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Pyometra and Metritis: Similarities and Differences

theriogenology & pediatrics

There are plenty of journal articles and book chapters written on both conditions, therefore my goal is to write practically and even, where appropriate, somewhat entertainingly.

In this post on pyometra and metritis, I will be using bitch as the main ‘character’, however, most of it applies to queen as well. Except, of course, that queen is an induced ovulator and therefore theoretically should never get pyometra if not bred or if living alone. Queens being queens, however, they indeed do spontaneously ovulate, and of course become at risk of pyometra just like the rest.



Clinical symptoms

In both conditions, a bitch or a queen is often presented with various degrees of vulvar discharge and therefore with suspicion of a uterine disease. It can vary from purulent to bloody, from having no odour to severely malodorous. There may be very little to no discharge in closed pyometra cases. Depending on level of endotoxemia, both diseases can present from bitch or queen being asymptomatic to being severely ill and in shock, which most likely correlates to duration of disease.

Vaginal cytology findings

Cytology of vaginal discharge will appear similar in both conditions: mostly PMNs and bacteria. No cornification on cytology indicates no estrogen presence. If we split hairs, I have seen both pyometra (purulent uterine fluid) and heat (cornified cells on cytology) at same time, but most commonly in granulosa-cell tumour cases, with ovaries producing both progesterone and estrogen at the same time). Otherwise, if there is cornification: likely no pyometra. There should never be vaginal cornification in metritis cases. If we split hairs again: there is a rise in estrogen at parturition, but not sufficient for cytology to appear like the bitch is in estrus.

Findings on ultrasound exam

Ultrasound exam is highly recommended every time uterine disease is suspected.

Uterus can be severely distended in some ‘closed’ pyometra cases, or just barely enlarged in open pyometra or in metritis cases. Without ultrasonography, diagnosis may be difficult, as radiographs will not differentiate between early pregnancy and pyometra and will not give much detail on uterine diameter. Severity of symptoms in both conditions does not always directly correlate with uterine size or the amount of uterine fluid, but rather with level of bacteremia and how much endotoxin is released into circulation during bacterial disintegration.


Leukocytosis, neutrophilia, +/-anemia are common in both conditions, except that in severe cases of pyometra there may be renal tubular damage with corresponding increase of renal values (theoretically could occur in metritis cases as well).


If the bitch is clinically ill, both conditions require immediate attempt to stabilize the bitch by addressing dehydration, acid-base and electrolyte abnormalities, hypotension, nausea, infection and endotoxemia.

There is indeed a saying that we should ‘never let the sun go down on pyometra’, but, honestly, same applies to acute metritis as well. Depending on level of endotoxemia, both diseases can present from bitch being asymptomatic to being severely ill and in shock. The sun should just never go down on the latter.




The main difference between the two conditions is when they occur in bitch’s reproductive cycle, which also influences one most important differences in the treatment.

Pyometra is a progesterone-mediated both uterine and systemic infection that by definition is diagnosed only in diestrus (within two months after the end of estrus), and most commonly occurs 3-4 weeks after the end of estrus when progesterone is expected to be at the highest. IT IS A HIGH PROGESTERONE CONDITION. 

Progesterone’s job is to keep the cervix closed and therefore uterine contents cannot evacuate properly if there is progesterone on board. Even in open pyometra cases, the cervix is just partially open.

If uterine fluid is present in any other stage of bitch’s cycle (like anestrus and estrus), it will most likely be mucometra or hydrometra, both non-infectious conditions. They both have good prognosis for the bitch’s health, but poor prognosis for future fertility.

Hemometra is also described as a condition of uterine distention with fluid, however, I would suspect it in rodenticide poisoning, or clotting disorder and not as a condition occurring spontaneously.


Metritis is an infectious disease that affects post partum bitches, most commonly 0-7 days after whelping, and is often referred to as acute post-partum metritis, or acute puerperal metritis. IT IS A LOW PROGESTERONE CONDITION.

Definition of metritis therefore is easy: the bitch must have whelped recently (regardless if pups survived or not) to have postpartum metritis.

A word on pathogenesis: pyometra develops not due to abnormally high progesterone concentrations, but more that endometrium in diestrus is hyper-reactive to any stimulation and the cervix is closed (because of high progesterone). Infection occurs when bacteria that moves into the uterus during proestrus and estrus cannot be cleared prior to complete closure of the cervix. Vagina is not a sterile environment, but uterus is. When functioning well, uterus is like self-cleaning oven, but it does malfunction now and then, especially in older bitches with cystic endometrial hyperplasia (CEH), which can then predispose to pyometra. Even if pyometra is a disease of older bitches, it is not uncommon to diagnose it in young ones, on their very first heat cycle. It is important to mention it to pet owners who now keep their bitches intact until full maturity as one of serious risks.

Post partum metritis can occur in association to retained placentas, retained pups, prolonged delivery. In most cases that we see in practice, however, it occurs in apparently healthy bitches after uneventful whelping and timely passing of placentas.

There is a school of thought that other conditions, like hypocalcemia, are likely predisposing factors. We often add calcium supplementation to treatment of bitches with post partum metritis that have large litters, even if calcium levels appear normal on bloodwork (as for testing, ionized calcium is better predictor than total calcium).

Note: despite common belief, retained placentas alone are rarely responsible for metritis: if all other defence mechanisms are working properly, placentas will be expelled just fine, even if it takes some time, up to a few days.


Main difference in treatment: in pyometra cases, progesterone sources (ovaries) need to be either removed surgically or progesterone levels need to be addressed medically. Without lowering progesterone, pyometra treatment will not be successful.

Therefore, gold standard for pyometra remains ovariohysterectomy. Referral hospitals and specialty clinic in North America may carry Alizin (Aglepristone), a progesterone receptor antagonist, a drug that can be successfully used to treat both open and closed pyometra (or terminate pregnancy) elegantly and with less side effects than using prostaglandins. Medical treatment is recommended for young and very valuable for breeding program bitches and queens. They need to be bred right during next estrous cycle, as some of them are at risk of developing pyometra again.

Metritis rarely (if ever) is treated surgically, except in non-responding cases. This is a common mistake, to remove uterus/ovaries in a post partum bitch as soon as metritis is diagnosed. Antibiotics and supportive care are at the core of metritis treatment. Hypocalcemia may be a possible comorbidity in cases of treatment resistant metritis.

The rest of supportive treatment is similar to pyometra treatment.

‘Sun going down’

From this ‘never let sun go down on pyometra’ expression it may appear that the bitch should be spayed immediately after diagnosis. Truth is, there are very few immediate (emergency) pyometra spays, because the most critical patients need addressing infection, hydration, hypotension, acid-base and electrolyte abnormalities first, otherwise anesthesia complications will increase exponentially.

On the other hand, if we diagnose ‘closed’ pyometra during routine pregnancy ultrasound at the end of the day in an apparently healthy bitch whose bloodwork does not indicate endotoxemia and renal involvement, we place them on antibiotics and schedule surgery the next morning. We educate owners on risk of uterine rupture and advise to keep the bitch quiet until then. The ‘sun going down’ reference, I suspect, comes from the times in the past when we attempted to treat pyometra in clinically ill bitches and queens only using antibiotics for days before finally spaying. Without addressing the main cause (high progesterone) they will deteriorate quickly.

In cases where the bitch is in shock, which happens with acute onset of metritis or pyometra with endotoxemia, even hours matter.

Interesting fact that I found preparing this post (although personally I have not observed this in the practice): endotoxemia is responsible for worst symptoms of pyometra and metritis and the toxins are released when large numbers of bacteria die and disintegrate, therefore the clinical symptoms can worsen when bitch is given antibiotics, as antibiotics do not affect endotoxin concentrations, but only kills bacteria, releasing more toxins into bloodstream. 



About the Guide: Rasa Levstein, DVM, DVSc, DACT

I am a small animal theriogenologist from Guelph, Ontario, Canada. My work revolves around breeding dogs, solving complex reproductive cases that colleagues share in form of referrals, and managing a canine frozen semen bank. I find the clinical aspect of reproduction fascinating and enjoy sharing my knowledge with veterinary students, colleagues, and breeders. Outside of work, I prioritize spending time with my family, reading, hiking, and horseback riding.


#theriogenology #practicepearls


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