Repeated Early Embryonic Loss in Mares: Causes and Work-Up

In short. One early loss is common and usually not investigated. Two or more in the same mare are a pattern, and a pattern has a cause: twins, a weak corpus luteum, an ageing lining, or an inflamed or infected uterus. The infection is the cause most often missed, because the routine swab finds only a third of them, and it is the cause that repeats until it is found. The work-up is short: timing, twins, progesterone, biopsy, and an activation culture if the biopsy is clean and the losses continue.

What counts as repeated early embryonic loss?

A pregnancy confirmed on the day 14 to 16 scan that is gone by day 40 to 45 is an early embryonic loss; the breeder says the mare absorbed. A single loss happens to fertile mares too, and most of them conceive again on the next cycle and carry to term. It is the second loss, or a loss in a mare that also has fluid, a clean swab and an empty season behind her, that should start a work-up before she is covered again. What the loss looks like from the owner's side is in mare absorbed at 30 days.

The causes, and the test for each

Cause	How it presents	How the vet checks	Can it be fixed?
Twins	Two vesicles on the day 14 to 16 scan, one or both lost later	Early scan and reduction of one vesicle	Yes, by finding them in time
Insufficient corpus luteum	Loss around day 30 to 40 with low progesterone	Progesterone blood test	Yes, progesterone support for the pregnancy
Ageing lining (fibrosis, cysts)	Older mare, losses at various stages	Biopsy with Kenney score, ultrasound for cysts	Partly: cysts can be removed, fibrosis cannot
Embryo quality	Older mare, older or frozen semen, irregular vesicle	History; no direct test	Reduced by good timing and semen
Stress, fever, transport, drugs	Loss after an event in the first weeks	History	Avoidable next time
Inflammation or infection of the uterus	Fluid on the scan, short cycles, previous endometritis, or nothing at all	Culture and cytology, biopsy, activation culture	Yes, treated on the culture result

Why the infection is the cause that repeats

An embryo can survive for a while in a uterus that is inflamed at a low level; the mare conceives, the scan shows a vesicle, and then the environment wins. Because a uterus that is being prepared for pregnancy dampens its own immune response, it is also a good place for bacteria to sit unnoticed. The infection is not cleared between cycles, so the same thing happens next time.

It is missed because the routine sample cannot see it. Culture from a swab found 34 % of infected mares in a direct comparison; culture from a biopsy found 82 % (Nielsen, Theriogenology 2005). In chronically infected mares, Streptococcus zooepidemicus sits in foci 300 to 500 micrometers below the epithelium (Petersen et al., Clinical Theriogenology 2009), and part of those bacteria are dormant and do not grow in 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). A mare can therefore have a clean culture and cytology, conceive, and still lose the pregnancy to an infection nobody has seen.

The work-up, in order

Timing. Was ovulation where it should have been relative to the cover, and was the vesicle the right size for its day?
Twins. Look at the day 14 to 16 scan again, or scan earlier next time.
Progesterone. A blood sample in the pregnancy; if it is low, support it next time.
Biopsy. Culture, cytology and histology from one sample. It finds far more infections than the swab and grades the lining.
Activation culture if the biopsy culture is clean and the losses continue, or if the mare has fluid, a previous endometritis, three or more foals, or is ten or older. The vet instills 10 ml of the bActivate medium in early estrus, samples after 48 hours and cultures with an antibiogram. bActivate is a diagnostic aid, not a treatment; a positive result is treated by the vet on the antibiogram, and the mare is covered in the following cycle. A negative result rules the dormant infection out, and that is a result too.
Cover with a fluid check 12 to 24 hours after, and scan at day 14 to 16, 25 to 30 and 40 to 45.

In the Hagyard series of 64 Thoroughbred problem mares that had failed for at least three cycles, 47 % were culture-positive after activation, 83 % became pregnant after treatment and covering, and 70 % of those with a known outcome foaled; there was no placebo group (Petersen and Bojesen, Clinical Theriogenology 2014).

Frequently asked questions

How many early losses before the mare should be investigated?
After the first loss in a young mare with no history, waiting one cycle is reasonable. After a second loss, or a first loss in a mare that is ten or older, has fluid on the scan, or has a clean swab and an empty season behind her, investigate before the next cover.

Can a uterine infection cause a mare to absorb even though she conceived?
Yes. A low-grade or dormant infection does not prevent conception every time; it makes the uterus unable to keep the pregnancy. Because the infection stays, the loss repeats.

Does a clean culture and cytology rule out infection as the cause?
No. It rules out an active infection on the surface of the lining. Deeper and dormant infections need a biopsy or an activation culture to be found.

What does the activation culture involve for the mare?
One instillation of 10 ml in early estrus, a second sample two days later, and treatment only if something grows. The cycle is used for diagnosis, and she is covered on the next one.

Sources

Nielsen JM. Theriogenology 2005;64:510-518.
Petersen MR et al. Clinical Theriogenology 2009;1:393-409.
Petersen MR et al. Veterinary Microbiology 2015;179:119-125.
Petersen MR, Bojesen AM. Clinical Theriogenology 2014;6(3):313-314.

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.