Placentitis - a reason for monitoring your pregnant mare

Foaling season is once again approaching and many pregnant mares have been left out on pasture to eat to their heart’s content over the winter. Often pregnant mares have minimal monitoring throughout most of their pregnancy, but there is a strong case to be made for closer monitoring, especially in mid to late gestation due to the risk of placentitis. Placentitis can lead to, at the very least, costly treatment and often to pregnancy loss. Placentitis can affect 3-5% of pregnancies and is the cause of about 60% of pregnancy losses due to abortion, stillbirth, or neonatal death (1).

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Placentitis is an infection of the placenta which is usually bacterial but can also be fungal, or a combination of the two. There are four types of placentitis, with the most common being ascending placentitis. This is an infection that starts in the vulva and then makes its way through the cervix and into the placenta. The other causes are nocardioform, diffuse, and multifocal, however these are quite uncommon in our practice area and have similar clinical signs, treatments, and outcomes. This infection of the placenta leads to placental insufficiency (where the placenta is no longer able to support the growing foal) and/or infection of the fetus, which then leads to abortion or the birth of a live but premature or very sick foal. 

 Placenta from a mare with placentitis showing a thickened and discoloured cervical star.

Placenta from a mare with placentitis showing a thickened and discoloured cervical star.

The outward clinical signs that may be seen in a mare with placentitis are white, mucoid vulvar discharge and premature udder development (“early bagging up”). Mares with a healthy pregnancy typically do not start to develop their udder until 2-4 weeks pre-foaling and with a problem pregnancy, bagging up will be seen earlier, around 6-12 weeks before the mare’s due date. Early bagging up is a sign commonly associated with placentitis, but is a general sign of a problem with the pregnancy, not specific to placentitis. Vulvar discharge is more specific to placentitis, however it is easily missed as the amount of discharge is variable and can accumulate under the tail and there is a greater chance of not noticing it. 

 Bagging up prior to foaling.

Bagging up prior to foaling.

So what should you do if you notice premature udder development or vulvar discharge in mid to late pregnancy? The first thing to do is to call your veterinarian. An early diagnosis and start to treatment will give your mare the best chance of maintaining her pregnancy, and therefore the best chance for a live foal. Your veterinarian can also help to determine if it is placentitis, or a different problem leading to the clinical signs. They will likely perform a palpation and ultrasound of the uterus to assess the well-being of the fetus and also the appearance of the cervix and placenta. If they diagnose placentitis, the treatment is antibiotics to treat the infection, anti-inflammatories to help decrease the damage to the placenta from inflammation, and a progesterone supplement to help maintain the pregnancy. Your veterinarian will also likely prescribe careful monitoring for signs that the mare is going into premature labour.

 Assessing the placenta on ultrasound.

Assessing the placenta on ultrasound.

Ideally, mares with placentitis will maintain their pregnancy until at least 320 days of gestation, until the foal has developed enough to be born live and hopefully not premature. Pregnancy should be attempted to be maintained for at least this long, although there is some evidence that mares with placentitis will give birth to viable foals around 310 days of gestation, likely due to rapid and early development of the fetus from the stress of placentitis. Signs that the mare is going to foal early or abort include the normal signs of foaling, but earlier than expected.  These include bagging up, streaming milk, relaxation of the ligaments around the vulva and tailhead, and restlessness.  

Mares with placentitis are also at a high risk for a red bag foaling (premature placental separation) and need to be monitored closely. It is very important that mares with placentitis be observed at foaling. In normal foaling, the first fluid filled sac seen at the vulva is the amnion which is a light blue to white, translucent membrane. This should rupture, releasing fluid, which is the water breaking. The foal should then be born within 20 minutes. In a red bag foaling, the chorioallantois appears at the vulva instead. This is a thick, red, velvety membrane. The appearance of this membrane is a time-sensitive emergency. The membrane must be opened immediately if it is seen, otherwise the foal will suffocate. The membrane can be torn, or cut with scissors, but care must be taken not to cut the foal while opening this membrane. Foals born through a red bag foaling may need to be treated for hypoxia (decreased oxygen) by a veterinarian.

 Normal (left) versus red bag (right) foaling. Red bag foalings require immediate intervention.

Normal (left) versus red bag (right) foaling. Red bag foalings require immediate intervention.

Foals born alive to mares that had placentitis are susceptible to the bacteria that were present in the uterus and may need antibiotics after birth. They must also be monitored closely for signs of infection and sepsis. These include: diarrhea, septic joints (hot, swollen joints with severe lameness), lethargy, inflamed umbilicus, and injected vessels around the eyes. Newborn foals can go downhill quickly when they become sick and should be seen as soon as possible if any of these signs are seen. It is recommended that all foals have a new-foal exam and a SNAP test done about 18-24 hours after birth (or sooner if problems are noticed). The SNAP test is a measure of the foal’s immunity. Foals do not acquire any immunity from their dam while in the uterus and must acquire it from nursing the mare’s colostrum (first milk) within the first 12-24 hours of life. A low immunity indicates a higher risk of infection and sepsis and is treated by giving the foal plasma transfusions.        

So if you suspect your pregnant mare has placentitis, or any other problem with her pregnancy, it is important to call your veterinarian as an early diagnosis is key to improving your chance of a successful pregnancy. Close monitoring of the pregnancy and the foal after it is born is very important in order to quickly deal with any problems that arise. While placentitis itself isn’t too common, it can have a big impact on your foaling season if even just one mare loses her foal to it.

(1) Canisso, I., Ball, B.A., Squires, E.L., & Troedsson, M.H.T. Comprehensive Review on Equine Placentitis. 2015. AAEP Proceedings. 61: 490-509.

(2) Macpherson, M.L. & Schlafer, D.H. Selected Topics in Reproductive Pathology: Mare II. 2015. AAEP Proceedings. 61: 276-294.