Pyometra
Pyometra is a condition in which the uterus fills with pus, typically in the weeks following a season. It arises when hormonal changes after ovulation create an environment in which bacteria can multiply unchecked within the uterine lining. The condition occurs almost exclusively in unspayed female dogs and cats, and tends to appear in middle-aged or older animals, though younger individuals can be affected. Owners often notice a combination of signs that may include drinking more than usual, lethargy, reduced appetite, or, in some cases, a thick discharge from the vulva. In other animals the cervix remains closed and no discharge is visible, which can make the picture less clear. The abdomen may appear distended, and some animals develop vomiting or seem generally unwell in ways that are difficult to pinpoint. This page explores what can signal pyometra, the mechanisms that allow infection to take hold, how the condition is investigated through examination and imaging, and the medical and surgical approaches used to address it. It also considers what influences outcomes and what owners may observe during recovery.
Why this matters now
Pyometra typically develops in intact (unspayed) female dogs and cats. In dogs it most commonly appears about four to six weeks after a season, with cases from as early as two weeks and as late as around four months afterwards. In cats signs most commonly appear within about four weeks of the start of a season. It is less common in cats, which mostly ovulate only after mating. The condition tends to affect middle-aged to older animals, though it can occur at any age in intact females. Risk increases with each successive heat cycle, and animals that have received hormone treatments may have elevated risk.
The condition can develop gradually with subtle early signs or progress rapidly to a serious state. In open pyometra, where the cervix remains open, discharge may be visible and provides an early warning. In closed pyometra, the cervix is sealed and infection accumulates without external signs, and affected animals can become severely ill over hours to a few days. Bacterial toxins absorbed into the bloodstream can cause sepsis, kidney injury and shock, and a distended uterus can rupture and spill pus into the abdomen, causing peritonitis. Pyometra does not clear on its own, and the toxic effects of the infection are fatal in many animals where treatment is delayed. Once the uterus is removed most dogs recover. In one series of 405 female dogs treated surgically in a non-specialist hospital, 394 (about 97 per cent) went home, and raised kidney values at diagnosis were linked with a longer stay and, less reliably, with a higher risk of death. In another study of dogs treated surgically, signs of sepsis at diagnosis were linked with complications: a low white cell count with peritonitis and a longer stay in hospital, and a high or low temperature with peritonitis. Whether the cervix is open or closed, how distended the uterus is and whether sepsis has begun cannot be judged at home, and the absence of discharge is not reassurance.
Signals & patterns
Early signals
Vaginal discharge
A cream, yellow, or bloody discharge from the vulva, often noticed on bedding or when the animal licks the area. Present only in open pyometra.
Excessive licking of the vulval area
The animal may pay more attention to cleaning this area than usual, sometimes indicating discharge or discomfort.
Reduced appetite
A gradual decline in interest in food is often one of the earliest signs of developing illness.
Increased thirst and urination
The toxins produced by the infection can affect kidney function, leading to increased water consumption and more frequent urination.
Later signals
Lethargy and weakness
As the infection progresses, the animal becomes increasingly tired and reluctant to engage in normal activities.
Vomiting
Systemic illness from the infection can cause nausea and vomiting as toxins circulate.
Abdominal distension
The uterus may enlarge significantly with accumulated pus, sometimes visible as a swollen belly.
Fever or subnormal temperature
Body temperature may be elevated initially but can become low as the animal deteriorates.
Collapse, abnormal gum colour or a painful abdomen
Sudden weakness or collapse, pale or dark red gums, fast breathing, or a tense and painful abdomen can indicate sepsis, or rupture of the uterus with pus spreading into the abdomen. In closed pyometra these changes can develop within hours.
Click to read about the biological mechanisms
How this is usually investigated
Diagnosis of pyometra combines clinical history, physical examination findings, and imaging. Identifying the condition promptly allows for appropriate intervention while the animal is still stable.
History and physical examination
Abdominal ultrasound
Abdominal radiographs
Blood tests
Vaginal cytology
Options & trade-offs
Treatment of pyometra focuses on removing the source of infection and providing supportive care. The approach depends on the animal's condition, the owner's breeding intentions, and the severity of illness.
Surgical removal (ovariohysterectomy)
Removal of the uterus and ovaries removes the infected uterus and, apart from rare cases where ovarian tissue is left behind, prevents recurrence. This is considered the standard treatment for most cases.
Trade-offs: Removes the source of infection and almost always prevents future episodes. Carries surgical and anaesthetic considerations, particularly in ill animals. Ends the animal's breeding potential permanently.
Medical management with hormone-based drugs
Hormone-based medications, such as aglepristone, which blocks the action of progesterone, and prostaglandins, which lower progesterone and make the uterus contract and expel its contents, are given alongside antibiotics. Neither is authorised in the UK for treating pyometra: aglepristone is authorised only for ending pregnancy in bitches, and prostaglandins only for farm animals and horses, so both are used off-licence under the veterinary medicines cascade. This approach is generally reserved for animals that are stable, not septic and intended for breeding, and improvement takes several days.
Trade-offs: May preserve breeding potential. Carries risk of incomplete resolution and recurrence with future heat cycles. Prostaglandins carry a risk of uterine rupture, particularly when the cervix is closed, and an animal can deteriorate during the days it takes for the uterus to empty, so this route involves repeated monitoring and scanning.
Stabilisation before surgery
Intravenous fluids and supportive care to improve the animal's condition before anaesthesia and surgery.
Trade-offs: Often needed for animals that are septic, dehydrated or have kidney changes. Because the infected uterus remains in place until surgery, stabilisation is usually measured in hours rather than days.
Antibiotic therapy
Antibiotics are typically used alongside primary treatment to address bacterial infection.
Trade-offs: Supports other treatments but cannot resolve pyometra alone due to the nature of the accumulated infection.
Common misconceptions
"Pyometra only affects elderly dogs"
While risk increases with age, pyometra can develop in any intact female after puberty. Middle-aged animals are commonly affected, but younger animals are not immune.
"If there is no vaginal discharge, the animal cannot have pyometra"
Closed pyometra occurs when the cervix is sealed, trapping infection inside with no external discharge. This form can be more dangerous as it may go unrecognised until the animal is seriously ill.
"Antibiotics alone can cure pyometra"
The infected material accumulating in the uterus cannot be effectively reached or cleared by antibiotics alone. Removal of the source of infection is typically necessary for resolution.
Related conditions
Vaginitis
Vaginitis shares the anatomical region and can sometimes produce a vulval discharge that may prompt owners to consider pyometra, particularly in intact females. The two can occasionally coexist, though vaginitis tends to occur in younger animals and does not typically cause the systemic signs seen with uterine infection.
Mastitis
Mastitis and pyometra are both bacterial infections of reproductive tissue that most often occur in intact females, and both may present with systemic signs such as lethargy, reduced appetite, and fever. They differ in the organ affected and the hormonal context, but the general pattern of tissue infection and inflammation shares common ground.
Pyelonephritis
Pyelonephritis involves bacterial infection ascending to the kidneys, and the signs, such as increased thirst, lethargy and non-specific malaise, can overlap with those of pyometra. Urinary infection with bacteria such as E. coli can accompany pyometra, and the systemic signs of either condition may prompt consideration of the other during investigation.
Endocarditis
Endocarditis is a bacterial infection that can arise when bacteria from elsewhere in the body enter the bloodstream and settle on heart valves. In cases of pyometra with significant bacterial load, there is a theoretical risk of bacteria spreading to distant sites, though endocarditis remains uncommon in this context.
Urinary Tract Infection in Dogs
Urinary tract infections in dogs can occasionally present with increased drinking and non-specific signs of unwellness, which may resemble the early picture of pyometra. The two conditions occupy different systems, but the overlap in clinical presentation means both may be considered during the diagnostic process in an unwell intact female dog.
Ovarian Remnant Syndrome
Ovarian tissue left behind after spaying can continue to produce hormones, and in rare cases this allows a pyometra to develop in the remaining stump of the uterus in an animal believed to be spayed.
Acute Kidney Injury
Pyometra can reduce kidney function through bacterial toxins and sepsis, and raised kidney values at diagnosis are linked with a poorer outcome. Kidney disease is also a look-alike, as both can cause increased thirst, reduced appetite and vomiting.
In an unspayed female dog or cat, being quiet, off food, drinking more or vomiting in the weeks to months after a season is the typical picture of pyometra, with or without a discharge. Closed pyometra produces no discharge at all, and fever is absent in many affected dogs, so neither a clean vulva nor a normal temperature rules it out. Collapse, pale or dark red gums, rapid breathing, a tense or painful abdomen, or a temperature below normal tend to mark sepsis or uterine rupture, and at that stage the outlook is poorer. The same early signs can come from other illnesses such as kidney disease or diabetes, and telling them apart relies on examination, blood tests and imaging. Spaying that removes the ovaries and uterus prevents pyometra in future, although, rarely, ovarian tissue left behind after spaying can allow infection in the remaining uterine stump.
Last reviewed: 9 October 2026 · Dr Alastair Greenway MRCVS