Disease Du Jour: AAEP’s Equine Infectious Abortion Field Diagnostic Guidelines 

In this episode of the Disease Du Jour podcast, Sarah Eaton, DVM, DACT, CVA, DABVP (Equine), joined us to discuss the American Association of Equine Practitioners’ new Equine Infectious Abortion Field Diagnostic Guidelines. Published in March 2026, these guidelines were authored by the AAEP Field Guidelines Subcommittee and supported and reviewed by the AAEP Infectious Disease Committee.

This episode of Disease Du Jour is brought to you by Bimeda.  

Important Infectious and Noninfectious Causes of Abortion in Horses 

“While the article is titled Infectious Abortion Field Diagnostic Guidelines, we don’t always know when they’re infectious or not, so we chose to include noninfectious causes in there so practitioners would have a good overview of what they were looking at in the field,” Eaton explained.  

Common noninfectious causes of equine abortion include: 

• Twins: Less common as practitioners have become more proficient with ultrasound and owners have grown more aware of the need to evaluate mares at 14-16 days of gestation.  

• Cord Torsion: This typically occurs later in gestation and is often a diagnosis of exclusion.  

• Fescue Toxicosis: This can lead to stillbirths or red bags and usually occurs late in the mare’s term.  

Infectious abortions are caused by bacterial or viral agents: 

• Bacterial: Streptococcus equi subspecies zooepidemicus and E. coli are the two major concerns. “These are the ones that are present in the environment,” Eaton said. “Mares tend to get ascending placentitis, so it goes up through the cervix and causes infection, leading to abortion.” 

• Viral: Equine herpesvirus is the leading concern. EHV-1 and EHV-4 can both cause abortion. “They’re slightly different in the patterns of the outbreaks, but they are definitely concerning from a herd-wide standpoint and can be really devastating to producers,” Eaton said.  

The guidelines emphasize that veterinarians should always consider infectious causes in every abortion until proven otherwise. “The reason we assume everything is contagious until proven otherwise is that it can be very difficult on initial presentation to determine whether something is contagious or not,” Eaton explained. “You’d hate for that first abortion to be deemed noncontagious and then find out that no, it was actually due to an infectious organism when you have abortion two, three, 10, or 15.” 

Diagnosing Causes of Equine Abortion 

The guidelines include a diagnostic flow chart that clearly outlines the samples veterinarians need to collect, how to package them, and what to expect on necropsy for various diseases based on the stage of gestation.  

“When we were working on it, we thought about those vets that see maybe one or two abortions a year,” Eaton said. “It’s not the main part of their practice, and they need someone to walk them through step by step.” 

The guidelines also walk veterinarians through the recommended steps for diagnosing and managing an abortion, beginning with taking a thorough history. Did the mare travel? Did she come in contact with other horses? What is her vaccination history? 

Next, the guidelines recommend examining the mare, starting with a general physical examination followed by an exam of the external reproductive tract. The veterinarian might also want to examine the mare’s internal reproductive tract, vagina, mammary glands, and abdominal wall. “The guidelines do walk you through what you’re looking for in each of those areas,” Eaton said.  

Once the physical exam is complete, the veterinarian should examine the placenta and/or fetus and collect samples from multiple sites.  

Eaton noted that the cause of abortion is successfully diagnosed in only about 30-60% of cases. “I would recommend that when you’re starting this out, before you even start the necropsy and collecting your samples, that you have a discussion with the owner that we may find nothing,” she said. “But that is actually good news, because it tells us it’s not something infectious that we need to worry about.” 

Listen to the podcast episode to learn more, including findings that raise concern for infectious abortion, prevention and biosecurity strategies, and sample handling and storage techniques. 

Access the Equine Infectious Abortion Field Diagnostic Guidelines here: aaep.org/wp-content/uploads/2026/03/Equine-Infectious-Abortion-FDG_final.pdf 

About Dr. Sarah Eaton 

Sarah Eaton, DVM, DACT, CVA, DABVP (Equine), graduated from the Purdue University College of Veterinary Medicine in 2006 with a focus on equine practice. After veterinary school, she completed a one-year internship in general equine practice and theriogenology at Durango Veterinary Clinic in Arizona. After her training, Eaton completed a theriogenology residency at the Western College of Veterinary Medicine under the guidance of Dr. Claire Card. She became a diplomate of the American College of Theriogenologists in 2009. 

After her residency, Eaton practiced in a mixed animal practice in British Columbia, an equine racetrack practice in New York, and a primary/referral practice in Michigan. In 2021, she left private practice to join the University of Arizona College of Veterinary Medicine faculty. 

Eaton became a diplomate of the American Board of Veterinary Practitioners (Equine) in 2021. She is a current member of the AVMA, AAEP, ABVP, SFT, and ACT. In addition to her teaching duties, Eaton serves on the AAEP’s Diversity, Equity, Inclusion (DEI) and Infectious Disease committees; chairs the Outreach committee for the ABVP; and is the faculty advisor for the University of Arizona CVM Integrative Medicine and Sports Medicine student club. 

Within the CVM, Eaton teaches equine clinical skills, theriogenology (all species), and portions of multiple large animal courses focusing on horses. 

Related Reading

• Research Spotlight: Equine Placentitis
• Advances in Stallion and Mare Fertility
• Associations Between Cytology/Bacteriology Findings and Live-Foal Rates in Broodmares

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