According tto Peter C. Sheerin, DVM, twins are a common occurrence in thoroughbreds, draft horses and warmbloods. Horses crossed with these breeds also have an increased likelihood of having twins. At one time, twins were the leading cause of non-infectious abortion in the mare, but ith the common use of ultrasound in pregnancy detection, that is no longer the case.
In the horse, the embryo moves around in the uterus until 17 days after ovulation. This is to allow pregnancy recognition to occur. The embryo secretes a substance that tells the mare she is pregnant and should not come back in heat.
This mobile phase is the best time to reduce twins. If the mare is examined early enough, (14 to 15 days after ovulation), one can come back and re-examine the mare later in the day or the next day if the twins are in a position where reduction would be difficult. Additionally, one of the techniques involves moving the embryo to the tip of the uterine horn prior to reduction. This would not be possible if the embryo has stopped moving.
When twins are detected, an attempt can be made to reduce one of the twins. Prior to or just after the procedure, the mare should be treated with an anti-inflammatory drug (most often Banamine) and a form of progesterone (most often injectable progesterone). The mareshould be maintained on a progestin (Regumate® or injectable progesterone) for a period of time after the reduction.
A retrospective research study looking at twin reductions and the subsequent live foal rates in 2003, 2006, 2007 and 2008. A list of control mares from the same farm, bred in the same year and had singleton pregnancies. There were 1493 twinning mares and 1378 control mares. The purpose of the research was to determine if the live foal rates were different in mares that had twins reduced and mares that carried a singleton pregnancy and if the age of the mare caused any differences in live foal rates. Additionally, treatments used at the time of twin reduction were evaluated.
Mares that had twins reduced had a lower live foal rate than mares carrying a singleton pregnancy. Eighty percent of the mares that had twins reduced had a live foal while 87% of the singleton mares did. The older the mare was, the more likely she was to lose her pregnancy. This happened in both groups. Mares that were older than 15 and had twins reduced had a lower live foal rate than younger mares (66.2 vs. 83.2). In the singleton pregnancies, a similar trend was noted (77.6% vs. 89%).
If a mare had a significant health issue during pregnancy such as colic surgery, medical colic requiring hospitalization, significant podiatry issue or other issues, she was less likely to have a live foal. Sixty percent of both the mares with twins reduced and singleton pregnancies that had other health issues had live foals. The healthy mares in each group had greater than 80% live foal rate.
The treatments given at the time of twin reduction were also examined. The most common treatment was Banamine® and injectable progesterone. Sixty seven percent of the mares received this treatment. Mares treated this way had a higher live foal rate than mares treated with any of the other treatments. Eighty five percent of mares given this treatment had live foals as compared to mares given other treatments where eighty two percent of mares had live foals.
Mares that did not receive any treatment had a live foal rate of 77 percent. Finally, it was determined that the pregnancy loss rate was similar if twin reduction occurred on 13 to 20 days after ovulation. The number of mares where twins were reduced after 18 days was small and we did not evaluate location of the twins.
So what does all this mean to mare owners? Twins can be successfully reduced between day 13 and 18 post ovulation. There will be a slightly higher pregnancy loss in mares that have twins reduced when compared to mares carrying a singleton pregnancy. If the twins are side by side or separated from each other, the pregnancy loss rate will be similar before 18 days of pregnancy.
Older mares have a higher rate of pregnancy loss. This age effect is more pronounced in mares where twins were reduced. If the mare has a significant health issue during pregnancy, she will have a higher pregnancy loss rate than a mare that was healthy during her pregnancy. Finally, treating the mares with anti-inflammatories and a progestin at the time of twin reduction will contribute to a higher live foal rate than no treatment.