Prenatal Losses in Sheep

In healthy sheep, fetal losses after pregnancy diagnosis is performed are typically low, ie, < 2%. However, early embryonic loss can be surprisingly high without an apparent problem at lambing (up to 30% of conceptions). Lambing percentage is calculated as the number of lambs born per ewe, but this does not account for how many embryos were originally conceived. If embryonic loss occurs before the next estrous cycle (about 17 days after breeding), the ewe will return to heat at the normal time. In these cases, she is often recorded as having “not conceived” or as a “rebreed”; however, she may have conceived and then lost the embryo. If embryonic death occurs before day 12, the reproductive cycle is not disrupted, and the ewe returns to heat normally. However, if embryonic death occurs after day 12, the cycle is prolonged, leading to a delayed return to heat, repeat marking by the ram, and potentially a more spread-out lambing period (1, 2).

Embryonic loss may involve the loss of all embryos, resulting in complete pregnancy failure, or partial loss in ewes carrying multiple embryos. Partial loss is more common and may result in a successful pregnancy but with fewer lambs than originally conceived, reducing the lambing percentage without obvious breeding problems. Several factors can contribute to embryonic loss, including congenital reproductive abnormalities, genetic defects, failure of proper hormonal signaling required to maintain pregnancy, disease, stress (such as handling or heat), and poor nutrition. The ewe’s nutritional status around conception is particularly important because it influences egg and embryo quality, early development, hormonal regulation of the estrous cycle, and the communication between the embryo and uterus necessary to establish pregnancy.

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 might appear as an observed abortion, abnormal discharge during pregnancy, open ewes at lambing, stillbirth, early neonatal death, or all of the above. Causes are most often infectious but might also be nutritional deficiencies, toxins, or trauma.

The following are the most commonly diagnosed causes of abortion:

• Chlamydia abortus

• Campylobacter jejuni

• Campylobacter fetus fetus

• Toxoplasma gondii

• border disease virus

• Coxiella burnetii

• Cache Valley virus

• Salmonella enterica serotype Abortusovis and other salmonellae

• iodine deficiency

• selenium deficiency

• some plant toxins (eg, locoweed)

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.