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Overview of Abortion in Large Animals

By

Ahmed Tibary

, DMV, PhD, DACT, Washington State University

Last full review/revision Apr 2021 | Content last modified Apr 2021

Abortion is the artificial termination of pregnancy after organogenesis is complete but before the fetus is viable. If pregnancy ends naturally before organogenesis, this is called early embryonic death. A full-term fetus that is delivered dead is a stillbirth, confirmed by lack of any indication of inflation of the lungs. Many causes of abortion can also cause stillbirths, premature or medically and physically compromised neonates, and occasionally mummified fetuses.

Determining the cause(s) of abortion in large animals is a difficult and often frustrating task. Numerous factors complicate diagnosis. Often, abortion follows an initial acute infection which then lingers for weeks or months. As such, the causative agent is often no longer present when abortion occurs and at the time of post-mortem examination. Expulsion may follow fetal death by hours or days, with lesions obscured by autolysis. It should be noted that fetal membranes and the aborted fetus are usually contaminated by environmental flora. Sporadic instances of abortion are likely the result of noninfectious (eg, toxic or genetic) causes, and currently much less is known about these etiologies compared with infectious causes. Furthermore, many veterinary diagnostic laboratories are currently equipped to focus only on infectious causes.

An additional diagnostic challenge for veterinarians faced with large animals with abortion is knowledge of proper specimen selection and handling. Practitioners are advised to seek assistance from the relevant diagnostic laboratory whenever possible. The best specimen is the complete fetoplacental unit in fresh condition, along with maternal serum. The placenta and fetus should be cleaned with water or saline, packed in clean plastic bags, chilled (but not frozen), and rapidly transported to the diagnostic laboratory. In most cases, autolysis proceeds at a much slower rate in fetuses than in carcasses of animals born alive. If chilled as soon as possible, most fetuses will be suitable for necropsy examination, even if they do not reach the laboratory for 1–2 days. 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, but often in instances of farms or herds affected it is more convenient to perform a necropsy and collect tissues samples for submission.

The specimens routinely used for testing vary somewhat between diagnostic laboratories, but a basic set of samples that will allow thorough examination includes stomach or abomasal 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 dam’s serum. These should be submitted in sterile containers to allow for microbiologic cultures. Because they are always contaminated, placentas should not be mixed with other tissues.

Representative samples of the following should also be submitted in 10% buffered formalin for histopathologic examination: lung, liver, heart, kidney, spleen, brain, skeletal muscle, thyroid, adrenal glands, intestines, and placenta. In a large majority of cases, gross lesions other than signs of autolysis (increased pleural and peritoneal fluid and blood-tinged subcutaneous edema) are not present. However, if lesions are found, fresh and formalin-fixed samples of affected tissues should be included.

Most agents, especially bacteria and fungi, infect the placenta and thus gain entry into the amniotic fluid, which is swallowed by the fetus. Stomach contents can be obtained aseptically, making it the best specimen for detection of fungi and most bacteria. Isolation from the stomach 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 (eg, fungi, Chlamydia, Coxiella) primarily affect the placenta; failure to include placenta decreases the probability they will be identified. Fetuses sometimes produce antibodies to 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 evidence of abortion caused by a particular agent unless background herd titer levels are known. High maternal titers may as likely be the reason an animal did not abort due to that agent, but absence of a titer can be used to exclude an agent. Antibody titers to agents with control programs (eg, Brucella abortus, pseudorabies virus) are always significant, even if the abortion was caused by something else. Demonstration of a fourfold increase in antibody titer is required to prove active infection by a specific agent. Often, abortion occurs weeks or months after initial infection of the dam, and her titer is stable or declining at the time of abortion. Paired serum samples obtained 2 weeks apart from 10% of the herd or a minimum of 10 animals often demonstrate seroconversion and provide evidence of active infection in the herd.

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