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Monitoring the pregnant mare

Equine Podiatry and Lameness Centre - Tuesday, September 15, 2020
Monitoring the pregnant mare

There is nothing quite as exciting as foaling season! With recent rain many pregnant mares have been happily turned out onto grassy paddocks. It can be a devastating loss however when spring comes around to find the mare that was bred last season and was putting on weight has lost her pregnancy.

The loss of any foal is devasting for both the mare and the owner. Abortions can occur at any time of gestation and can be due to many different reasons. Abortions may occur sporadically as a one off or affect multiple mares at a single farm. Early identification of a problem pregnancy may be able to save the foal’s life and prevent future pregnancy problems. There are numerous conditions that may cause abortions in mares including poor nutrition, stress, cord twists, twin pregnancies and infections.

Cord twist / stress

Twisted umbilical cords or cords with poor blood flow are common factors in abortions between 6 and 8 months gestation. It is thought that that the cord twists due to increased fetal movement from stress such as the mare having a colic episode. The cause of cord twists and poor blood flow remain largely unknown. If your mare is pregnant and requires veterinary treatment it is advised to have her pregnancy assessed when things have settled down. Medication is available to help blood flow in these high-risk pregnancies.

Twin pregnancies

Twins used to be the most common cause of abortion in horses. With the widespread use of ultrasound for early pregnancy detection the rate of twins has dramatically declined.

Twins if they survive are likely to suffer foaling difficulties which may damage the mare and not allow her to be bred again. The foals themselves are generally smaller, weaker and more susceptible to infection if they survive.

It is sometimes not possible to detect twins on ultrasound due to cysts, different times of egg release, or limited facilities. To reduce the risk of your mare having twins speak to and follow the advice of your equine reproduction veterinarian.

Placentitis (Infection)

Placentitis is an infection of the membrane surrounding the foal and may affect between 3-5% of pregnancies. Infections are usually caused from bacteria but may involve fungal organisms. Infectious agents gain access to the placenta either through travelling up from the vagina and through the cervix or through infections in the mare’s blood stream or gastrointestinal tract travelling to the uterus.

Equine Amnionitis and Fetal Loss (EAFL) is a form of placentitis that occurs after the mare ingests caterpillars. The processionary caterpillar (Ochrogaster lunifer) is thought to be the main culprit. The tiny hairs on the caterpillars pierce through the intestinal tract, penetrate the uterus, and may cause abortions within three to four days after eating the caterpillar, but can be up to 5 months.

Chlamydia is a bacteria carried by birds and can cause respiratory disease in people and abortions in mares. Groups of mares and veterinarians were found with the bacteria in the Hunter Valley in 2015.Testing of mares is underway to get more information about this bacteria. Due to the potential for human illness care must be taken when handling aborted fetal material. Herpes virus is a common virus that occurs worldwide. The virus can be spread by respiratory droplets and may cause respiratory illness, abortion or neurological signs. Abortions are normally spontaneous with no warning signs and may occur from 2 weeks to several months after infection.

Herpes can also live inside the mare and re-activate at times of stress. Aborted fetuses contain large amounts of the virus and outbreaks (abortion storms) have occurred on large farms. It is recommended that any aborted fetus is treated as possibly infected with herpes and care is taken when handling the foal and placenta. Vaccinations are available and recommended on large breeding farms.

Veterinarian checks

Monitoring pregnant mares is a skilled veterinary task and it is best to be discussed with your Equine Veterinarian. Your veterinarian may perform a physical examination to assess the mare’s health along with samples of her blood or milk for hormone levels. Ultrasound examination may be used to assess the fetal membranes and the foal’s heart rate. Ultrasound can also be used to examine the size of the foal and blood flow through the cord. As the foal moves around inside the mare this is often a time-consuming task and requires good facilities, patience and a skilled veterinarian.

In human pregnancies sampling of the fetal fluid is commonly performed. Sampling of fluid around the foal is still under investigation in the mare and is not commonly performed yet.

What should you do if your mare has aborted?

We strongly encourage aborted fetuses and membranes to be submitted to the laboratory for routine testing of herpes virus and chlamydia. Performing an autopsy on the fetus and membranes may provide information into the cause of abortion and help future pregnancies. The mare should be examined by your Equine Veterinarian to make sure she is healthy, and no membranes or infection remain in the mare which can be life threatening. If you find a fetus and or membranes it is critical that careful hygiene is followed. This includes wearing gloves and face masks and disinfecting any equipment that you may have come into contact. If you have any questions about abortions or mare reproduction contact your Equine Veterinarian.

Signs to monitor your mare

Overall health - Pregnant mares are often allowed to gain as much weight as they want, but weight from the growing foal doesn’t occur until 9 to 11 months gestation. It is important that brood mares do not gain too much weight as obesity may increase the risk of dystocia (difficulty foaling).

Udder development - In a normal pregnant mare the udder starts to get bigger from 1 to 4 weeks prior to foaling. If you notice your mare developing a bag earlier than this it can be a sign of impending abortion or infection within the mammary gland (mastitis).

Tail - Giving birth is messy! If you notice any discharge on the tail this may be an early sign of infection or abortion. In a normal pregnant mare prior to foaling you will notice the slope of the vagina change and softening around the vagina as the muscles relax.

Check for caterpillars - The processionary caterpillar can form nests or bags. The nests may be tent like at the bottom of native trees or have a golden yellow (money bag) appearance in branches.

Written by Dr Liz Barter - BVSc