Non-Infectious Causes of Pregnancy Loss in Horses

When discussing equine abortion, infectious diseases often receive the most attention. However, several non-infectious factors can also lead to pregnancy loss in mares, significantly affecting breeding success and farm economics. From twinning and hormonal imbalance to stress and poor management, these hidden risks can compromise equine reproductive health even in the absence of infection. 

Twinning: A Major Threat to Pregnancy 

Twinning is one of the leading non-infectious causes of abortion in mares. Multiple ovulations increase the chances of twin pregnancies, but the equine uterus is poorly designed to support two fetuses successfully. As pregnancy advances, limited uterine space and inadequate nutrient supply often result in late-term abortion or the loss of one or both foals. 

The use of ultrasonography for early pregnancy diagnosis has greatly reduced twinning-related abortions by allowing veterinarians to identify and manage twin pregnancies early. Studies also suggest that mares with better nutritional status may have a higher likelihood of twinning, while maiden and barren mares may show increased incidence of twin foals^1,2. 

Aging Mares and Early Pregnancy Loss 

Age is another important factor influencing reproductive success in horses. Although fertilization rates may remain normal, older mares experience significantly higher rates of early embryonic loss compared to younger mares^1,3. 

Most pregnancy failures occur between conception and day 65 of gestation, commonly referred to as “early pregnancy loss.” Reduced oviductal and uterine environment quality in aging mares can negatively affect embryo development and survival^3,4,5. 

Progesterone Deficiency and Pregnancy Failure 

Progesterone plays a critical role in maintaining pregnancy in mares. Produced initially by the corpus luteum, this hormone supports the uterine environment needed for embryo survival. 

Low progesterone levels may occur due to inadequate luteal function or increased prostaglandin release, resulting in pregnancy failure. Conditions such as endometrial cysts may interfere with embryo migration and disrupt maternal recognition of pregnancy. 

Hormonal testing, including progesterone, pregnant mare serum gonadotropin (PMSG), and estrone sulfate assessment, can help evaluate pregnancy status and reproductive health. Hormonal therapy may also help prevent abortion associated with progesterone deficiency^1,6. 

Umbilical Cord Torsion 

Umbilical cord torsion occurs when excessive twisting obstructs blood flow through the umbilical vessels, compromising fetal oxygen supply¹. In severe cases, this can lead to fetal death and abortion. 

Certain stallions may produce foals with longer umbilical cords, increasing the risk of torsion. Because this condition often develops during late gestation, regular monitoring of pregnant mares during the final trimester is essential. Ultrasonography can help assess fetal movement and umbilical cord health^1,8. 

Mare Reproductive Loss Syndrome (MRLS) 

Mare Reproductive Loss Syndrome (MRLS) is an environmentally associated condition linked to early and late fetal loss in mares. Large outbreaks reported in the United States caused devastating economic losses in the equine industry. 

Research identified exposure to Eastern tent caterpillars as a major contributing factor during MRLS outbreaks. Environmental conditions such as higher temperatures and low humidity may further increase risk. Preventing pregnant mares from exposure to hairy caterpillars remains one of the most effective preventive strategies^1,9,10. 

Mycotoxins in Feed 

Contaminated feed can also contribute to reproductive problems in horses. Mycotoxins such as aflatoxin, fumonisin, ochratoxin, zearalenone, and deoxynivalenol are commonly found in moldy feed and may cause reproductive and immune-related disorders¹. 

Modern diagnostic techniques including ELISA and PCR help detect mycotoxin contamination. Proper feed storage, quality control, and detoxification strategies are important in reducing exposure risk¹. 

Management and Stress Factors 

Poor management practices continue to be major contributors to pregnancy loss in mares. Nutritional deficiencies, mechanical injuries, overcrowding, and inadequate stable conditions can negatively affect reproductive health. 

Transportation stress is another overlooked factor. Long-distance transport may elevate cortisol levels in pregnant mares, increasing the risk of abortion. Minimizing stress and ensuring proper nutrition and handling throughout gestation are essential for successful foaling outcomes. 

Prevention Starts with Good Management 

Many non-infectious causes of equine abortion can be reduced through proactive reproductive management. Key preventive measures include: 

• Early pregnancy scanning 

• Monitoring for twin pregnancies 

• Regular hormonal evaluation 

• Balanced nutrition and clean feed 

• Stress reduction during pregnancy 

• Routine veterinary examinations 

• Careful monitoring of aging mares 

Final Takeaway 

Non-infectious causes of pregnancy loss in horses are often underestimated but can have a major impact on breeding success. Twinning, hormonal imbalance, aging, umbilical cord torsion, environmental stress, and poor management all play important roles in equine abortion. Early detection, proper nutrition, regular veterinary care, and strong reproductive management remain the foundation for improving pregnancy outcomes in mares. 

 References 

1. Li L, Li S, Ma H, Akhtar MF, Tan Y, Wang T, Liu W, Khan A, Khan MZ, Wang C. An overview of infectious and non-infectious causes of pregnancy losses in equine. Animals. 2024 Jul 2;14(13):1961. https://doi.org/10.3390/ani14131961  

2. Alamaary M, ALI A. Abortion and uterine prolapse in a Thoroughbred mare with twin pregnancy: Clinical and laboratory findings and treatment approach. Journal of equine science. 2020;31(4):95-9. https://www.jstage.jst.go.jp/article/jes/31/4/31_2018/_pdf  

3. De Mestre AM, Rose BV, Chang YM, Wathes DC, Verheyen KL. Multivariable analysis to determine risk factors associated with early pregnancy loss in thoroughbred broodmares. Theriogenology. 2019 Jan 15;124:18-23. https://rvc-repository.worktribe.com/OutputFile/1384223  

4. Carluccio A, Bucci R, Fusi J, Robbe D, Veronesi MC. Effect of age and of reproductive status on reproductive indices in horse mares carrying mule pregnancies. Heliyon. 2020 Oct 1;6(10). https://www.cell.com/heliyon/pdf/S2405-8440(20)32018-1.pdf  

5. Kahler A, McGonnell IM, Smart H, Kowalski AA, Smith KC, Wathes DC, de Mestre AM. Fetal morphological features and abnormalities associated with equine early pregnancy loss. Equine Veterinary Journal. 2021 May;53(3):530-41. https://beva.onlinelibrary.wiley.com/doi/pdfdirect/10.1111/evj.13340  

6. Raghupathy R, Szekeres-Bartho J. Progesterone: a unique hormone with immunomodulatory roles in pregnancy. International journal of molecular sciences. 2022 Jan 25;23(3):1333. https://www.mdpi.com/1422-0067/23/3/1333  

7. Lawson JM, Verheyen K, Smith KC, Bryan JS, Foote AK, de Mestre AM. The equine umbilical cord in clinically healthy pregnancies. Equine veterinary journal. 2024 Jul;56(4):742-50. https://beva.onlinelibrary.wiley.com/doi/pdfdirect/10.1111/evj.14055  

8. Christoffersen M, Nielsen SB, Madvig CB, Agerholm JS. Potential risk factors for fetal loss due to umbilical cord torsion in the mare. Theriogenology. 2024 Jan 15;214:182-6. https://www.sciencedirect.com/science/article/pii/S0093691X23004284  

9. Swerczek TW. An alternative model for fetal loss disorders associated with mare reproductive loss syndrome. Animal Nutrition. 2020 Jun 1;6(2):217-24. https://www.sciencedirect.com/science/article/pii/S2405654520300305  

10. Campos I, Batista B, Matos AC, Dutra F, Gomes G, Pinna A, Leite J, Ferreira A. Pregnancy loss due to amnionitis in anglo‐arabian mare—Case report. Reproduction in Domestic Animals. 2020 Apr;55(4):438-41. https://www.academia.edu/download/123604800/rda.1362520250704-1-b8aa58.pdf