Overview of Abortion in Large Animals
Abortion is the termination of pregnancy after organogenesis is complete but before the expelled fetus can survive. If pregnancy ends before organogenesis, it is called early embryonic death. A dead, full-term fetus is a stillbirth (its lungs are not inflated). Many etiologies of abortion also cause stillbirths, mummification, and weak or deformed neonates.
The etiologic diagnosis of abortion in livestock is a difficult and often frustrating task. The diagnostic success rate is relatively low and variable: 30%–40% for bovine, 60%–65% for ovine, 35%–40% for porcine, and from <10% to 90% in equine abortion cases submitted to diagnostic laboratories. The diagnostic success rate in camelids is very low except in outbreaks. Numerous factors complicate diagnosis. Often, abortion follows initial infection by weeks or months, so the causative agent is no longer apparent when abortion occurs. Expulsion may follow fetal death by hours or days, with lesions obscured by autolysis. Fetal membranes and the aborted fetus are usually contaminated by environmental agents before examination. Many sporadic abortions are likely the result of noninfectious (ie, toxic or genetic) causes, about which much less is known than infectious causes; many diagnostic laboratories are not equipped or staffed to deal with these causes of abortion.
Another problem in determining the cause of abortions is improper or inadequate specimen selection and handling. 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 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 their placentas. It is best to submit calves and foals whole, but in many cases it is more convenient to perform a necropsy and collect 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 (see Collection and Submission of Laboratory Samples). 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 4-fold 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 wk apart from 10% of the herd or a minimum of 10 animals often demonstrate seroconversion and provide evidence of active infection in the herd.