Placentitis--General Information

The troublesome problem of placentitis was the subject of an in-depth session at the 50th annual American Association of Equine Practitioners (AAEP) Convention in Denver, Colo., Dec. 4-8, 2004. Michelle LeBlanc, DVM, Dipl. ACT, of Rood and Riddle Equine Hospital in Lexington, Ky., set the tone for the session when she told her listeners that "The single most important cause of premature delivery of a foal is placentitis. It accounts for nearly one-third of late-term abortions and fetal mortality in the first day of life."

And, she added, 90% of the placentitis cases stem from bacteria entering the uterus via the vagina and then breaching the cervical barrier.

The title of LeBlanc's talk was, "Ascending Placentitis: What We Know About Pathophysiology, Diagnosis, and Treatment."

While the disease is insidious, she said, horse owners and practitioners have the advantage of being alerted by some outward signs, enabling them to begin a treatment protocol aimed at saving the foal. Two of the classic tip-offs on the part of the pregnant mare, she said, are vaginal discharge and premature udder development.

The goal of the practitioner, she said, is to manage these mares in such a way that pregnancy is prolonged. "If premature birth can be delayed for a few weeks after clinical signs of placentitis develop," she informed her listeners, "a foal may be born significantly premature, but survive with limited neonatal care."

A paper published in the Proceedings with LeBlanc as chief author discussed in detail what occurs within the uterus during pregnancy and parturition.

During her talk, she spent a portion of her time discussing a study that took place earlier in her career when she was on the staff at the University of Florida. The goal of the study was to determine whether measuring changes in maternal plasma progestins and determining placental thickness or separation were valid protocols in determining whether placentitis was posing a threat of abortion.

During the study, LeBlanc said, plasma progestin concentrations and transrectal ultrasonography findings were compared between pony mares carrying normal pregnancies and mares that received a cervical inoculation of Streptococcus equi between 270 and 293 days gestation.

Peter Sheerin, DVM, Dipl. ACT, also of Rood and Riddle Equine Hospital, picked up the narrative at that point and presented the following information: Uterine-placental thickness (CTUP, or combined thickness of the uterus and placenta) was measured by transrectal ultrasonography in all mares beginning on 224 days gestation. Experimental mares were scanned weekly until inoculation and every 48-72 hours after inoculation with bacteria. A combined uterine-placental thickness in excess of 1.2 centimeters was considered abnormal.

Progestins were measured every three days in control mares from 265 days gestation until parturition and every day in the inoculated mares. Plasma progestins were compared before and after inoculation with bacteria in experimental mares and between groups.

Assessment of the plasma progestin data, Sheerin said, revealed that a minimum of three plasma samples taken 48-72 hours apart in the experimental mares was needed to identify individuals that exhibited a change in progestin concentration from a baseline sample taken the day before experimentation.

In the experimental mares, he said, a plasma progestin profile was considered to be abnormal--either in rising or falling levels--if the three progestin samples taken after the baseline sample increased or decreased by more than 50 percent above or below the baseline value.

(It was later pointed out that this evaluation method must be used with caution after 310 days of gestation. After that time, progestins begin to rise and continue to rise until parturition. If one measures progestins after 310 days of gestation and the progestin concentration is decreasing, then there may be a problem, LeBlanc said.)

"In clinical practice," Sheerin reported, "one may examine a pregnant mare after plasma progestins have begun an abnormal rise or fall. In the latter situation, if the value obtained for the progestin sample lies outside the normal reference range for the laboratory and the second and third samples are also out of the range, it is likely that the fetus is stressed."

All of the control mares in the study, Sheerin said, delivered healthy foals. Only two of the mares that were inoculated with bacteria delivered live foals; both were somewhat premature. One was born after 313 days of gestation and one was born after 314 days of gestation. They were described as being viable and precociously mature.

Sheerin also reported:

"Four of the 15 mares with placentitis could not be identified clinically, because they did not exhibit a vaginal discharge or precocious mammary development. Fourteen of 15 mares exhibited changes in their plasma progestin profiles. Plasma progestins decreased sharply in the seven mares that aborted within seven days of inoculation and increased in seven of eight mares that carried their fetuses for more than 15 days after inoculation.

He also reported that: "All inoculated mares exhibited histological changes in the cervical star region of the placenta consistent with ascending placentitis. Nine of 15 inoculated mares had a combined uterine-cervical thickness in excess of 1.2 centimeters before delivery. Four of the seven mares that aborted less than seven days after inoculation and two mares that carried for more than 15 days had CTUP less than 1.2 centimeters."

Sheerin drew these conclusions from the study:

"Fourteen of 15 mares were identified when both transrectal ultrasonography and plasma progestin profiles were performed. However, four of the 15 inoculated mares did not exhibit clinical signs. Therefore, ultrasonography and plasma profiles are useful diagnostically only if the mare exhibits either vaginal discharge or premature udder development."

In a later treatment presentation as part of the in-depth series, Margo MacPherson, MS, DVM, Dipl. ACT, of the University of Florida, dealt with treatment protocols for placentitis. In a large clinical study regarding treatment of mares diagnosed with placentitis in Kentucky, she said, investigators examined records of 477 mares. Fifteen were diagnosed with placentitis.

The clinical signs presented included early udder development, vaginal discharge, and increased CTUP. The mean gestational age at diagnosis was 8.6 months.

The mares studied were treated with a combination of systemic antibiotics--trimethoprim sulfa, ceftiofur, or penicillin and gentamicin--along with pentoxifylline, altrenogst (Regumate) and non-steroidal anti-inflammatory agents (NSAIDs.) The afflicted mares were treated from discovery of the disease until they either aborted or delivered a live foal. Eleven of the 15 diagnosed and treated mares delivered live foals. Birth weights of the surviving foals from mares treated for placentitis, MacPherson said, were similar to foals from non-affected mares.

She also reported on another study that indicated that trimethoprim sulfa and pentoxifylline, either alone or in combination, can delay pre-term delivery in mares with placentitis.

"Data from these two studies," MacPherson concluded, "suggest that antibiotic and anti-inflammatory treatment may positively impact pregnancy outcome in mares with placentitis."

Also figuring prominently into the equation when placentitis is involved, LeBlanc said in her opening presentation, is inflammation, which can set off a cascade of events that concludes with premature birth. LeBlanc explained it something like this: Bacteria or bacterial products in fetal membranes stimulate cell-mediated immune mechanisms with subsequent release of pro-inflammatory cytokines. The pro-inflammatory cytokines stimulate release of prostaglandins from the endometrium, which in turn initiate uterine contractions.

Thus, there is a need for a drug therapy that can combat the bacterial invasion that precipitated placentitis and also negate the inflammatory action of cytokines. One study, MacPherson reported, involved treating primates with placentitis using antibiotics only--ampicillin. That treatment protocol, she said, was effective in eradicating bacteria from the amniotic fluid, but "it did not block elevations in amniotic fluid cytokines, prostaglandins, or uterine contractions."

However, MacPherson added: "Concentrations of amniotic fluid cytokines and prostaglandins were suppressed in primates treated with ampicillin, dexamethasone, and indomethacin. It seems that combined therapy is needed to stem bacterial infection and to suppress the subsequent inflammatory response." A similar study has not been conducted in the horse.

Also effective in staving off premature labor, MacPherson said, are progestins, such as altrenogest.

MacPherson's conclusion in her presentation was this: "Effective treatments for placentitis in mares are still elusive. Data from studies involving humans and non-human primates indicate that combined therapies with antibiotics, anti-inflammatory agents, and progestin therapy show the most promise in interrupting pre-term labor. Preliminary data in horses support this concept."

About the Author

Les Sellnow

Les Sellnow is a free-lance writer based near Riverton, Wyo. He specializes in articles on equine research, and operates a ranch where he raises horses and livestock. He has authored several fiction and non-fiction books, including Understanding Equine Lameness and Understanding The Young Horse, published by Eclipse Press and available at or by calling 800/582-5604.

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