Fluid in the Mare's Uterus After Breeding: What It Means

Is fluid after covering normal?

Yes, for a short time. Semen and the extenders it comes in provoke an inflammation in every mare. That is how the uterus cleans itself: white blood cells come in, fluid is produced, and the uterine muscle pushes it out through a cervix that is still open. In a healthy mare the scan is clear again within about 48 hours, and the embryo, which arrives from the oviduct on day five or six, finds a quiet uterus.

The mares breeders write about are the others. The scan on day two or three still shows a line or a pocket of fluid. The vet flushes, gives oxytocin, and the fluid is gone, and the next cycle it is back. That pattern has a name, delayed uterine clearance, and it is the most common reason a mare that cycles normally and is covered at the right time still comes back empty.

Why does my mare hold fluid?

Because her uterus cannot empty itself the way it should. The usual reasons, and they add up:

• Age and foals. The uterine muscle loses tone and the lining loses its lymphatic drainage with the years and with each foaling. A mare of ten or more, or a mare with several foals, holds fluid more often than a young maiden mare.
• A uterus that hangs. In older, multiparous mares the uterus sinks below the pelvic brim, so fluid has to run uphill to get out.
• A cervix that does not open properly. Common in the older maiden mare: the cervix stays tight, and the fluid stays in.
• Frozen semen. It provokes a stronger inflammation than fresh or chilled semen, so a mare that copes with fresh semen may hold fluid after frozen.
• Infection. An inflamed lining produces fluid on its own, before any covering. Fluid during oestrus in a mare that has not been covered is almost always infection or an anatomy that lets air and bacteria in.

What does the vet do about post-breeding fluid?

The standard programme, and it works in most mares:

1. Scan 12 to 24 hours after covering. Fluid found early is easy to deal with; fluid found on day four is not.
2. Lavage. Warm saline in, fluid and debris out, repeated until the fluid runs clear. The lavage also removes the bacteria that came in with the semen. More in uterine lavage in the mare.
3. Oxytocin. Small doses to make the uterus contract and push out what is left. Some vets use a longer-acting drug the day after.
4. A sample. Culture and cytology from the fluid or from the lining, so that antibiotics are given only when there is something to treat.
5. Antibiotics only on a positive culture, chosen from the sensitivity test.

Done at the right time, this gets a large share of fluid mares in foal. The problem is the mare where it does not: flushed every cycle, sample clean every time, empty at the end of the season.

The fluid mare with a clean sample

A clean culture and cytology rules out an active infection on the surface of the lining. It does not rule out an infection in the lining. In a direct comparison, culture from a swab found 34 % of infected mares and culture from a biopsy 82 % (Nielsen, Theriogenology 2005). In chronically infected mares, Streptococcus zooepidemicus was found in foci 300 to 500 micrometres below the epithelium, where the swab does not reach (Petersen et al., Clinical Theriogenology 2009). Part of those bacteria are dormant and do not grow in culture at all; after activation with a culture medium the organism was detectable in 64 % of mares, after saline in 8 % (Petersen et al., Veterinary Microbiology 2015).

That is why recurring fluid with a clean swab is the profile in which the search should go one step deeper: a low-volume lavage or a biopsy for the sample, and, if the mare has failed two or more cycles, an activation culture. The vet instils 10 ml of the bActivate medium during early oestrus, samples again after 48 hours and cultures normally, with an antibiogram. bActivate is a diagnostic medium, not a medicine: it makes a dormant infection visible so the vet can treat it on the culture result and cover the mare in the following cycle.

The organism is nearly always the same, and nearly always treatable: in 28,887 endometrial samples analysed in Germany, beta-haemolytic streptococci made up 79.7 % of positive cultures and were penicillin-sensitive in 99.5 % of cases (Köhne et al., 2024).

What this looked like in practice

At Hagyard Equine Medical Institute in Kentucky, 64 Thoroughbred problem mares that had failed to conceive for at least three cycles were examined this way. 47 % were culture-positive after activation, 83 % became pregnant after treatment and covering, and 70 % of the mares with a known outcome foaled. The study had no placebo group; the figures describe the whole sequence of diagnosis and veterinary treatment (Petersen and Bojesen, Clinical Theriogenology 2014). Historically, problem mares of this kind foaled at 15 to 50 % against 80 to 85 % in the general population (Bosh et al., 2009).

Frequently asked questions

How long after covering should the uterus be clear of fluid?
Within about 48 hours in a healthy mare. Fluid on the scan two or three days after covering is delayed uterine clearance and should be treated, and a mare that holds fluid every cycle should be examined for the cause before she is covered again.

Is fluid in the uterus always an infection?
No. After covering it is the normal reaction to semen, and in older mares it is often poor drainage rather than bacteria. Fluid before covering, or in a mare that has not been covered, usually is an infection. Only a sample can tell, and a swab is the least sensitive way of taking it.

My mare is flushed every cycle and still not in foal. What now?
Ask your vet for a sample that reaches deeper than the swab, a low-volume lavage or a biopsy, and, if she has failed two or more well-timed covers, for an activation culture with bActivate before the next cover. Seven questions to check first: is my mare a candidate?

Does bActivate treat the fluid?
No. It is a diagnostic aid that wakes dormant bacteria so a standard culture can find them. The treatment is your vet's, chosen from the antibiogram, and the lavage and oxytocin for the fluid stay as they are.

Sources

• Nielsen JM. Endometritis in the mare: a diagnostic study comparing cultures from swab and biopsy. Theriogenology 2005;64:510-518.
• 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 et al. Activation of persistent Streptococcus equi subspecies zooepidemicus in mares with subclinical endometritis. Veterinary Microbiology 2015;179:119-125.
• Petersen MR, Bojesen AM. Clinical Theriogenology 2014;6(3):313-314 (Hagyard field series).
• Köhne M et al. Journal of Equine Veterinary Science 2024;105008.
• Bosh KA et al. Equine Veterinary Journal 2009 (foaling rates in Thoroughbred mares).

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.