Swine abortion diagnosis: Are you stressed?
Emily Brouwer, Tim Pasma
Animal Health Laboratory, University of Guelph, Guelph, ON.
AHL Newsletter 2024;28(3):12.
Swine veterinarians and pathologists are frequently asked to identify the cause of reproductive failure/abortion in swine. Fetuses that are submitted to the Animal Health Laboratory are examined by a pathologist, and a routine set of tissues are sampled for histology, bacterial culture, and virology testing.
The pathologist identifies morphologic changes, either grossly or histologically, and integrates these findings with the results of ancillary testing. A relatively high proportion of these cases will have no specific diagnosis: percentages of non-diagnostic cases have ranged from 25.9 % to 66.7 % over the last five years (Graph 1).
Graph 1. Proportion of swine abortion cases with no diagnosis (2019-2023)
Although receiving a report without an etiologic diagnosis is inherently frustrating for both the submitting veterinarian and the pathologist, it is important to recognize that not all causes of abortion are infectious, and not all causes of abortion will cause grossly or histologically detectable lesions.
Non-infectious causes of abortion include a nebulous collection of factors including seasonal effects, ambient temperatures, stress factors, and toxic substances. One such example is heat stress, where high ambient temperatures or increased humidity lead to redistribution of blood flow in the dam at the expense of uterine and placental perfusion, thus increasing the risk of pregnancy failure. In addition, heat stress at the time of breeding and implantation can lead to embryonic mortality, increased return to estrus, and small litter sizes. In a similar vein, seasonal infertility in pigs occurs in late summer to early autumn and is associated with photoperiod and high temperatures. This physiologic process is thought to be a vestige of seasonal breeding in ancestral wild boars, and aberrations in progesterone—essential for maintaining pregnancy—linked to the alternation in daylight.
Stress in the dams is also linked to pregnancy failure, and encompasses various environmental and social factors, including movement and comingling. The pathogenesis of pregnancy loss secondary to stress is poorly understood, but proposed mechanisms include cortisol-mediated effects on the pituitary gland and subsequent downstream effects on corpora lutea, as well as increased basal body temperature, and increased contractility of the uterus in times of excitation.
Maternal illness in the sows can also result in abortion, with no appreciable lesions in the fetuses or fetal tissues. Assessing the sows at the time of an abortion outbreak is critical, and evidence of systemic disease in the sows should be investigated concurrently with routine abortion diagnostic testing. In some circumstances (e.g., influenza A virus infection), maternal illness does not result in viremia and fetal infection, but rather the maternal systemic inflammatory response disrupts the hormonal balance required for maintenance of pregnancy.
Various toxins are associated with failure of pregnancy. One toxin of note, carbon monoxide (CO), can lead to abortion and stillbirth with no clinical signs in the sows, as there is greater affinity of carbon monoxide for fetal hemoglobin versus maternal hemoglobin. CO toxicosis in fetuses results in cherry red discolouration of the tissues, and is confirmed with carboxyhemoglobin testing in fetal thoracic fluid. Mycotoxins are also implicated in porcine failure of pregnancy, though the experimental evidence does not implicate any particular mycotoxin in abortion, and mycotoxin ingestion resulting in abortion in an otherwise healthy sow is considered rare. The notable exception amongst mycotoxins, however, is zearalenone. This estrogenic mycotoxin tends to impact conception and litter size, rather than causing abortion.
The unifying trait for the above-listed conditions is that (with the exception of carbon monoxide poisoning) the pathologist will see no characteristic gross or histologic lesions, the battery of ancillary tests will identify no significant pathogens, and a report will be sent off with the diagnosis of “idiopathic abortion”. The diagnostic workup for common causes of abortion includes PCR testing for PRRSV, PCV 1, 2 and 3, parvovirus, and potentially, Leptospira spp. Bacterial culture is also performed on placenta, lung and stomach content in order to identify possible infectious causes. Negative results in these cases, while often frustrating, are important pieces of information in ruling out infectious causes.
In order to maximize the diagnostic utility of the submitted specimens, the Animal Health Laboratory recommends submitting up to three litters of aborted fetuses, taking care to include placentas and keeping the fetuses chilled (or frozen, if necessary). PCR and bacteriologic testing should be pooled by litter. A thorough history, taking note of sow health, is always appreciated. AHL
References
1. Althouse GC, Kauffold J, Rossow S. Diseases of the Reproductive System In: Diseases of Swine, 11th ed. JJ
Zimmerman, LA Karriker, A Ramirez, KJ Schwartz, GW Stevenson, J Zhang, eds. John Wiley & Sons, Inc.2019:384-391.
2. Maes D, Peltoniemi O, Malik M. Abortion and fetal death in sows. Reprod Domest Anim. 2023;58 Suppl 2:125-136. doi: 10.1111/rda.14436. PMID: 37724658.
3. Tibary A. Abortion in Pigs. In: Merck Veterinary Manual, reviewed/revised 2021. https://www.merckvetmanual.com/reproductive-system/abortion-in-large-animals/abortion-in-pigs Accessed August 13, 2024.