Also Known As – Twinning
Introduction
Most twin pregnancies in horses are dizygotic and therefore the result of multiple ovulations. Some mares are prone to multiple ovulation and can therefore repeatedly conceive twin pregnancies.
Due to the nature of the equine placenta, which is dependent upon surface area of attachment, the majority of twin pregnancies are not viable as placentation is inadequate, and therefore they result in abortion of one or often both foetuses.
Embryos in the horse are mobile until approximately day 15 of pregnancy, a key event for Maternal Recognition of Pregnancy - Anatomy & Physiology, at which point they implant in one of the uterine horns. 75% of twins that fixate in the same uterine horn will naturally reduce to a single pregnancy.[1] This does not occur when conceptuses implant in separate horns.
Twin pregnancies may still be the most common cause of abortion in mares.
Distribution
Worldwide
Signalment
Twin pregnancies occur more frequently in older mares and in thoroughbreds and draft breeds.
Mares that develop twin pregnancies tend to conceive twins in subsequent pregnancies also.
Clinical Signs
Clinical signs prior to abortion may be very subtle and premature mammary gland development may be the only clue. Some mares may exhibit behavioural changes and systemic signs.
Visible abortions usually occur between 7 and 9 months of gestation. Unicornual twins (implanted in the same horn) may abort earlier due to earlier competition than those in separate horns (bicornual).
Foetuses are often of grossly different sizes when aborted due to earlier death of one twin.
Diagnosis
Pregnant mares should be ultrasound scanned trans-rectally at 13-15 days and again prior to 30 days of gestation to accurately diagnose twin pregnancies. Manual reduction of a second conceptus should be performed by 30 days for the optimal chance of the remaining foetus developing normally.
It is important that twin pregnancies are differentiated from endometrial cysts and so mares for breeding should be ultrasound scanned trans-rectally prior to mating so that any existing cysts can be identified and mapped. This avoids confusion as cystic structures can look very similar to early pregnancies. If records are unavailable, rescanning of a suspicious structure after 2 days should reveal a noticeable increase in size if it is a conceptus and if scanned during the migratory phase then it is also likely to have moved to a different location within the uterus. After 24 days of gestation, an embryonic heartbeat will be detectable, confirming that the structure is indeed a pregnancy.
Treatment
Twins detected during migration are easily separated and one can then be manually manipulated to the tip of the uterine horn and crushed. The remaining vesicle develops normally in 90% of cases.[1]
If this procedure is used after 30 days of gestation then the fluid released is more likely to disrupt the remaining pregnancy. If both conceptuses have already implanted in the same horn of the uterus, it can be very difficult to crush one without also damaging the other. Survival rates fall significantly and progressively after 30 days of gestation. At 30 days, 85% of singleton foetuses are carried successfully to term.
Management after endometrial cup formation at 40 days is obviously very difficult, as it any pregnancy loss at this stage of gestation.
Other possible options for reduction of one twin have been used, including dietary restriction, surgical removal, intracardiac injection of potassium chloride and transvaginal allantocentesis but are far less frequently used than manual reduction early in gestation due to lower success rates.
It is recommended that pregnancy development is closely monitored after manual reduction of a twin pregnancy.
Control
Breeding from mares known to be prone to multiple ovulation should be performed carefully and with scrupulous ultrasound examination at least twice prior to 39 days of gestation.
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References
Merck Veterinary Manual, Management of Reproduction: Horses: Abortion, accessed online 25/07/2011 at http://www.merckvetmanual.com/mvm/index.jsp?cfile=htm/bc/181607.htm