In healthy sheep, fetal losses after pregnancy diagnosis is performed are typically low, ie, < 2%. However, embryonic loss can be surprisingly high without an apparent problem at lambing (up to 30% of conceptions). Embryonic death before day 12 does not disturb the normal cycle length, whereas embryonic death after this time increases cycle length and may appear as repeat marking and a stretched-out lambing period. Medically important rates of embryonic loss are due to issues mentioned previously, but some losses occur in healthy ewes, with higher rates in more prolific breeds.
Fetal loss in the second and third trimester is generally uncommon in healthy flocks. However, when it is abnormally frequent and due to disease or injury, it may appear as an observed abortion, abnormal discharge during pregnancy, open ewes at lambing, stillbirth, early neonatal mortality, or all of the above. Causes are most often infectious but may also be nutritional deficiencies, toxins, or trauma.
The most commonly diagnosed causes of abortion are:
Abortion rates of 25%–30% are not unusual in outbreaks of abortion attributed to these causes; in general, a flock abortion rate >5% is considered abnormal and should be investigated.