Mare Abortion and Placentitis: Causes and Prevention

Most mare abortions have an identifiable cause, and infection leads the list, with ascending bacterial placentitis at the front. An early warning sign such as an udder that develops weeks too soon or a vulvar discharge is a reason to call your vet the same day, because early treatment can save the foal.

This guide is for breeders in France, Ireland and Italy. It covers the infectious and non-infectious causes of abortion, the early signs, how the vet measures placental thickness, treatment principles, what to do after a loss, and how to prevent it in 2026 and beyond.

How common is abortion in mares, and what causes it?

Abortion is fetal loss between about 40 and 320 days of gestation. After about 320 days, a dead fetus is a stillbirth. A veterinary review estimates current abortion incidence at 8% to 19%, and notes that most are isolated events rather than outbreaks, thanks to better farm management.

The largest French dataset is an Anses/Resumeq autopsy study of 851 aborted foals (2010-2019):

In that study, bacteria caused 82% of infectious abortions, and Streptococcus zooepidemicus was the most frequently isolated bacterium. Other series report that 30 to 50% of abortions never receive a final diagnosis, which is why submitting samples matters (see below).

What are the infectious causes of abortion?

Ascending bacterial placentitis

This is the most commonly diagnosed cause in many breeding areas. About 90% of placentitis cases are ascending infections: bacteria from the vulva and vagina pass through the cervix and inflame the placenta. The organisms most often involved are Streptococcus equi subsp. zooepidemicus, Escherichia coli and Klebsiella.

Placentitis is estimated to affect 3-7% of pregnant mares. Even on well-managed Thoroughbred farms, placentitis-related abortion can reach up to 5%. The ascending form is most common between 6 and 9 months of gestation.

A Swedish case series of 17 mares argued that an inadequate vulvar seal is an underrecognised trigger. Poor perineal conformation is therefore a risk factor to discuss with your vet before breeding.

EHV-1 (equine herpesvirus 1, rhinopneumonia)

EHV-1 is the most important viral cause of abortion. In the French study it caused 5-8% of abortions, usually in months 9-11. Other reviews report a wider range, 15% to 26% of fetal losses, mostly between months 4 and 10. The mare often shows no warning sign at all, and the virus can reactivate under stress.

Vaccination at 5, 7 and 9 months of gestation is the standard prevention. It reduces the risk and viral spread but does not eliminate it, so biosecurity remains essential.

Nocardioform placentitis and fungal placentitis

Nocardioform placentitis and fungal (mycotic) placentitis are less common than the ascending bacterial form. They tend to be diagnosed on placental examination and lab work, which is another reason to keep the placenta after an abortion.

Leptospirosis

Leptospira-related placentitis spreads through the blood (hematogenous route) and often causes abortion between 7 and 9 months. L. pomona is the most common abortion serotype. Shedders can carry the organism for up to three months. Leptospirosis is a zoonosis, so wear gloves and wash thoroughly when handling aborted material.

Other infectious agents

Equine viral arteritis (EVA) can cause abortion, with rates approaching 60% in naive populations. Mares vaccinated against EVA must be kept apart from unvaccinated mares for three weeks, and vaccination must happen at least three weeks before breeding. Discuss serological status with your vet before vaccinating.

What are the non-infectious causes?

Non-infectious causes made up 25% of cases in the French study. Their relative weight varies a lot between studies, from 10.5% to 72%.

Twins

Multiple ovulations occur in an estimated 8 to 15% of equine ovulations. The mare's uterus cannot sustain two fetuses well. Twins fell from 29% of Newmarket Thoroughbred abortions in the 1970s to 3% in 2001, which shows what early ultrasound and pinching of one embryo can achieve.

Fescue

Fescue is mostly a concern on tall fescue pastures. Gestation can be prolonged by an average of 27 days, and about 90% of affected mares develop agalactia. Late-gestation mares should come off infected fescue at least 3-4 weeks before the due date. Check your pasture with your vet or advisor.

What early warning signs should you watch for?

Ascending placentitis often gives clues weeks before an abortion, while the mare stays bright, with normal temperature and general condition.

• Premature udder development or milk dripping. Normal udder development appears about 2-4 weeks before foaling. An udder that fills well ahead of that is a red flag, sometimes as early as 5 months.
• Vulvar discharge, especially pus or mucus.
• Unusual weight pattern or discharge on the tail.
• Edema on all four legs together with fever or discharge, which points to a more serious infection.

These signs can resemble early labor. Do not wait to see if they settle. Check the udder and vulva daily during the last two months of pregnancy, and call the vet at the first sign.

How does the vet measure placental thickness (CTUP)?

The combined thickness of the uterus and placenta (CTUP) is measured by transrectal ultrasound. Only the vet does this, and it is the key tool for confirming placentitis. The measurement is taken ventrally, just cranial to the cervical star, averaging at least three readings while the foal is quiet.

Reference upper limits (PubMed Central review and Equine-Reproduction.com):

A value above these limits, together with fluid changes, placental separation or abnormal fetal findings, supports a diagnosis of placentitis. The vet may also check hormones. Blood progesterone drops rapidly in placentitis, and normal plasma progesterone is 3-6 ng/ml from day 110 to 300.

For mares with a history of loss, serial ultrasound of the placenta and fetus can begin from about 150 days. High-risk mares may need re-evaluation every 14-30 days.

How is placentitis treated?

Treatment is decided by the vet, and usually starts before lab results come back. The principles from the veterinary literature are:

1. Antimicrobials chosen for good placental perfusion, such as trimethoprim sulfamethoxazole or penicillin-based combinations.
2. Anti-inflammatories such as flunixin meglumine or phenylbutazone, and pentoxifylline in some protocols.
3. Progestogen support with altrenogest, usually continued until foaling. Note that evidence is not unanimous: one experimental study found no benefit from altrenogest.
4. Caslick vulvoplasty performed or extended where the vulvar seal is poor.

The Swedish series of 17 mares treated with trimethoprim sulfa twice daily plus acetylsalicylic acid, with a Caslick, ended with 17 live, surviving foals. The earliest, most severe cases still delivered septicaemic foals. Early detection makes the difference.

Clenbuterol is debated: it may relax the cervix and has been linked to earlier delivery, so leave that decision to your vet. Foals born before 300 days are unlikely to survive, and survival improves with each day closer to 320. Never medicate on your own.

What should you do after an abortion?

Act fast and protect the other mares.

1. Call your vet immediately.
2. Isolate the mare from other pregnant mares, and restrict movement of people and equipment between groups.
3. Keep the fetus and the placenta. Do not bury or discard anything. Store it cool, in a clean, sealed container or bag, and send it to the lab through your vet. Fetal autopsy and placental examination are what identify the cause.
4. Wear gloves and wash hands and boots, since some agents (leptospirosis, chlamydia) can infect people.
5. Disinfect the stall and remove bedding safely.
6. Ask your vet about reporting obligations and testing for EHV-1 and other agents in your country, especially if several mares are in contact.
7. Record everything in the mare's file: date of loss, gestation day, signs seen, treatments and lab results. This history guides the next breeding decision.

In Breedio, each mare's full history is kept in her record, so you can reread the timeline later with your vet.

How can you prevent abortion and placentitis?

• Vaccinate against EHV-1 at 5, 7 and 9 months of gestation, and keep contact horses vaccinated too (every six months for horses that mix with pregnant mares).
• Do a breeding soundness exam before the season, and correct poor perineal conformation or cervical damage. A Caslick procedure reduces fecal contamination.
• Scan early for twins, and treat them well before day 45.
• Remove late-gestation mares from infected fescue.
• Keep nutrition balanced. Deficiencies in selenium, vitamin E and iodine raise risk.
• Check the udder and vulva daily in the last two months.
• Use ultrasound screening from 8-9 months, and earlier for mares with a history of loss or uterine infection. Mares with previous placentitis are often given preventive treatment from about 7 months.
• Limit stress and mixing of groups, and apply biosecurity to every new arrival.

How does Breedio help?

The Gestation Tracker follows each mare through the roughly 340 days of pregnancy, calculates gestation phases from veterinary thresholds, and sends notifications at key milestones. That helps you plan the vaccination months and the late-pregnancy checks without relying on memory, and it works offline in the barn. Track Your Mares and keep a complete history for each one. For the stallion side of your plan, browse the stallion registry on Breedio.

Breedio keeps your calendar and records clean so that you spot a deviation sooner.

Key takeaways

• Infection causes most abortions, and ascending placentitis is the main culprit.
• Premature udder development or vulvar discharge needs a same-day call to the vet.
• CTUP is measured by the vet, with limits of 8, 10 and 12 mm by stage of gestation.
• After a loss, isolate the mare and keep the fetus and placenta for the lab.
• Vaccination, twin screening, a sound vulvar seal and daily checks are your best prevention.