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Understanding Pyometra: Part I. Natural history and etiopathogenesis

Mar 05, 2019

Hagman R.  Pyometra in small animals.  Vet Clin North Am Small Anim Pract 2018;48(4):639-661.

Cherokee_SmokePyometra refers to an acute or chronic bacterial infection of the post-estrus uterus accompanied by accumulation of inflammatory suppurative exudate (pus) within the uterine lumen. Sexually intact adult female cats and dogs are susceptible to the development of this relatively common condition.  Pyometra occurs less frequently in queens than in bitches, as the former are induced ovulators and experience a significant association between seasonal light cycles and estrus frequency, unlike dogs.  In cats, therefore, there may be less influence of progesterone dominance on the etiology of pyometra than in dogs, where pyometra develops during the luteal phase.  Repeated estrus cycles that do not result in pregnancy can increase a cat's predisposition to development of pyometra, as can the use of synthetic or natural hormones and other medications designed to control estrus or terminate pregnancy. 

Approximately 2.2% of queens are diagnosed with pyometra prior to 13 years of age.  The mean age at diagnosis is 5.6 years, and the incidence of feline pyometra increases with the animal's age, especially after age 7 years.  Pyometra in both species is more common in countries where elective gonadectomy of healthy dogs and cats is not routinely performed.

Progesterone is the key hormone that promotes the development of uterine infection, generally by ascending opportunistic bacteria. A uterus under the influence of progesterone is prepared for pregnancy but is also predisposed to microbial growth if a pregnancy does not occur, as progesterone facilitates growth and proliferation of endometrial glands, increases local fluid secretion, decreases myometrial contractions, and suppresses intrauterine leukocyte responses.  

Cystic endometrial hyperplasia (CEH), a clinically silent condition when uncomplicated, is more common in older bitches and queens, is also a luteal phase disease, and can predispose to the development of pyometra in either species.  However, not all CEH progresses to pyometra, and these disorders can develop independently.  Although less is known about risk factors and protective factors involved in feline pyometra, and development of this condition in cats is less well studied than in dogs, the pathogenesis of feline pyometra is assumed to be similar to that in dogs.

The most common infectious agent in both feline and canine pyometra is Escherichia coli, which is found in approximately 71% of culture-positive feline pyometras. Staphylococcus spp. and Streptococcus spp. are associated with 8% and 19% of culture-positive feline pyometra cases; other bacteria are involved in the remaining 2%.  This closely parallels bacterial culture findings in canine pyometra.  Approximately 20% of bacterial cultures in feline pyometras result in no growth.  Mixed bacterial infections have been reported in dogs but not in cats.  Bacteriologic culture of vaginal discharge in an affected animal is of no utility, as the same bacteria are present in the vaginas of healthy animals. Although pyometra is generally considered to be the result of ascending infection from gastrointestinal bacteria, hematogenous spread of infecting bacteria is considered possible in dogs.  Given that cats, especially intact cats given to free-roaming lifestyles and random matings, are frequently involved in encounters that involve fighting and biting, hematogenous spread of bacteria to a predisposed feline uterus is possible as well.

Pyometra can result in life-threatening complications due to the systemic and local inflammatory responses engendered by bacteria and their products such as endotoxins.  Systemic inflammatory response syndrome, septic shock, disseminated intravascular coagulation, multiple organ dysfunction, and death can ensue if treatment is not pursued quickly. Pyometra is therefore a medical emergency in both species and veterinary care must be sought immediately when the condition is suspected. [PJS]

See also:

Hagman R, Strom Holst B, et al.  Incidence of pyometra in Swedish insured cats.  Theriogenology 2014;82:114-120.

Kenney KJ, Mattheisen DT, et al.  Pyometra in cats: 183 cases (1979-1984).  J Am Vet Med Assoc 1987;191:1130-1132.

cystic endometrial hyperplasia disseminated intravascular coagulation endotoxin estrus luteal phase progesterone septic shock

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