In healthy sheep, fetal losses after pregnancy diagnosis is performed are normally low, ie, <2%. However, embryo loss can be surprisingly high without an apparent problem at lambing (up to 30% of conceptions). Embryo death before day 12 does not disturb the normal cycle length, whereas embryo death after this time increases cycle length and may appear as repeat marking and a stretched-out lambing period. Pathologic levels of embryo loss are due to issues mentioned previously, but some losses occur in healthy ewes, with higher levels in more prolific breeds.
Fetal loss in the second and third trimester is generally low in healthy flocks. However, when it is abnormally high and due to a pathologic process, 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 or toxins. The most commonly diagnosed causes of abortion are Chlamydia abortus, Campylobacter jejuni, Campylobacter fetus fetus, Toxoplasma gondii, border disease virus, Coxiella burnetii, Cache Valley virus, Salmonella Abortusovis and other salmonellae, iodine deficiency, selenium deficiency, and some plant toxins (eg, locoweed). Abortion rates of 25%–30% are not unusual in these outbreaks, but in general an abortion rate >5% is considered abnormal and should be investigated.