Pregnancy loss in animals can be divided into three categories: early embryonic loss, abortion, and stillbirth. Abortion is the termination of a pregnancy after fetal organogenesis has been completed but before the fetus can survive outside the uterus. Pregnancy that ends before organogenesis is called early embryonic loss. A full-term fetus that is delivered dead is classified as a stillbirth, which can be confirmed by the absence of lung inflation. Many causes of abortion can also lead to stillbirths, medically compromised neonates, fetal monsters, or mummification.
The causes of abortion are either infectious or noninfectious. Despite thorough investigation, the definitive cause cannot be established in many abortion cases. Several factors complicate diagnosis, including absence of the pathogen at the time of abortion and sample collection, autolysis of the fetus, contamination with environmental microbiota, and submission of improper specimens to diagnostic laboratories.
The best specimens for pathogen isolation in cases of abortion are the fresh, intact placenta; the fetus; and maternal serum. Tissue samples should be cleaned and chilled (but not frozen) before being transported to a diagnostic laboratory. If chilled immediately, most fetuses are suitable for necropsy, even if they do not reach the laboratory for 1–2 days.
Pearls & Pitfalls
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Fetal pigs, sheep, and goats are usually small enough to transport or ship whole with the placenta. If there are multiple fetuses, three to five should be submitted with the placentas. It is best to submit calves and foals whole; when whole farms or herds are affected, however, it is often more convenient to perform necropsies and collect tissue samples for submission.
The fetal specimens routinely used for testing in cases of abortion vary somewhat between diagnostic laboratories. A basic set of samples that enables thorough examination includes gastric contents; heart blood or fluid from a body cavity; unfixed lung, liver, kidney, and spleen (some laboratories also request tissues such as thyroid glands, thymus, heart, brain, abomasum, and stomach); placenta (if available); and the dam’s serum. These samples should be submitted in sterile containers to allow for microbiological cultures.
Representative samples of the following, in 10% buffered formalin, should also be submitted for histological examination in abortion cases: fetal lung, liver, heart, kidney, spleen, brain, skeletal muscle, thyroid, adrenal glands, intestines, and placenta. Usually, gross lesions other than signs of autolysis (increased pleural and peritoneal fluid and blood-tinged subcutaneous edema) are not present. If lesions are found, however, fresh and formalin-fixed samples of affected tissues should be included.
Most agents that cause abortion, especially bacteria and fungi, infect the placenta and thus gain entry into the amniotic fluid, which is swallowed by the fetus. Gastric contents are the best specimen for detecting fungi and most bacteria because they can be obtained aseptically. Isolation from gastric contents is much easier than from the placenta, which is always heavily contaminated. Lungs, liver, spleen, and kidneys are also good for culture.
Several agents that cause abortion (eg, fungi, Chlamydia bacteria, Coxiella bacteria) affect primarily the placenta. Failure to include the placenta in specimens for examination decreases the probability that these agents will be identified. If the placenta is not available, a caruncle can be removed from the uterus and submitted.
Fetuses sometimes produce antibodies against certain agents (eg, bovine viral diarrhea virus, Neospora spp, Leptospira spp), and fetal serum or fluid from a body cavity can be tested for antibodies. The presence of precolostral antibodies is evidence of in utero exposure.
A single antibody titer in the dam rarely provides definitive evidence that abortion was caused by a particular agent, unless background herd titers are known. In fact, high maternal titers might indicate prior exposure and immunity rather than causation, and they might even be the reason the animal did not abort from that agent. However, the absence of a titer can be useful for excluding an agent.
Maternal antibody titers against pathogens with established control programs (eg, the bacterium Brucella abortus, pseudorabies virus) are always important, even if the abortion was ultimately caused by a different factor. Demonstration of a 4-fold rise in antibody titer is required to confirm active infection by a specific agent.
Often, abortion occurs weeks or months after the dam's initial infection, by which time her antibody titer could be stable or declining. Paired serum samples collected 2 weeks apart from 10% of the herd or from at least 10 animals can often demonstrate seroconversion and provide evidence of active infection within a herd.
For More Information
de la Concha-Bermejillo A, Romano J. Laboratory use in pregnancy loss diagnosis. Clin Theriogenol. 2021;13(3):211-220.
Taylor RF, Njaa BL. General approach to fetal and neonatal loss. In: Njaa BL, ed. Kirkbride's Diagnosis of Abortion and Neonatal Loss in Animals. 4th ed. John Wiley & Sons; 2012:1-12.
