Ultrasound of the Empty Mare: What the Vet Sees

The pregnancy check is done by ultrasound about two weeks after ovulation. When the probe shows no vesicle, the mare is empty. But the image does not stop there: it also shows the state of the uterus and the ovaries, and that is often where the explanation lies. This post describes what the veterinarian sees on the scan of an empty mare, what fluid in the uterus means, what the scan cannot show, and what the next step is.

What does the vet see on the scan of an empty mare?

The rectal probe shows three things:

The uterus. Its size, the appearance of the wall and above all its content. During estrus the endometrium takes on a "cartwheel" pattern, a normal edema. Outside estrus the uterus should be homogeneous and empty. Fluid, cysts or a thickened wall are findings that need an explanation.
The ovaries. Growing follicles, the dominant follicle before ovulation, the corpus luteum after. They tell whether the mare cycles normally and whether covering happened at the right time.
The cervix. Closed outside estrus, open during it. A cervix that does not close lets in air and bacteria.

What the ultrasound does not show is the cause of an infection. It shows its consequences.

What does fluid in the uterus mean?

It is the most common finding in the empty mare, and its meaning depends on the moment:

Before covering. Fluid during estrus signals inflammation or a uterus that empties poorly. The cause should be found before covering, otherwise the embryo arrives in a hostile environment.
After covering. A little fluid in the hours that follow is normal: semen provokes a brief inflammation that a healthy uterus clears in one or two days. If the fluid is still there later, that is post-breeding endometritis, and the veterinarian treats it: uterine lavage, oxytocin, and antibiotics only if a culture justifies them. More in uterine lavage in the mare.
Without any covering. Fluid in a mare that has not been covered is almost always an infection or an anatomy that lets air in.

Fluid says there is a problem. It does not say which bacterium, or where it sits.

What happens when the vesicle has disappeared?

When a first scan showed a vesicle and the next one does not, the mare has resorbed. The most frequent causes are a uterine infection, an insufficient corpus luteum and abnormalities of the embryo. The infection is the one that repeats from cycle to cycle if it is not found.

What the ultrasound cannot show

The bacteria. A mare can have a uterus that looks clean on the scan and still harbor a chronic infection, for two reasons:

The bacteria are in the tissue, not in the lumen. In chronically infected mares, Streptococcus zooepidemicus lodges in foci 300 to 500 micrometers beneath the epithelium (Petersen et al., Clinical Theriogenology 2009). Neither the probe nor the swab reaches them. In a direct comparison, swab culture found 34% of uterine infections, biopsy culture 82% (Nielsen, Theriogenology 2005). More in why a clean swab does not rule out infection.
Part of the bacteria are dormant. They do not grow in standard culture. After activation with a culture medium the organism was detectable in 64% of mares, after saline in 8% (Petersen et al., Veterinary Microbiology 2015).

And the organism is almost always the same: in 28,887 endometrial samples analysed in Germany, beta-hemolytic streptococci accounted for 79.7% of positive cultures, penicillin-sensitive in 99.5% of cases (Köhne et al., 2024).

What is the next step after an "empty" scan?

In order, so that nothing is treated blind:

Check the timing. Did the estrus scans show an ovulation at the right moment relative to covering?
Check the semen. Quality, storage, the stallion's results with other mares.
Examine the uterus. Swab with culture and cytology, and a biopsy if the veterinarian finds it useful.
If the swab is clean and the mare stays empty: culture after activation. The veterinarian instills the bActivate medium, samples 48 hours later and cultures normally, with an antibiogram. bActivate is a diagnostic medium, not a medicine.
Treat what was found, according to the antibiogram, then cover in the following cycle with a fluid check.

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. The indications from the breeder's side are on the when to use page; the protocol on the how to use page.

Frequently asked questions

From when does the ultrasound confirm pregnancy?
The embryonic vesicle is visible about two weeks after ovulation. The veterinarian re-checks afterwards to confirm the pregnancy continues and to rule out twins.

Does fluid on the scan always mean infection?
No. Just after covering, a little fluid is a normal reaction. Fluid that persists, or fluid without any covering, signals a problem that needs an explanation.

Does a normal scan rule out a uterine infection?
No. The scan shows fluid and the wall, not bacteria. A chronic infection in the tissue can give a uterus that looks normal.

Can bActivate be seen on the scan?
No, and that is not its role. bActivate is a culture medium the veterinarian instills so that dormant bacteria grow, then cultures 48 hours later. It is a diagnostic step, not a treatment.

Sources

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
Nielsen JM. Endometritis in the mare: a diagnostic study comparing cultures from swab and biopsy. Theriogenology 2005;64:510-518. PMID 15978661
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
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, 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.