Many factors can cause pregnancy loss in mares

After months of careful planning and anticipation, the loss of a mare’s pregnancy can be devastating.

Any number of things can cause fetal death in mares and it is worth knowing why in case preventive measures are available for future breedings.

For starters, mares have to be healthy and well-nourished to carry a pregnancy to term. Any severe illness, such as colic, can harm the fetus. Diseases of the uterus can also end the pregnancy and possibly kill the mare. For instance, uterine arteries can rupture, and the uterus may twist.

In otherwise healthy pregnancies, trouble with delivery can harm the foal. For example, if the foal enters the birth canal in an abnormal position, the birth may not proceed without assistance. Prolonged deliveries result in reduced oxygen to the foal, poor doing once the foal is born and potential death. Fetuses may also die due to malformations such as heart defects.

Conditions that specifically affect the fetal membranes deprive the fetus of oxygen. Their long umbilical cord is prone to excessive twisting, which can choke off the blood supply. And with twin pregnancies, there is not enough area of connection to support both fetuses. This is why equine twins rarely survive. Insufficient placentation in twin pregnancies causes late-term stillborns. Ultrasound examination of bred mares can identify twin pregnancies and selectively reduce one of the fetuses to avoid this.

There are infectious causes of pregnancy loss — nasty microbes that either kill the fetus outright or damage the fetal membranes.

Infections with equine herpes virus are probably the most common infectious cause in Western Canada. The same herpes virus also causes respiratory infections and neurological disease. Many horses are infected without outward signs of illness and can spread the virus.

Stillbirths from herpes virus typically occur toward the end of gestation. Individual mares can abort, or outbreaks may affect several mares in the herd. Affected foals are stillborn with areas of tissue death in the lungs and liver.

Infections may target just the placenta. Most often bacteria track up the birth canal into the uterus, leading to infection and pregnancy loss. Although rare, systemic bacterial infections can travel through the blood to also damage the fetus and membranes.

While the loss of a foal is devastating, there may be some solace in knowing what went wrong. The best way to get an answer is to have an autopsy of the foal and placenta. At a diagnostic laboratory, a veterinary pathologist will examine the organs and placenta for signs of disease with the ultimate goal of reaching a diagnosis.

In horses, and other livestock, the placenta is a vitally important piece of the puzzle. Many types of pregnancy loss start and end in these fetal membranes. The examination of the placenta is also important for live-born foals that don’t survive because the causes of the condition known as poor doing may be the same as those that cause stillbirths.

It is a good practice to save the placenta from any birth, even if apparently normal, just in case. Immediately following delivery, gather up the fetal membranes, seal them in several layers of leak-proof bags and freeze them. The packages will be there if the foal dies and can be disposed of after a few months if the foal does well. Wear gloves when handling the placenta and be sure to thoroughly wash your hands.

Even with a diagnostic work-up by a pathology lab, the cause may be undetermined. Researchers speculate that in these instances, something happens with the mare that is not related to illness. Whether the cause involves the mare, damage to the placenta or the foal itself, understanding what went wrong may give closure and help with future prevention.

Dr. Jamie Rothenburger is a veterinarian who practices pathology and is an assistant professor at the University of Calgary’s Faculty of Veterinary Medicine.
Twitter: @JRothenburger

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