Uterine Lavage in the Mare: When and Why

A uterine lavage (uterine flush) is a treatment carried out by the veterinarian: body-warm fluid is run into the uterus through a catheter and drained again until it comes back clear. The aim is to remove fluid, debris and bacteria from the uterus. This post explains when a lavage makes sense, which fluid is used, what a lavage solves, and what it does not solve.

When does the veterinarian flush the uterus?

There are three situations in which a uterine lavage is common:

• Fluid in the uterus after covering. Semen and covering trigger a short inflammatory reaction. A healthy uterus clears it within one to two days. In mares that cannot, fluid stays behind, and the embryo does not survive in it. The veterinarian then flushes shortly after covering, often together with oxytocin so that the uterus contracts and empties.
• Fluid before covering. Fluid in the uterus during estrus points to inflammation or to a uterus that empties poorly. A lavage before covering gives the embryo a clean environment.
• As part of treating endometritis. With a confirmed infection the veterinarian flushes to remove bacteria and inflammatory products, then treats with antibiotics chosen from the antibiogram. More on the condition itself in endometritis in mares.

Which fluid is used?

Usually physiological saline or lactated Ringer's, body-warm, in portions of half a liter to a liter, repeated until the fluid runs clear. The veterinarian judges what comes back: cloudy fluid, flakes or pus say something about severity. Antibiotics are not a standard ingredient of the flushing fluid. They belong there when a positive culture with an antibiogram exists, not as a precaution. In the German analysis of 28,887 endometrial samples, the streptococci that cause most uterine infections were 99.5% penicillin-sensitive, but 3.1% of all positive cultures were already multi-resistant (Köhne et al., 2024). Blind flushing with antibiotics makes that last number bigger.

What does a uterine lavage solve?

A lavage does exactly what the name says: it removes what lies loose in the uterine cavity. Fluid, debris, semen residue and bacteria that float free in the lumen. For fluid after covering that is often enough, and many mares get in foal quite normally after a timely flush.

What does a uterine lavage not solve?

Two things, and they explain why some mares stay empty despite repeated flushing:

• Bacteria in the tissue. In chronically infected mares, Streptococcus zooepidemicus sits in foci 300 to 500 micrometers beneath the epithelium of the uterine lining (Petersen et al., Clinical Theriogenology 2009). Flushing fluid does not reach there.
• Dormant bacteria. Part of these bacteria are in a dormant state in which they do not grow in culture. The swab does not find them, so they are not treated. After activation with a growth medium the organism was detectable in 64% of mares, after saline in 8% (Petersen et al., Veterinary Microbiology 2015).

That is why a lavage is a treatment of symptoms and not a diagnosis. When a mare builds fluid again after flushing, or stays empty, the next question is not "flush again" but "which bacterium is behind it, and where does it sit".

How does bActivate relate to a lavage?

Both go into the uterus through a catheter, and there the similarity ends. bActivate is a diagnostic growth medium: the veterinarian instills it into the uterus and leaves it there so that dormant bacteria start growing again. 48 hours later the vet takes a perfectly normal swab and cultures it as usual, with an antibiogram. The result is a culture that shows what really sits in the uterus, including what the first swab missed. bActivate is not a medicine and not a treatment; the treatment follows afterwards, targeted, from the antibiogram.

At Hagyard Equine Medical Institute in Kentucky, 64 Thoroughbred problem mares that had been barren for at least three cycles were examined. 47% (30 of 64) were culture-positive after activation, 83% (53 of 64) became pregnant after targeted treatment and breeding, and 70% of the mares with a known outcome (32 of 46) delivered a live foal. The study had no placebo group; the figures describe the whole sequence of diagnosis and treatment, not the effect of a single product (Petersen and Bojesen, Clinical Theriogenology 2014). Every result is on the clinical evidence page.

What are the risks of a uterine lavage?

With clean technique they are small. What matters is sterile handling, body-warm fluid and not too much pressure. Repeated flushing without a diagnosis carries a different risk: it costs cycles. Every cycle spent on a flush without covering is one cycle less in the season. What to do when the mare does not get in foal despite flushing is on the when to use page; the protocol for the veterinarian is on the how to use page.

Frequently asked questions

How often can a mare be flushed?
The veterinarian decides per mare and per cycle. After covering, flushing is often done on one or two consecutive days. If a mare needs flushing again in every cycle, that is a reason to look for the cause, not to keep flushing.

Can I flush the uterus myself?
No. A uterine lavage is a sterile procedure with a catheter through the cervix and belongs in the veterinarian's hands.

Do antibiotics belong in the flushing fluid?
Only with a positive culture and an antibiogram, and then the drug that follows from it. Not as a precaution.

Is bActivate a lavage?
No. bActivate is a growth medium that stays in the uterus so that dormant bacteria grow and can be detected 48 hours later in a normal culture. It is a diagnostic step, not a treatment.

Sources

• Köhne M, Hegger A, Tönissen A, Heusinger A, Hader C, Görgens A, Sieme H. Frequency of potentially pathogenic bacterial and fungal isolates among 28,887 endometrial samples from mares. 2024.
• Petersen MR, Nielsen JM, Lehn-Jensen H, Bojesen AM. Streptococcus equi subspecies zooepidemicus resides deep in the chronically infected endometrium of mares. Clinical Theriogenology 2009;1:393-409
• Petersen MR, Skive B, Christoffersen M, Lu K, Nielsen JM, Troedsson MHT, Bojesen AM. Activation of persistent Streptococcus equi subspecies zooepidemicus in mares with subclinical endometritis. Veterinary Microbiology 2015;179:119-125. DOI 10.1016/j.vetmic.2015.06.006
• Petersen MR, Bojesen AM. Equine subclinical endometritis caused by dormant beta-hemolytic streptococci. Clinical Theriogenology 2014;6(3):313-314 (congress abstract, 64 problem mares at Hagyard Equine Medical Institute, no placebo group).

Written by the Bojesen & Petersen Biotech ApS team. Medical oversight: Prof. Anders Miki Bojesen DVM PhD (University of Copenhagen) and Dr. Morten Rønn Petersen DVM PhD Dipl. ACT.