Risks of infection
The placental structure, including the amnion and chorioallantois, is vital for exchanging nutrients, gases and waste products
between a pregnant mare and her foal. To protect this important structure, viscous mucus produced from the cervix helps form
an impermeable barrier—a cervical plug—to keep bacteria and other foreign organisms from invading the fetus' sterile environment,
including the delicate placental membranes.
In some mares, however, a compromised reproductive tract can allow bacterial or fungal infection of the placenta. The result
is often early embryonic loss, late-term abortion or premature birth. Foals that survive birth may be weak and die soon after
Placentitis may occur in mares of any age but most often occurs in underweight or older mares in which the conformation of
the vulva allows bacterial ascension through the cervix. Previous cervical injury also may allow bacteria to breach the cervical
barrier, increasing the potential for placentitis.
Infection usually congregates at the cervical star, though it can also penetrate farther, invading the entire placental structure.
Once inflammation and infection spread, the mare's body produces prostaglandins, which can cause uterine contraction and abortion.
Inflammation may also cause the placental tissue to thicken and pull away from the uterine lining, decreasing nutrients and
oxygen to the foal. In some cases, premature separation of the placenta from the uterus leads to a "red-bag" delivery. Instead
of rupturing the placenta at the cervical star, emerging foals become wrapped in the placental membranes, causing suffocation
With nocardioform placentitis, infection occurs at the base of the uterine horn, not at the cervical star as in other forms
of placentitis. The associated lesions most commonly occur "in the body of the placenta at the bifurcation of the horns. The
affected chorion is covered by a thick, light brown, tenacious exudate. The actinomycetes associated with nocardioform placentitis
do not reach the fetus, and fetal lesions are limited to those of placental insufficiency."2
Macpherson attributes the 2011 spike in nocardioform infection to Amycolatopsis species and Crossiella equi. Although these species are common in soil, as a 2012 study says, "It is not known if these bacteria are present in soil
or any other place and whether mares become infected with these actinomycetes in the stall environment or in pasture."2
In this recent outbreak, abortion occurred exclusively during the last trimester of gestation and affected mostly thoroughbreds,
although there were some standardbred, saddlebred and quarter horse cases. "Most of the reported cases from 2010 to 2011 involved
placental lesions in term foals with relatively fewer abortions occurring," says Barry Ball, DVM, PhD, DACT, of the University
of Kentucky Gluck Equine Research Center. "It is likely that some of the increased incidence of the disease was due to improved
surveillance and reporting of this type of lesion by owners and veterinarians," he says.
Diagnosing placentitis can be difficult and begins with the basics—obtaining a complete mare history (including previous pregnancies)
and the foaling due date. "Essentially the mare with placentitis is presenting with premature mammary gland development because
she's developed a 'bag' and she's not due to foal for several weeks or months," Macpherson says. "Now you can rule out whether
the mare has ascending placentitis or whether, among other causes, she's experiencing a twin pregnancy."
In addition to observing premature mammary gland development, examining the vulvar area is an important part of the general
physical examination, as vulvar discharge would likely indicate placentitis, Macpherson says. "In my experimental model, where
I infect mares' placental area by putting bacteria in their cervix, we see vulvar discharge before we see mammary gland development,"
she says. "In the clinical setting, however, we see the mammary glands come up long before we ever see the vulvar discharge.
That's probably because most people aren't looking frequently at the mare's perineal region. So if she has bits of purulent
material in her vulva, she swishes her tail and it's gone."
One of the primary tools Macpherson uses to diagnose placentitis is transrectal ultrasound, looking at the cervical star region
for the combined thickness of the uterus and placenta (CTUP). In previous studies,3 doctors measured this region to determine what was normal at eight, nine, 10 and 11 months of gestation. They also looked
at mares with placentitis and determined that affected mares would have an increase in the CTUP. "So the transrectal ultrasound
can help us see if there is a change or an increase in those measurements or if there is any separation of the placenta at
the cervical star region," Macpherson says. "We also look at the character of the fetal fluids to see if there are abnormal
changes suggestive of an infection."
Macpherson also uses transabdominal ultrasound to look at the CTUP from a different view and help diagnose nocardioform placentitis,
which results in a broad mucoid discharge and separation of the placenta. "That is something you cannot diagnose with transrectal
ultrasound," she says. "Transabdominal ultrasonography gives us the best view and is also our best means of detecting twins.
Finally, transabdominal ultrasound examination is a great tool for assessing fetal viability through measurement of fetal
heart rate and activity level."
At the West Coast Equine Reproductive Symposium, Jennifer Ousey, BSc, MSc, PhD, research scientist at Rossdale and Partners
in Suffolk, UK, discussed the use of Doppler ultrasonography to assess the equine fetus and placenta.
"A colleague of mine did not find it a useful tool," says Macpherson, "but people have postulated that there is a change of
blood flow to the placenta's infected areas." This technique certainly warrants further investigation.
In addition to ultrasound, some laboratory tests may help diagnose placentitis, including mammary secretion testing for electrolytes,
which are noted to change close to delivery. Doctors can also measure serum progesterone concentrations over several days
of gestation. "Both of these lab tests give practitioners some information about the pregnancy's status," Macpherson says.
Among the newer diagnostic laboratory techniques is the measurement of serum amyloid A concentrations. Researchers from The
Ohio State University and the University of Kentucky have shown exciting results from measuring serum amyloid A protein concentrations
from normal pregnant mares4 and mares with placentitis.5,6 These inflammatory proteins may rise in conjunction with placental infections, providing a useful biomarker of disease.