Pyometra in Dogs: Symptoms, Causes, and Veterinary Care Explained

Understanding Pyometra in Dogs

Pyometra is an infection of the uterus, sometimes called „pyo” in veterinary practice, and primarily affects intact female dogs—those that have not been spayed. While it most commonly occurs in middle-aged to older dogs, cases have been documented in puppies as young as nine months. The canine uterus is uniquely structured: it divides into two horns, allowing multiple fetuses to develop simultaneously. This anatomy influences how hormones act during the reproductive cycle.

Upon reaching sexual maturity, a female dog enters estrus—the period when she becomes fertile and receptive to mating—and this cycle repeats roughly every six months. If fertilization occurs, gestation lasts about 63 days, after which she gives birth to a litter. However, due to hormonal fluctuations linked to estrus and mating, a false pregnancy, or pseudopregnancy, can occur, where the body prepares for pregnancy even without fertilization. This condition, though not involving actual pregnancy, produces similar uterine changes, including thickening of the uterine lining.

Reproductive Cycle and Risk Factors

Each time a dog goes through estrus, the cervix relaxes and opens, and the uterine lining thickens in preparation for a possible pregnancy. Normally, the cervix acts as a protective barrier, preventing feces, debris, and harmful bacteria from entering the uterus. But when the cervix is open, this barrier weakens, increasing the risk of contamination. If fluid and bacteria accumulate over time—typically within two to three weeks—an infection can develop, leading to pyometra.

Types of Pyometra

There are two main forms: open pyometra and closed pyometra. Open pyometra is the most common and, relatively, the least dangerous. In this case, infection develops in the uterus, but the cervix remains open, allowing pus to drain. Affected dogs are often diagnosed quickly because owners notice a foul-smelling discharge from the vulva—yellow, greenish, or brown—or observe excessive licking of the genital area. Although pus drainage reduces immediate danger, the open form can progress to the closed form, or vice versa.

Closed pyometra is a more severe and dangerous condition. Here, the cervix closes completely, sealing the infection inside the body. Since pus cannot drain, it accumulates, causing the uterus to swell and enlarge. This increased pressure can lead to serious complications, including sepsis—blood infection—and uterine rupture, both associated with a high mortality rate. Because external signs are often absent in early stages, closed pyometra is difficult to diagnose early, increasing the risk to the animal.

Symptoms of Pyometra

Symptoms typically appear between two and three weeks after the end of an estrus cycle. In dogs with open pyometra, vulvar discharge is a clear indicator, but some fastidious animals may lick away the discharge before owners notice, making it essential to watch for other signs. These include increased water consumption and urination, heavy or rapid breathing, fever, decreased appetite, vulvar discharge, excessive genital licking, restlessness, lethargy, vomiting, gums that are dark red or bright red, abdominal pain or discomfort, weakness, and, in severe cases, collapse.

Causes and Contributing Factors

The etiology of pyometra is closely tied to the repeated reproductive cycle. With each estrus, the cervix opens and the uterine lining thickens, and over time, this tissue partially loses its ability to contract and expel fluids and contaminants. Older dogs—especially those over seven years—are most vulnerable, but the condition is not excluded in younger dogs, including those as young as a few months.

The bacterium most frequently involved in pyometra is Escherichia coli, commonly known as E. coli, responsible for approximately 90% of cases. This bacterium usually originates from fecal contamination. Factors such as obesity, a recessed vulva, chronic diarrhea, and excessive hair that frequently traps feces or debris increase the likelihood of bacteria reaching the vagina and uterus, creating a favorable environment for infection. Studies have also suggested that diet may influence the development of pathogenic bacteria, though the exact mechanisms require further research.