Pyometra is a secondary infection that occurs as a result of hormonal

changes in the female's reproductive tract. During estrus ("heat"), white

blood cells, which normally protect against infection, are inhibited from

entering the uterus. This allows sperm to safely enter the female's

reproductive tract without being damaged or destroyed by these immune

system cells. Following estrus ("heat") in the dog, progesterone hormone

levels remain elevated up to two months and cause thickening of the

lining of the uterus in preparation for pregnancy and fetal development.

If pregnancy does not occur for several consecutive estrus cycles,

the uterine lining continues to increase in thickness until cysts often

form within the tissues (a condition called Cystic Endometrial Hyperplasia).

The thickened, cystic lining secretes fluids that create an ideal environment

for bacteria to grow in. Additionally, high progesterone levels inhibit the

ability of the muscles in the wall of the uterus to contract and expel

accumulated fluids or bacteria. The combination of these factors often leads to infection.





The use of progesterone-based drugs can cause changes in the uterus similar to the estrus cycle. In addition, estrogen or synthetic estrogen drug will increase the effects of progesterone on the uterus. Drugs containing both estrogen and progesterone are sometimes used to treat certain conditons of the reproductive system. Any intact female receiving hormones must be carefully monitored for the development of pyometra.





The cervix is the gateway to the uterus. It remains tightly closed except during estrus, when it relaxes to allow sperm to enter the uterus. If the cervix is open or relaxed, bacteria that are normally found in the vagina can enter the uterus easily. If the uterus is normal, the uterine environment is adverse to bacterial survival; however, when the unterine wall is thickened or cystic, perfect conditions exist for bacterial growth. In addition, when these abnormal conditions exist, the muscles of the uterus cannot contract properly either due to thickening of the uterine wall or the hormone progesterone. This means that bacteria that enter the uterus cannot be expelled.





Pyometra may occur in any sexually intact young to middle-aged dog; however, it is most common in older dogs. After many years of estrus cycles without pregnancy, the uterine wall undergoes the changes that promote this disease. Pyometra usually occurs 2-8 weeks after the last estrus ("heat cycle").







The clinical signs depend on whether or not the cervix remains open. If it

is open, pus will drain from the uterus through the vagina to the outside. Pus

or abnormal discharge is often seen on the skin or hair under the tail or on 

bedding and furniture where the dog has recently laid. Fever, lethargy,

anorexia, and depression may or may not be present.

If the cervix is closed, pus that forms is not abe to drain to the outside. It

collects in the uterus ultimately causing the abdomen to distend. The 

bacteris release toxins that are absorbed into the bloodstream. Dogs with 

closed pyometra become severely ill very rapidly. They are anorexic, very 

listless and very depressed. Vomiting or diarrhea may also be present.

Toxin released by the bacteria affect the kidney's ability to retain fluid. Increased urine production occurs, 

and many dogs drink an excess of water to compensate. Increased water consumption may occur in both open and closed pyometra.





Dogs that are examined early in the course of the disease may have slight vaginal discharge and show no 

signs of illness. However, most dogs with pyometra are seen later in the illness. A very ill female dog with a

history of recent "heat" that is drinking an increased amount of water should be suspected of having

pyometra. This is especially true if there is a vaginal discharge or a painful, enlarged abdomen.


Dogs with pyometra usally hae a severe elevation of the white blood cell count and often have an elevation of

globulins (a type of protein often associated with the immune system) in the blood. The specific gravity

(concentration) of the urine is generally low due to toxic effects of the bacteria on the kidneys. However, these changes are non-specific and may be present in any dog with a major bacterial infection.


If the cervix is closed, radiographs (x-rays) of the abdomen will often identify the enlarged uterus. If the cervix is open, there will often be such minimal uterine enlargement that the radiograph may be inconclusive. An ultrasound examination may be helpful in identifying an enlarged uterus and differntiating that from a normal pregnancy. Ultrasound changes that indicate pyometra include increased unterine size, thickened uterine walls, and fluid accumulation within the uterus.






The preferred treatment is to surgically remove the infected uterus and ovaries, or perform an ovariohysterectomy ("spay"). Dogs diagnosed in the early stage of the disease are very good surgical candidates. The surgery is more complicated than a routine spay at this stage. However, most dogs are 

diagnosed with pyometra when they are quite ill resulting in a more complicated surgical procedure and a 

longer period of hospitalization. Intravenous fluids are required to stablize the dog before and after surgery. Antiobiotics are usually given at home after surgery.



© Copyright 2009 Lifelearn Inc. Used and/or modified with permission under license.

Alta Vista Veterinary Hospital is a Cenral Phoenix Animal Hospital 





What else can cause changes in the uterus?


How does bacteria get into the uterus?


When does pyometra occur?

What are the clinical signs of pyometra?


How is pyometra diagnosed?


How is pyometra treated?