Reproduction in horses is all about the delivery of a live foal! Before you get all excited about looking at the placenta and fetus, know the normal structures. Check out these of my web pages. Normal functional anatomy of placentas and normal equine placenta.

The foaling rate of a large number of mares is about 50-60% and up to 80% in some studies. Failure of pregnancy in horses is high!

In a study of a large number of stallions and mares, Lane et al (2016) found that 67% of mares foaled. Rose et al (2018) found that in a study of 1754 mares, embryonic mortality was 8% and abortion and stillbirth was 4.5%. 83% of pregnancies resulted in a live foal. Embryonic mortality is therefore the biggest contributor to failure of pregnancy.

Cooper et al (2021) examined statistics of 857 brood mares from 88 farms in Ontario, Canada and found that in one year (2015) there was 54 FOP and 28 were embryonic mortality (they called it early embryonic mortality), 6 were in mid gestation and 20 were late gestation; there were 2 perinatal deaths. Similar ratios were reported for 2014, although the total was 30.

The number of mares becoming pregnant and then the number of those mares that fall is increasing with assisted reproductive technologies (Allen and Wilsher 2018)


Allen WR, Wilsher S. Half a century of equine reproduction research and application: A veterinary tour de force. Equine Vet J. 2018; 50: 10-21.

Cooper CJ, Arroyo LG, Pearl DL, Hewson J, Lillie BN. Survey of the equine broodmare industry, abortion, and equine herpesvirus-1 vaccination in Ontario. Can Vet J. 2021 Feb;62(2):124-132. PMID: 33542550; PMCID: PMC7808208.

Lane EA, Bijnen M, Osborne M, More SJ, Henderson I, Duffy P, Crowe MA. Key Factors Affecting Reproductive Success of Thoroughbred Mares and Stallions on a Commercial Stud Farm. Reprod Domest Anim. 2016; 51: 181-187.

Rose BV, Firth M, Morris B, Roach JM, Wathes DC, Verheyen KLP, de Mestre AM. Descriptive study of current therapeutic practices, clinical reproductive findings and incidence of pregnancy loss in intensively managed thoroughbred mares. Anim Reprod Sci. 2018; 188: 74-84.


Failure of Pregnancy

Failure of pregnancy, or pregnancy loss, is when the embryo or fetus is lost. It encompasses embryonic mortality, abortion, stillbirth, early pregnancy loss, late term abortion and other terms, with overlapping of the definitions of each.

Embryonic mortality

Embryonic mortality is defined as the death of the embryo. The general point of separation between embryo and fetus is either when the conceptus begins to move, or when it attains the phenotype to allow identification of the species.

Embryonic mortality, also called early pregnancy loss, occurs before day 65 of gestation. Others use 34 days as the dividing line between embryo and fetus. It is estimated that approximately 8% of pregnancies fail in the first 65 days and most are lost in the first 42 days (6.4%) whereas 1.6% are lost between days 43 and 65 (Rose et al 2018). Embryonic mortality at 8% as compared to 4.5% for abortion and stillbirth was the biggest contributor to pregnancy loss.

Little is known of embryonic mortality, however there are some principles and possibilities. These will be discussed here.

Embryonic mortality is traditionally divided into early and late embryonic mortality. Early embryonic mortality is considered to be related to the health of the uterine tube and endometrium. Endometritis has a major effect in the mare. Late embryonic mortality can have a variety of causes including genetic and epigenetic anomalies.

The embryo spends some time in the uterine tube. Lesions of the uterine tube are not well described. Partial obstruction from dead embryos, debris or inflammation can prevent pregnancy.

Once in the uterus, uterine environment becomes very important. The embryo produces a substance that allows Maternal Recognition of Pregancy (MRP). The embryo must move throughout the uterus and endometrial cysts, foreign objects, adhesions, exudates/luminal fluid, endometrial edema and abnormal motility of the myometrium will block this. Embryonic loss will result.

The endometrial cups are formed about day 36-38. Failure of formation will result in loss of the conceptus at about day 80-95. This is why cross species abortion occurs.

Kahler et al (2021) Examined 27 embryos that were considered early pregnancy loss and found evidence of intrauterine growth retardation in three of the eight fetuses examined. One embryo in 13 had failure of closure of the neural tube and there was subcutaneous hemorrhage in 14 of 27.

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 Vet J. 2021; 53:

Nath LC, Anderson GA, McKinnon AO. Reproductive efficiency of Thoroughbred and Standardbred horses in north-east Victoria. Aust Vet J. 2010; 88: 169-175.

Rose BV, Firth M, Morris B, Roach JM, Wathes DC, Verheyen KLP, de Mestre AM. Descriptive study of current therapeutic practices, clinical reproductive findings and incidence of pregnancy loss in intensively managed thoroughbred mares. Anim Reprod Sci. 2018; 188: 74-84.

Wilsher S. The uterus and early pregnancy failure in the mare. Eq Vet Edu 2019; 31:214-224


Abortion is a general term for failure of pregnancy. There are myriad causes and these are detailed below in the SPECIFIC CAUSES of FAILURE of PREGNANCY.

Fetal maceration

Maceration of the equine fetus is a rare occurrence. Mostly, only fragments of bones remain. The initial cause of fetal death is seldom identified.

McNaughten JW,  Wallace RC. Theriogenology Question of the Month. Jv Amer Vet Med Assoc 2019; 254: 209-211




There are several large reviews of failure of pregnancy so that the number and type of causes of failure of pregnancy can be determined.


Bain (1969) reported on 2562 fetal losses in New Zealand. He found abortion in 15% of mares 3-6 yrs aborted, 19.3% of mares 7-9 yrs, 21.5% of mares 10-12, and 22.2% of mares 13 and older. Causes were not listed.

Platt (1973) reported statistics from the UK. The reported statistics included

Abortion rate according to maternal age

Age 3-6 - 6.9%

Age 7-10 - 11.2%

Age 11-14, 10.7%

Age 15-18 - 15.4%

Age >18 - 28.6%

Abortion in maiden mares vs successful pregnancy.

Maiden mares had a 13.6% abortion rate. Mares that foaled the previous season had a 6.5% abortion rate.

Causes of Abortion in 166 mares


EHV - 5

Placentitis 22


Twinning - 48

fetal anomalies - 5

Body pregnancy and Hydrops - 3

goitre - 1


Dystocia - 12

Unknown - 14

No diagnosis - 34

Autolysis - 22


Giles et al (1993) did a large study in Kentucky. The reported on 3527 cases up to 1991.


placentitis 27.8%

herpesvirus 4%

fetal sepsis


Fetal lesions

dystocia/asphixia 19%

anomalies 9.9%

diarrhea 1%

Placental lesions

Torsion 3.4%

improper separation and edema 7%


No diagnosis 16.9%


Hong et al (1993) did a large study in Kentucky. They examined 1211 aborted or stillborn foals and found


placentitis 19.4%

herpesvirus 3.3%



Fetal lesions

dystocia/asphixia 19.5%

anomalies 8.5%

diarrhea 2.7%

Placental lesions

Edema 4.3%

Torsion 4.5%

improper separation 4.7%

Other 6%

No diagnosis 16.9%

Tengelsen et al (1997) reported on 290 abortions in Michigan


EHV - 26 - 8.9%

Placentitis - - 12.7%

Septicemia - 6.2%


Anomalies - 4.8%

Vitamin E Selenium deficiency - 56%

Umbilical abnormalitis - 4.8%

Twins - 2%


Smith et al (2003) reported on 1252 fetuses and neonatal foals over a 10 year period in the UK. They found the following:

39% had umbilical cord lesions of which torsion was 33% and 3% were cervical pole necrosis

10% placentitis

7% Equid alpha herpesvirus infection

6% twinning

14% stillbirth

3% were neonatal infections in the absence of placentitis.

8% were no diagnosis


Marenzoni et al (2012) reported on 103 cases of abortion, stillbirth and neonatal death.

Non infectous causes were

Umbilical cord torsion - 5.8%

Congenital anomalies - 0.9%

Placental mineralisation - 1.9%

Placental insufficiency - 0.9%

body pregnancy - 1.9%

Premature separation - 2.9%

Dystocia - 0.9)


EHV-1 - 21%

EIA - 1.9%

Bacterial infection (sepsis) - 19.4%

Bacterial infection (Placentitis) - 8.7%

No diagnosis - 35.9%


Roach et al (2021) reported on a population of mares in the UK and Ireland. There were 5 breeding seasons, 1802 mares and 3586 pregnancies. 3516 pregnancies were reported with followup. 92.7 % of pregnant mares foaled and had a live foal at 24 hours after parturition. Failure of pregnancy or perinatal mortality was 7.3% (257 fetuses).

Bain AM. Foetal losses during pregnancy in the thoroughbred mare: a record of 2,562 pregnancies. N Z Vet J. 1969; 17: 155-158.

Causes of failure of pregnancy in the 116 pregnancies were investigations were done

Umbilical cord 1.5% - 52 cases - 44.8% of FOP (majority were torsion)

Noninfectious placental disease 0.4% - 13 cases - 11.2% (placental edema, villous atrophy, chorioallantoic mineralisation)

infectious placentitis 0.3% - 12 cases - 10.3% (ascending and hematogenous spread)

EHV 0.3% - 11 cases - 9.5%

Fetal anomalies and sepsis 0.2% - 6 cases - 5.2%

No diagnosis 0.4% - 13 cases - 11.2%

Cantón et al (2023) reported on 1774 cases from California.

This report is different to the majority is having a total idiopathic abortion percentage of 71%. 11 % were noninfectious and 19 % were infectious.

Infectious causes (19 %)

Bacteria - 13%

Virus - 5%

Mycotic - 1%

Noninfectious (11 %)

Umbilical torsion - 4%

Malformation - 2 %



Giles RC, Donahue JM, Hong CB, Tuttle PA, Petrites-Murphy MB, Poonacha KB, Roberts AW, Tramontin RR, Smith B, Swerczek TW. Causes of abortion, stillbirth, and perinatal death in horses: 3,527 cases (1986-1991). J Am Vet Med Assoc. 1993; 203: 1170-1175.

Cantón GJ, Navarro MA, Asin J, Chu P, Henderson EE, Mete A, Uzal FA. Equine abortion and stillbirth in California: a review of 1,774 cases received at a diagnostic laboratory, 1990-2022. J Vet Diagn Invest. 2023; 35: 153-162.

Hong CB, Donahue JM, Giles RC Jr, Petrites-Murphy MB, Poonacha KB, Roberts AW, Smith BJ, Tramontin RR, Tuttle PA, Swerczek TW. Equine abortion and stillbirth in central Kentucky during 1988 and 1989 foaling seasons. J Vet Diagn Invest. 1993; 5: 560-566.

Marenzoni ML, Lepri E, Casagrande Proietti P, Bietta A, Coletti M, Timoney PJ, Passamonti F. Causes of equine abortion, stillbirth and neonatal death in central Italy. Vet Rec. 2012; 170: 262.

Pozor M. Equine placenta – A clinician's perspective. Part 2: Abnormalities. Equine vet Educ 2016; 28: 394-404

Platt H. Aetiological aspects of abortion in the thoroughbred mare. J Comp Pathol. 1973; 83: 199-205.

Prickett ME. Abortion and placental lesions in the mare. J Am Vet Med Assoc. 1970; 157: 1465-1470.

Roach JM, Foote AK, Smith KC, Verheyen KL, de Mestre AM. Incidence and causes of pregnancy loss after Day 70 of gestation in Thoroughbreds. Equine Vet J. 2021; 53: 996-1003.

Smith KC, Blunden AS, Whitwell KE, Dunn KA, Wales AD. A survey of equine abortion, stillbirth and neonatal death in the UK from 1988 to 1997. Equine Vet J. 2003; 35: 496-501.

Tengelsen LA, Yamini B, Mullaney TP, Bell TG, Render JA, Patterson JS, Steficek BA, Fitzgerald SD, Kennedy FA, Slanker MR, Ramos-Vara JA. A 12-year retrospective study of equine abortion in Michigan. J Vet Diagn Invest. 1997; 9: 303-306.

Whitwell KE. Investigations into fetal and neonatal losses in the horse. Vet Clin North Am Large Anim Pract. 1980;2: 313-331.


General pathogenesis

Many cases of infectious FOP are from ascending placentitis.

Fedorka et al (2021) found that [IL-6] was increased and that IL-6 activated the classical JAK/STAT pathway, thereby acting as anti-inflammatory, anti-apoptotic, and pro-survival. The IL-6R was expressed within the chorioallantois, indicating a paracrine
signaling pathway of maternal IL-6 to fetal IL-6R.


Fedorka CE, Scoggin KE, El-Sheikh Ali H, Loux SC, Dini P, Troedsson MHT, Ball BA. Interleukin-6 pathobiology in equine placental infection. Am J Reprod Immunol. 2021; 85: e13363.

General reviews

Hong et al (1993) and Canisso et al (2015) provide a comprehensive review of placentitis.

Hong et al (1993) reported that 68.6% of placentitis cases had an agent recovered. There were may different bacteria.

Canisso I, Barry A. Ball BA, Erol E, Squires EL, Troedsson M. Comprehensive Review on Equine Placentitis. AAEP Proceedings 2015; 61: 490-509

Hong CB, Donahue JM, Giles RC Jr, Petrites-Murphy MB, Poonacha KB, Roberts AW, Smith BJ, Tramontin RR, Tuttle PA, Swerczek TW. Etiology and pathology of equine placentitis. J Vet Diagn Invest. 1993; 5: 56-63.




Bacterial placentitis has several different pathogenesies. One very common mode is by transcervical infection of the placenta - ascending infection. To reach the placenta, bacteria from the vagina must pass the cervical barrier. This barrier is partly physical (the cervical epithelium and stroma, and the cervical mucus plug.


Loux SC, Scoggin KE, Troedsson MH, Squires EL, Ball BA. Characterization of the cervical mucus plug in mares. Reproduction. 2017; 153: 197-210.

Actinobacillus spp

Family Pasteurellaceae; Actinobacillus equuli subsp equuli and subsp haemolyticus

Bacteria of the genus Actinobacillus are normal inhabitants of the alimentary and reproductive tract of horses. They can cause septicemia and reproductive disease (endometritis and failure of pregnancy). They are often recovered with Streptococcus equi subsp zooepidemicus.



Layman QD, Rezabek GB, Ramachandran A, Love BC, Confer AW. A retrospective study of equine actinobacillosis cases: 1999-2011. J Vet Diagn Invest. 2014; 26: 365-375.


Anaplasma phagocytophaglum

Equine anaplasmosis is a tick born infection. On Ontario, there is a seroprevalence of about 1% (Neely et al. 2021).

Neely M, Arroyo LG, Jardine C, Moore A, Hazlett M, Clow K, Archer H, Weese JS. Seroprevalence and evaluation of risk factors associated with seropositivity for Borrelia burgdorferi in Ontario horses. Equine Vet J 2021; 53: 331-338

Arcanobacterium hippocolae

Bemis et al (2021) report on a stilbirth where there was placentitis with this bacterium. Isolation from stomach content and lung was also reported.

Bemis DA, Bryant MJ, Kania SA, Newman SJ. Isolation of Arcanobacterium hippocoleae from a case of placentitis and stillbirth in a mare. J Vet Diagn Invest. 2008; 20: 688-691.

Bacillus safensis

Kelley et al (2021) reported on a mare that aborted at 7 months gestation. There was a 30 x 12 cm region of fibrinosuppurative placentitis at the junction of the pregnant horn and the body. This is the location of nocardioform placentitis.

Kelley DE, Hodo CL, Aceino A, Lawhon SD, Hinrichs K, Brinsko SP. Abortion due to Bacillus safensis in a mare. Equ Vet Edu 2021; 33: e28-e30

Bartonella henselae

Johnson et al (2009) reported a single case of failure of pregnancy at 240 days gestation. At post mortem there are foci in the liver, kidney, lung and adrenal. On histology there was multifocal necrosis and vasculitis with gram-negative bacteria. The bacterium was visible as sShort spirals on WSS stain

Johnson R, Ramos-Vara J, Vemulapalli R. Identification of Bartonella henselae in an aborted equine fetus. Vet Pathol. 2009; 46: 277-281.

Borrelia burgdorferi

Lyme disease is a tick born infection. On Ontario, there is a seroprevalence of about 16% (Neely et al. 2021).

Neely M, Arroyo LG, Jardine C, Moore A, Hazlett M, Clow K, Archer H, Weese JS. Seroprevalence and evaluation of risk factors associated with seropositivity for Borrelia burgdorferi in Ontario horses. Equine Vet J 2021; 53: 331-338


Cellulosimicrobium cellulans

Bolin et al (2004) reported on cases of this bacteria. There were were 8 cases with isolation from fetuses and placentas, 5 fetuses and 4 placentas. 12 were abortions, 3 were premature and 2 were full term. The fetal lesions were in hte lung, liver and placenta. There was multifocal necrosis in the liver, and placentitis. The placentitis was either on the cervical star or the 'anterior surface' of the body of the placenta. This latter location had a brown mucoid exudate, just like nocardioform placentitis. Lung lesions were pyogranulomatous pneumonia.

Bolin DC, Donahue JM, Vickers ML, Giles RC, Harrison L, Jackson C, Poonacha KB, Roberts JE, Sebastian MM, Sells SE, Tramontin R, Williams NM. Equine abortion and premature birth associated with Cellulosimicrobium cellulans infection. J Vet Diagn Invest. 2004; 16: 333-336.

Chlamydia abortus

Ricard et al (2023) examined 99 placentas in western Canada and identified Chlamydia 16s RNA in 26 cases and 22 were Chlamydia abortus. 17 had no final diagnosis. The other 9 had placentitis of unknown origin, 3 had bacterial abortion, 1 was EHV1 and 1 had placental separation.


Ricard RM, Burton J, Chow-Lockerbie B, Wobeser B. Detection of Chlamydia abortus in aborted chorioallantoises of horses from Western Canada. J Vet Diagn Invest. 2023; 35: 359-365.

Chlamydia psittaci

Chlamydia psittaci is not a widely recognised cause of failure of pregnancy in horses. It fits in the 'possible', sporadic and 'occasional' group.

Placentitis with chorioallantoic placentitis and amnionitis affecting regions at the attachment of the cord is of the mononuclear type, considered lymphocytes and histiocytes, was found. Fetal hepatitis and pneumonia also occurs in a high number of cases.


Szeredi et al (2005) reported finding Chlamydia psittaci in the trophoblasts of mares who had aborted. They identified Chlamydia psittaci in 64 of 77 cases of abortion and considered it the cause in 11 cases.

Taylor et al (2017) found Chlamydia psittaci by PCR in 37 of 184 'equine reproductive loss events". Nothing further was reported. There was no evidence of transmission to humans.

Jelocnik et al (2018) provided evidence of the a dove origin of Chlamydia psittaci as a cause of abortion in horses.

Akter et al (2020) reported on 2 cases of abortion (and one weak foal) that had PCR positive lung and placenta. Histological lesions were not reported as they did not have fixed tissue.

Bauman et al (2020) reported on 2 cases in Switzerland. Both cases were EHV-1 positive too. One was Chlamydia psittaci and the other Chlamydia abortus postive. Necrosuppurative lesions were found in the placenta.

Anstey et al (2021) reported on the epidemiology of Chlamydia psittaci infection in horses in Australia - the Hunter Valley area.

Begg et al (2022) wrote a detailed report on the lesions in Chlamydia psittaci infected equine fetuses and placentas of 46 cases. There were 7 abortions, 26 premature births and 13 neonatal loss. Placentitis including amnionitis, and chorionitis was the main lesion - and it was not just at the cervical star/pole region. Lymphocytes and histiocytes were the main population of cells. In the fetus 19 of 38 cases had hepatitis and pneumonia occurred in 26 of 38.

Anstey et al (2022) reported on transmission to incontact mares. Close contact mares to those that aborted did not test positive.





Akter R, Stent AW, Sansom FM, Gilkerson JR, Burden C, Devlin JM, Legione AR, El-Hage CM. Chlamydia psittaci: a suspected cause of reproductive loss in three Victorian horses. Aust Vet J. 2020; 98: 570-573.

Anstey S, Lizárraga D, Nyari S, Chalmers G, Carrick J, Chicken C, Jenkins C, Perkins N, Timms P, Jelocnik M. Epidemiology of Chlamydia psittaci infections in pregnant Thoroughbred mares and foals. Vet J. 2021; 273: 105683.

Anstey SI, Jenkins C, Jelocnika M. Suspected chlamydial foetal loss highlights the need for standardised on-farm protocols. Australian Vet J 2022; 100: 600-604

Begg AP, Carrick J, Chicken C, Blishen A, Todhunter K, Eamens K, Jenkins C. Fetoplacental pathology of equine abortion, premature birth, and neonatal loss due to Chlamydia psittaci. Vet Pathol. 2022; 59: 983-996.

Baumann S, Gurtner C, Marti H, Borel N. Detection of Chlamydia species in 2 cases of equine abortion in Switzerland: a retrospective study from 2000 to 2018. J Vet Diagn Invest. 2020; 32: 542-548.

Bocklisch H, Ludwig C, Lange S. Chlamydia as the cause of abortions in horses. Berl Munch Tierarztl Wochenschr. 1991; 104: 119-124.

Borel N, Polkinghorne A, Pospischil A. A Review on Chlamydial Diseases in Animals: Still a Challenge for Pathologists? Vet Pathol. 2018; 55: 374-390

Forster JL, Wittenbrink MM, Häni HJ, Corboz L, Pospischil A. Absence of Chlamydia as an aetiological factor in aborting mares. Vet Rec. 1997; 141: 424.

Glávits R, Molnár T, Rády M. Chlamydia-induced abortion in a horse. Acta Vet Hung. 1988; 36: 33-36.

Jelocnik M, Jenkins C, O'Rourke B, Barnwell J, Polkinghorne A. Molecular evidence to suggest pigeon-type Chlamydia psittaci in association with an equine foal loss. Transbound Emerg Dis. 2018; 65: 911-915.

Szeredi L, Hotzel H, Sachse K. High prevalence of chlamydial (Chlamydophila psittaci) infection in fetal membranes of aborted equine fetuses. Vet Res Commun. 2005 Mar;29 Suppl 1:37-49.

Taylor KA, Durrheim D, Heller J, O'Rourke B, Hope K, Merritt T, Freeman P, Chicken C, Carrick J, Branley J, Massey P. Equine chlamydiosis-An emerging infectious disease requiring a one health surveillance approach. Zoonoses Public Health. 2017

Corynebacterium pseudotuberculosis

Poonacha and Donahue (1995) reported on a mare that aborted her foal at 254 days gestation. The liver of the fetus was swollen and had multiple variably sized white foci that were miliary abscesses. The placenta has placentitis of the cervical star. Microscopic lesions were in the lung, liver kidney lymph node and chorioallantois. Corynobacterium pseudotuberculosis was cultured.

Poonacha KB, Donahue JM. Abortion in a mare associated with Corynebacterium pseudotuberculosis infection. J Vet Diagn Invest. 1995 Oct;7(4):563-4. doi: 10.1177/104063879500700428. PMID: 8580187.

Coxiella burnetti

Failure of pregnancy because of Coxiella is infrequently reported. Infection is possible. A seroprevalence based on a review of the literature (Marenzoni et al 2013) suggests 15.8%.

Marenzoni et al (2013) did not find any Coxiella RNA in 122 cases of abortion.

Akers et al (2020) reported identifying 21 cases on 600 cases of abortion (4%) where Coxiella was detected but the load was low and it was not necessarily a cause of abortion - no histology was done.


Akter R, Legione A, Sansom FM, El-Hage CM, Hartley CA, Gilkerson JR, Devlin JM. Detection of Coxiella burnetii and equine herpesvirus 1, but not Leptospira spp. or Toxoplasma gondii, in cases of equine abortion in Australia - a 25 year retrospective study. PLoS One. 2020; 15: e0233100.

Marenzoni ML, Stefanetti V, Papa P, Casagrande Proietti P, Bietta A, Coletti M, Passamonti F, Henning K. Is the horse a reservoir or an indicator of Coxiella burnetii infection? Systematic review and biomolecular investigation. Vet Microbiol. 2013; 167: 662-669.


Leptospira interrogans

Abortion due to Leptospira interrogans represents up to 13% of Failure of Pregancy in mares in some locales. Leptospira interrogans serovar Pomona is probably the most common isolate. In Kentucky, the common serovar is Leptospira interrogans serovar Pomona type kennewicki and about 6% of FOPs are leptospira related based on PCR testing and serology for serovar determination.

Foals infected in late gestation with Leptospires usually abort or are stillborn, but weak foals with icterus, hepatomegaly and petechia can be found. Leptospires may be found in the placenta, umbilical cord, kidney, and liver. Placental disease includes edema and areas of necrosis, hydrallantois, vasculitis or diffuse placentitis occasionally develops. The fetal liver may be swollen, have yellow discoloration. Microscopic lesions include multifocal necrosis and lymphocytes and plasma cells may be in the portal triads. Some have giant cells. Other lesions include interstitial nephritis, pulmonary haemorrhage, myocarditis and meningitis. Funisitis with diffuse yellowish discoloration of the umbilical cord may be seen also.


Divers TJ, Chang YF, Irby NL, Smith JL, Carter CN. Leptospirosis: An important infectious disease in North American horses. Equine Vet J. 2019; 51: 287-292.

Erol E, Jackson CB, Steinman M, Meares K, Donahoe J, Kelly N, Locke S, Smith JL, Carter CN. A diagnostic evaluation of real-time PCR, fluorescent antibody and microscopic agglutination tests in cases of equine leptospiral abortion. Equine Vet J. 2015; 47: 171-174.

Hong CB, Donahue JM, Giles RC Jr, Petrites-Murphy MB, Poonacha KB, Roberts AW, Smith BJ, Tramontin RR, Tuttle PA, Swerczek TW. Etiology and pathology of equine placentitis. J Vet Diagn Invest. 1993; 5: 56-63.

Neorickettsia ristici

Phylum: Proteobacteria, Class Alphaproteobacteria; Order Rickettsiales, Family Anaplasmataceae, Genus Neorickettsia, Species Neorickettsia ristici

Neorickettsia ristici (previously Ehrlichia ristici) is the causative agent of Potomic horse fever or Equine monocytic ehrlichiosis. Mare infected with this bacterium/rickettsias can abort and the fetus has lymphohistiocytic lesions in multiple organs including enterocolitis, hepatitis, myocarditis and mesenteric lymphadenitis. Organisms are found in many different organs.

Long et al (1997) reported on 2 cases.

Coffman et al (2008) reported on one case.


Coffman EA, Abd-Eldaim M, Craig LE. Abortion in a horse following Neorickettsia risticii infection. J Vet Diagn Invest. 2008; 20: 827-830.

Long MT, Goetz TE, Whiteley HE, Kakoma I, Lock TE. Identification of Ehrlichia risticii as the causative agent of two equine abortions following natural maternal infection. J Vet Diagn Invest. 1995; 7: 201-205.


Nocardioform actinomycete organisms of nocardioform placentitis

There are numerous species of nocardioform bacteria cultured from nocardioform placentitis including Amycolatopsis kentuckyensis, Amycolatopsis lexingtonensis, Amycolatopsis pretoriensis and Crossiela equi. Streptomyces atriruber and Streptomyces silaceus have also been isolated from cases of nocardioform placentitis.

It is not known how the infection occurs and attempts to reproduce the disease were unsuccessful.

Lesions occur in an unusual location. Rather than being at the cervical star, they tend to be a distinct region of thick light brown tenacious exudate on the chorion at the bifurcation of the horns on the lateral and ventral surface of the placenta. This brown tenaceous material gives rise to the descriptive diagnosis of 'mucoid' placentitis.


Canisso IF, Ball BA, Erol E, Claes A, Scoggin KE, McDowell KJ, Williams NM, Dorton AR, Wolfsdorf KE, Squires EL, Troedsson MH. Attempts to induce nocardioform placentitis (Crossiela equi) experimentally in mares. Equine Vet J. 2015; 47: 91-95.


Mycobacterial failure of pregnancy is very rare. There are several single case reports


Johnson et al (2012) reported on a case of abortion due to Mycobacterium runyon group IV. There was brown discolouration of the amnion and allantois, and marked edema. Bacilli were in trophoblasts but there was little inflammation.


Johnson AK, Roberts JF, Hagan A, Wilborn RR, Dujovne G, Sells SF, Donahue JM. Infection of an equine placenta with a novel mycobacterial species leading to abortion. J Vet Diagn Invest. 2012; 24: 785-790.

Salmonella spp

Mayhew et al (2021) reported on a mare that aborted at 10 months gestation. The placental lesions were grossly and histologically around the cervical star and there was fetal sepsis. Salmonella enterica subsp. arizonae was cultured from the placenta, lung and fetal stomach. The placental lesions included bacteria within the chorioallantoic membranes and within vessels.


Mayhew KK, Clarke L, Howerth EW. Salmonella enterica subsp. arizonae-associated abortion in a mare. Equine Vet Educat 2021;

Stenotrophomonas maltophilia

There are many bacteria that cause a syndrome of focal placentitis not at the cervical star. These create a brown tenacious exudate that is variously called mucoid placentitis or focal mucopurulent placentitis. The pathogenesis is not known.

Gomes VCL, Del Piero F, Langohr IM, de Aguiar LH, A. Anderson A, Sones JL, Pinto CR. Equine focal mucopurulent placentitis associated with Stenotrophomonas maltophilia Equine Vet Educ 21021; 33: 452


Streptococcus equi subsp. zooepidemicus

A common bacteria found in ascending placentitis in mares is Streptococcus equi subsp. zooepidemicus.

Fedorka et al (2019) inoculated mares with this bacterium in the cervix and measured cytokines in maternal and fetal circulation (no effect was detected), in the amniotic fluid (changes identified) and allantoic fluid (no change), but the chorioallantois and endometrium had a proinflammatory response. Fetal organs (liver and spleen) had an immunomodulating change. Fedorka et al (2021) studied T helper and T reg cells and found that there were increases in T helper cells in the chorioallantois suggesting fetal response. T helper cells increased and T reg cells decreased.


Fedorka CE, Ball BA, Scoggin KE, Loux SC, Troedsson MHT, Adams AA. The feto-maternal immune response to equine placentitis. Am J Reprod Immunol. 2019; 82: e13179

Fedorka CE, El-Sheikh Ali H, Walker OF, Scoggin KE, Loux SC, Dini P, Troedsson MHT, Ball BA. The imbalance of the Th17/Treg axis following equine ascending placental infection. 2021; 144: 7 103268.


Aspergillus funigatus

Geotrichum candidum



Encephalitazoon cuniculi

van Rensburg et al (1991) reported on one case of encephalitozoon infection in an stillborn foal. Lesions and organisms were in the kidney.

van Rensburg IB, Volkmann DH, Soley JT, Stewart CG. Encephalitozoon infection in a still-born foal. J S Afr Vet Assoc. 1991; 62: 130-132.

Neospora caninum

Neospora caninum is a common abortogenic agent of cattle. Little is known of the importance in horses. Serology for Neospora caninum, which cross reacts with Neospora hughesi, has been done in horses. About 20% of 296 slaughtered horses in Wyoming had a positive serum titre.

Dubey and Porterfield reported on a case of equine abortion due to Neospora sp.Only lung was examined histologically and tachyzoites were visible there.


Duby JP, Porterfield ML. Neospora caninum (Apicomplexa) in an aborted equine fetus. J Parasit 1990; 76: 732-734

Dubey JP, Romand S, Thulliez P, Kwok OC, Shen SK, Gamble HR. Prevalence of antibodies to Neospora caninum in horses in North America. J Parasitol. 1999; 85: 968-969.


Neospora hughesi

Anderson et al (2019) reported on a confirmed case at 280 days gestation. Lesions were in the lungs, liver and heart.

Anderson JA, Alves DA, Cerqueira-Cézar CK, da Silva AF, Murata FHA, Norris JK, Howe DK, Dubey JP. Histologically, immunohistochemically, ultrastructurally, and molecularly confirmed neosporosis abortion in an aborted equine fetus. Vet Parasitol. 2019; 270: 20-24.




Equid alphaherpesvirus 1

The most common cause of viral abortion and therefore of herpesviral failure of pregnancy is Equid alphaherpesvirus I aka equine herpesvirus 1 (EHV-1). EHV-1 is in the family Herpesviridae, subfamily Alphaherpesvirinae, genus Varicellovirus.

A horse is usually infected through the respiratory mucosa from whence virus spreads to the local lymphoid tissue, the mandibular lymph node, and then spreads within lymphocytes throughout the body. Viraemia lasts about 14 days and subsequently target tissues are infected. Within the uterus, endothelial cells of arterioles in the pregnant uterus are infected. Small infarcts occur and it is through these that virus enters the fetus to produce a fetal viraemia. Failure of pregnancy is from the effects on endothelial cells in the pregnant uterus and fetal disease.

Latent infection in lymphoid tissues and trigeminal ganglia occurs. Triggers for recrudescence are largely unknown.


Lunn DP, Davis-Poynter N, Flaminio MJ, Horohov DW, Osterrieder K, Pusterla N, Townsend HG. Equine herpesvirus-1 consensus statement. J Vet Intern Med. 2009; 23: 450-461.

Marenzoni ML1, Bietta A, Lepri E, Casagrande Proietti P, Cordioli P, Canelli E, Stefanetti V, Coletti M, Timoney PJ, Passamonti F. (2013) Role of equine herpesviruses as co-infecting agents in cases of abortion, placental disease and neonatal foal mortality. Vet Res Commun (2013) 37: 311–317.

Smith KC, Borchers K. (2001) A study of the pathogenesis of equid herpesvirus-1 (EHV-1) abortion by DNA in-situ hybridization. J Comp Pathol 2001; 125: 304-310.

Alphaarterivirus equid (equine arteritis virus [EAV])

EAV is of the Order Nidovirales, Family Arteriviridae, Genus Arterivirus. It produces a variety of clinical effects in adults but in mares, infertility, early embryonic mortality, abortion and stillbirth.

The virus causes a necrotising vasculitis/vascular necrosis. The virus is endotheliotropic and has vascular medial trophism. It causes myometritis and edema. This with vasculitis and thrombosis causes hypoxia of the placenta. This results in chorionic detachment and then expulsion of the fetus. The fetus becomes infected but the lesions are subtle and included arterial smooth muscular necrosis.

Most pregnant mares abort fetuses without fetal or placental lesions. The virus infects smooth muscle cells of the myometrium and induces necrotic myometritis.

Some fetuses become viremic and have fibrinoid vasculitis, interstitial pneumonia, and renal tubular necrosis.

Many stallions become inapparent clinical carriers of the virus after the acute phase. Spread to mares is venereal or directly horizontal.

Mares clear the virus quickly but stallions develop persistent infection of the testis.

Vet Pathol  34: 287-296

Del Piero F. Equine viral arteritis. Vet Pathol. 2000; 37: 287-296.

Equine encephalosis virus

Order Reovirales; Family Reoviridae; Genus Orbivirus;

This virus is an Orbivirus similar to African horse sickness. The virus is found in South Africa and Israel. Abortion can occur
South Africa and now Israel



Genetic abnormalities

Chromosomal abnormalities

Genetic abnormalities including chromosomal translocations are possible causes of failure of pregnancy, usually early embryonic mortality. It is called Repeted Early Embryonic Loss (REEL) or equine early embryonic loss (EEEL) or early pregnancy loss (EPL).

Whitwell (1980) reported on a personal communication with JG Matthews who found 1 case of mosiacism in 30 aborted foals.

Ghosh et al (2016) reported on a case where the Arabian mare had an autosomal translocation. She had REEL.



Ghosh S, Das PJ, Avila F, Thwaits BK, Chowdhary BP, Raudsepp T. A Non-Reciprocal Autosomal Translocation 64,XX, t(4;10)(q21;p15) in an Arabian Mare with Repeated Early Embryonic Loss. Reprod Domest Anim. 2016; 51: 171-174.

Whitwell KE. Investigations into fetal and neonatal losses in the horse. Vet Clin North Am Large Anim Pract. 1980; 2: 313-331.


Equine familial isolated hypoparathyroidism (EFIH)

This syndrome results in postnatal hypocalcemia and subsequent muscle fasciculation and seizures.Originally it was called idiopathic hypocalcemia, as calcium infusion would result in clinical improvement. It was found that foals had a nonsense variant of Rap Guanine Nucleotide Exchange Factor 5 (RAPGEF5c.2624C>A p.Ser875*; EquCab 3.0. chr4: g.54108297G>T 


Elcombe ME, Bellone RR, Magdesian KG, Finno CJ. Prevalence of the RAPGEF5 c.2624C>A and PLOD1 c.2032G>A variants associated with equine familial isolated hypoparathyroidism and fragile foal syndrome in the US Thoroughbred population (1988-2019). Equine Vet J. 2023;55(4):666-671. doi:10.1111/evj.13883

Fragile foal syndrome

Fragile foal syndrome is a cutaneous asthenia/Ehler-Danlos-like syndrome. The mutation is a missense mutation in the PLOD1 (c.2032G>A; p.Gly678Arg gene) that effects collagen fibril formation. Homozygous animals are affected.


Elcombe ME, Bellone RR, Magdesian KG, Finno CJ. Prevalence of the RAPGEF5 c.2624C>A and PLOD1 c.2032G>A variants associated with equine familial isolated hypoparathyroidism and fragile foal syndrome in the US Thoroughbred population (1988-2019). Equine Vet J. 2023;55(4):666-671. doi:10.1111/evj.13883


Other fetal causes of failure of pregnancy

Ascites (NOS)


Congenital hydrocephalus

Internal hydrocephalus

Non communicative

Obstruction of mesencephalic aquiduct (Sylvius)

Obstruction of Foramen of Monro a (interventricular)

Obstruction of foramina of Luschka (lateral apeture of 4th ventricle

Communicative type

Friesan foals with hydrocephalus have obstruction of the jugular foramens and cerebral vessels filled with blood. It is autosomal recessive inheritance. There is a nonsense mutation in B3GALNT2.

Other breeds, such as Belgian draft horses can have the same mutation.

Kolb DS, Klein C. Congenital hydrocephalus in a Belgian draft horse associated with a nonsense mutation in B3GALNT2. Can Vet J. 2019; 60: 197-198.

Sipma KD, Cornillie P, Saulez MN, Stout TA, Voorhout G, Back W. Phenotypic characteristics of hydrocephalus in stillborn Friesian foals. Vet Pathol. 2013; 50: 1037-1042.


External hydrocephalus

Increased production

Reduced uptake

Craniofacial anomalies

There are myriad anomalies of the skull.

Meloschisis (bilateral oblique facial clefts) was reported by Agerholm et al (2017)


Agerholm JS, Pedersen HG, McEvoy FJ, Heegaard S. Bilateral oblique facial clefts, rudimentary eyes and hydrocephalus in an aborted equine foetus. Reprod Domest Anim. 2017; 52(4): 692-695



Giant cell hepatopathy.

Car and Anderson (1988) reported on 3 foals of 132 aborted fetuses that had giant multinucleated hepatocytes, confirmed by electron microscopy. The cause was not determined.

Car BD, Anderson WI. Giant cell hepatopathy in three aborted midterm equine fetuses. Vet Pathol. 1988; 25: 389-391.


Glycogen storage disease - Glycogen branching enzyme deficiency

About 3% of horses with glycogen branching Enzyme deficiency abort. Most die in the neonatal and post weaning periods - up to 18 weeks.

Affected horses have spherical or ovoid inclusions in skeletal muscle. These are PAS +.


Render JA, Common RS, Kennedy FA, Jones MZ, Fyfe JC. Amylopectinosis in fetal and neonatal Quarter Horses. Vet Pathol. 1999; 36: 157-160.


Fetal neoplasms are very rare.


Neu (1993) reported on a fetus with hepatoblastoma. It was born dead at term.

deVries et al (2013) reported on a foal that died with peritoneal metastases from a hepatoblastoma.


Neu SM. Hepatoblastoma in an equine fetus. J Vet Diagn Invest. 1993; 5: 634-637.

de Vries C, Vanhaesebrouck E, Govaere J, Hoogewijs M, Bosseler L, Chiers K, Ducatelle R. Congenital ascites due to hepatoblastoma with extensive peritoneal implantation metastases in a premature equine fetus. J Comp Pathol 2013; 148: 214-219.

Placental mixed germ cell tumor

Brockenstedt et al (2015) reported a case of equine placental mixed germ cell tumor with supposed metastasis to the liver of a foal. Metastasis occurred and the foal was euthanised at 52 days of age

Bockenstedt MM, Fales-Williams A, Haynes JS. Equine placental mixed germ cell tumor with metastasis to the foal. Vet Pathol 2015; 52: 360-363.


Is exceedingly rare for teratoma to be found in the placenta or the foal enutero.

Gurfield and Benirschke (2003) reported a case of placental teratoma in a fetal foal.

Binanti et al (2013) reported on a case where a teratoma was attached to the umbilical cord. It was 20 x 30 x 20 cm no description of the exact location on the umbilical cord was provided.From the photograph it appears to be close to the umbilicus of the foal.


Binanti D, Livini M, Riccaboni P, Sironi G. A case of umbilical cord teratoma in an aborted foal. J Vet Diagn Invest. 2013; 25: 173-175.

Gurfield N, Benirschke K. Equine placental teratoma. Vet Pathol. 2003 Sep;40(5):586-588.

Polymorphisms of p53

Leon et al (2012) reported on a mare that had repete early embryonic loss (REEL). She had single nucleotide polymorphism of p53.


Leon PM, Campos VF, Thurow HS, Hartwig FP, Selau LP, Dellagostin OA, Neto JB, Deschamps JC, Seixas FK, Collares T. Association between single nucleotide polymorphisms in p53 and abortion in Thoroughbred mares. Vet J. 2012; 193: 573-575.

Thyroid hyperplasia and musculoskeletal deformities (Congenital hypothyroidism-dysmaturity syndrome).

Foals with or without musculoskeletal deformities may have microscopic thyroid hyperplasia yet no clinically visible goitre. A variety of musculoskeletal deformities occur. It is essential for the thyroid gland to be examined histologically in every failure of pregnancy.

Affected horses are newborn or are aborted. Foals have microscopic hyperplasia of the thyroid glands and multiple congenital musculoskeletal deformities. The thyroid gland are of normal size. Musculoskeletal lesions include mandibular prognathia, inappropriately ossified carpal and tarsal bones, flexural deformities of the forelimbs and ruptured tendons of the common digital extensor muscles. Despite the fact that they are of either normal or lengthened gestation, they show signs of immaturity including a short soft coat and lax joints. Aborted foals may have osteopetrosis.

Cases were initially reported in Western Canada but have been found in a variety of other locations in Canada and internationally including Europe.

Allen A (1995) Hyperplasia of the thyroid gland and musculoskeletal deformities in two equine abortuses. Canadian Vet. J. 36: 234-236

Allen AL, Doige CE, Fretz PB, Townsend HG. Hyperplasia of the thyroid gland and concurrent musculoskeletal deformities in western Canadian foals: reexamination of a previously described syndrome. Can Vet J. 1994; 35: 31-38.

Allen AL, Townsend HG, Doige CE, Fretz PB. A case-control study of the congenital hypothyroidism and dysmaturity syndrome of foals. Can Vet J. 1996; 37: 349-351; 354-358.

Koikkalainen K, Knuuttila A, Karikoski N, Syrja P, Hewetson M (2014) Congenital hypothyroidism and dysmaturity syndrome in foals: First reported cases in Europe. Equ Vet Educat 2014; 26: 181-189




Placental abnormalites

Placental edema

Allantoic pouches (polyps, vesicles)

Bulbous polypoid structures of the allantois are occasionally seen in placentas. One or 2 small ones is incidental and of no particular significance. Multiple large examples are filled with clear fluid, and they usually indicate placental edema or local allantoic dysregulation of fluid - They are also reported in mare that abort foals that were derived from somatic cell nuclear transfer (SCNT) cloning.

Causes of placental edema should be investigated - see placental edema.

Morresey PR. Allantoic vesicles: Only a problem when they are a problem? Equine vet. Educ. 2009; 21: 145-146

Pozor MA, Sheppard B, Hinrichs K, Kelleman AA, Macpherson ML, Runcan E, Choi YH, Diaw M, Mathews PM. Placental abnormalities in equine pregnancies generated by SCNT from one donor horse. Theriogenology. 2016; 86: 1573-1582.

Body Pregnancy

A small number of conceptus may attach in the body of the uterus. In this location the fetus does not develop in the correct location and may expand out through the cervix or be sufficiently restricted to be aborted.

Jobert et al (2005) reported on 30 body pregnancies diagnosed on ultrasound of the location of the vesicles within the uterine body. 9 of the body pregnancies were in the cranial uterus - 21 were in the caudal uterine body. Of these 21, 13 were lost by 43 days of gestation, one at 4 months, and one by 9 months. 3 pregnancies were terminiated. 10 pregnancies went to term. The photograph of a body pregnancy had the insertion of the cord in the normal location! Of the 9 in the cranial uterine body, 2 were aborted and 7 were caried to term.


There is ongoing research into the proteome of the fluids in the uterus in early pregnancy, and studies of the peptides of the fetal maternal intaction.


Bastos HBA, Martinez MN, Camozzato GC, Estradé MJ, Barros E, Vital CE, Vidigal PMP, Meikle A, Jobim MIM, Gregory RM, Mattos RC. Proteomic profile of histotroph during early embryo development in mares. Theriogenology. 2019; 125: 224-235.

Jobert ML, Leblanc MM, Pierce W.Pregnancy loss rate in equine uterine body pregnancies. Equ Vet Educat 2005; 17: 163-165

Quinn BA, Hayes MA, Waelchli RO, Kennedy MW, Betteridge KJ. Changes in major proteins in the embryonic capsule during immobilization (fixation) of the conceptus in the third week of pregnancy in the mare. Reproduction. 2007; 134: 161-170.

Whitwell KE. Morphology and pathology of the equine umbilical cord. J Reprod Fertil Suppl. 1975; 23: 599-603.

Premature placental separation (PPS)

Separation of the placenta in the first stages of pregnancy is identified as a red bag delivery. Normally, the placenta ruptures during parturition and the placenta only separates after birth. In premature separation, rupture occurs after the placenta separates.

PPS is common in mare reproductive loss syndrome (endophyte toxicosis) and placentitis where there is placental edema and thickening particularly if there is fibrosis.

Foals born when the placenta separates prematurely are more likely to develop significant hypoxia.


Murase H, El-Sheikh Ali H, Ruby RE, Scoggin KE, Ball BA. Transcriptomic analysis of the chorioallantois in equine premature placental separation. Equine Vet J. 2023; 55: 405-418.


Cord abnormalities

Abnormal length

The normal length of equine umbilical cord is discusses in the section of the normal umbilical cord

Issues can occur with the umbilical cord is shorter than normal (less than 36cm in thoroughbreds). If this causes an issue it is either because the placenta pulls from the endometrium causing a larger avillus area of the insertion of the cord.

A cord that is longer than normal (>83 cm) has several potential issues.

  1. Altered blood flow. Longer cords have greater resistance to blood flow.
  2. Strangulation. A cord that is too long is more likely to wrap around around a limb or the neck or body.
  3. Excessive torsion. There are twists normally in a cord - up to 3 -5. Twists can obstruct the urachus only or the urachus and vessels. In both cases the urachus is dilated behind the twist. In vascular obstruction there is ballooning and edema of the cord.
  4. Necrosis of the body of the placenta (aka infarction of the placenta).


Foote AK, Ricketts SW, Whitwell KE. A racing start in life? The hurdles of equine feto-placental pathology. Equine Vet J Suppl. 2012 Feb;(41):120-129.

Smith KC, Blunden AS, Whitwell KE, Dunn KA, Wales AD. A survey of equine abortion, stillbirth and neonatal death in the UK from 1988 to 1997. Equine Vet J. 2003l; 35: 496-501.

Whitwell KE, Jeffcott LB. Morphological studies on the fetal membranes of the normal singleton foal at term. Res Vet Sci. 1975; 19: 44-55.

Whitwell KE. Morphology and pathology of the equine umbilical cord. J Reprod Fertil Suppl. 1975; 23: 599-603.

Single umbilical artery

There should be 2 umbilical arteries.

Girodroux et al (2017) reported finding a single umbilical artery in a foal. The cord was too long but the single artery was not considered a cause of clinical issues.


Girodroux M, Lores M, Vilaregut L, Wilsher S. A single umbilical artery and omphalophlebitis in an Arabian foal. Equine Vet Educ 2019; 31:6-12

Whitwell KE. Morphology and pathology of the equine umbilical cord. J Reprod Fertil Suppl. 1975; 23: 599-603.

Strangulation by yolk sac remnant

Although very rare, the yolk sac remnant can wrap around the umbilical cord, causing obstruction.

Herniation of intestines into the umbilical cord

This is a rare lesion in foals.

Whitwell KE. Morphology and pathology of the equine umbilical cord. J Reprod Fertil Suppl. 1975; 23: 599-603.


These are the result of change in the fluid volume of the allantois and amnion. Little is known of their cause. Leptospirosis is a reported cause.

Mitchell et al (2019) reported on the succesful birth of a foal in a mare with hydrallantois.


Mitchell ARM, Delvescovo B, Tse M, Crouch EE, Cheong SH, Castillo JM, Felippe MJB, Ainsworth DM, de Amorim MD. Successful management of hydrallantois in a Standardbred mare at term resulting in the birth of a live foal. Can Vet J. 2019; 60: 495-501.

Processionary caterpillars/Mare Reproductive Loss Syndrome/Equine Amnionitis and fetal loss

Mare Reproductive Loss Syndrome of horses first reported in Kentucky and Equine Amnionitis and Fetal Loss in Australia are diseases related to ingestion of processionary caterpillars. The association of abortion and these caterpillars was based on observational and epidemiological factors, and the lesions initially identified were infectious and inflammatory. It is now known that the setae of caterpillars penetrate the intestine and migrate laterally. If they penetrate the pregnant uterus, they cause failure of pregnancy by carrying environmental type bacteria into the uterus and conceptus to cause infection, including endometritis, chorioallantoic placentitis and amnionitis and funiculitis.


Todhunter KH, Cawdell-Smith AJ, Bryden WL, Perkins NR, Begg AP. Processionary caterpillar setae and equine fetal loss: 1. Histopathology of experimentally exposed pregnant mares. Vet Pathol 2014; 51: 1117-1130.

Todhunter KH, Cawdell-Smith AJ, Bryden WL, Perkins NR, Begg AP. Processionary caterpillar setae and equine fetal loss: 2. Histopathology of the fetal-placental unit from experimentally exposed mares. Vet Pathol 2014; 51: 1131-1142.




Fescue endophyte toxicosis Neotyphodium coenophialum

Pregnant mares who ingest Paspalum infected with Claviceps purpurea, and Festuca elatior (tall fescue) infected with the endophyte Neotyphodium coenophialum. The seed head contains the most indole-diterpenoid mycotoxins.



Putnam MR, Bransby DI, Schumacher J, Boosinger TR, Bush L, Shelby RA, Vaughan JT, Ball D, Brendemuehl JP. Effects of the fungal endophyte Acremonium coenophialum in fescue on pregnant mares and foal viability. Am J Vet Res. 1991; 52: 2071-2074.

Riet-Correa F, Rivero R, Odriozola E, Adrien Mde L, Medeiros RM, Schild AL. Mycotoxicoses of ruminants and horses. J Vet Diagn Invest. 2013 Nov;25(6):692-708.


Placental teratoma

Gurfield and Benirschke (2003) reported on an aborted fetus with multiple nodules over 50% of the placenta. They were primitive teratomas.

Gurfield N, Benirschke K. Equine placental teratoma. Vet Pathol. 2003; 40: 586-588.


The dogma that there is a limited amount of exchange area of the equine uterus and that slight reduction in surface area will result in failure of pregnancy is typified by the rareity of live birth of twin foals. Anaya et al (2018) reported on Spanish horses where 23 twin pregnancies resulted in live foals out of 21,097 (0.066% prevalence). There were 23 live births.

Twin pregnancy may be aborted or one foal may become mummified. That mummy may be encorporated within the placenta of the other foal and the one viable foal may then have a placenta that surrounds that mummified foal. Rapacz-Leonard and Pazdzior-Czapula (2021) report on a case of encorporation with resultant placental changes that they said was similar to fetal-foot placental area of degeneration (PAD)


Anaya G, Fernández ME, Valera M, Molina A, Azconab F, Azorc P, Soléc M, Moreno-Millán M, Demyda-Peyrás D. Prevalence of twin foaling and blood chimaerism in purebred Spanish horses. The Vet J 2018; 234: 142-144

Rapacz-Leonard A, Paździor-Czapula K. Large placental area of degeneration in a twin pregnancy in a mare (Equus caballus). Equine Vet Edu 2021 34 e54-e59.

Reduced Placental histotrophic and hemotrophic function

Nutrition of the fetus and placenta through the endometrium occurs through a combination of histotrophic and hemotrophic means. Secretion of the endometrial glands is required for the uterine "milk" that is taken up by trophoblasts at the base of the microcotyledons. Adequate microcotyledonary surface area is required for hemotrophic transfer.

Endometrial disease particularly fibrosis and alteration of endometrial glands is likely to result in altered placental development and the resulting reduced transfer and surface area. Ferreira et al (2015) recorded a reduced blood flow early in gestation in older mares and in mares with endometrial degeneration as indicated by categorization of endometrial biopsies.

Endometrial biopsy is the only effective way to measure alteration to endometrial structure that could influence placental surface area. Quantitative means to measure surface area in a diagnostic setting is not established. Endometrial biopsy therefore remains an important component of investigation of failure of pregnancy.


Ferreira JC, Canesin HS, Ignácio FS, Rocha NS, Pinto CR, Meira C. Effect of age and endometrial degenerative changes on uterine blood flow during early gestation in mares. Theriogenology. 2015 Oct 15;84(7):1123-30. doi: 10.1016/j.theriogenology.2015.06.013. Epub 2015 Jun 29. PMID: 26194699.


Maternal causes of Failure of Pregnancy


Age of mare

Older mares are more likely to abort their foals. Platt (1973)


Bain AM. Foetal losses during pregnancy in the thoroughbred mare: a record of 2,562 pregnancies. N Z Vet J. 1969; 17: 155-158.

Platt H. Aetiological aspects of abortion in the thoroughbred mare. J Comp Pathol. 1973; 83: 199-205.


Mare with an abortion in a previous pregnancy are more likely to abort the next pregnancy - 13.6% abortion rate vs 6.5% of mares that had a live foal.

Platt H. Aetiological aspects of abortion in the thoroughbred mare. J Comp Pathol. 1973; 83: 199-205.


Santschi et al (1991) reported that 13 of 74 pregnant mares that survived their colic aborted. 36 of 74 mares who had surgery had live births. Hypoxia was an important determinant of survival of foals.

Boening and Leendertse (1993) reported that the abortion rate of 115 mares referred for colic was 16.4%. 34 mares of 85 surgical cases (20.5%) aborted.

Chenier and Whitehead (2009) reported on 153 cases where abortion occurred after colic. Anaesthetic time (especially >3 hours) and hypotension were important determinants of outcome. Endotoxemia and hypoxia were not.


Boening KJ, Leendertse IP. Review of 115 cases of colic in the pregnant mares. Equine Vet J. 1993; 25: 518-521.

Chenier TS, Whitehead AE. Foaling rates and risk factors for abortion in pregnant mares presented for medical or surgical treatment of colic: 153 cases (1993-2005). Can Vet J. 2009; 50: 481-485.

Santschi EM, Slone DE, Gronwall R, Juzwiak JS, Moll HD. Types of colic and frequency of postcolic abortion in pregnant mares: 105 cases (1984-1988). J Am Vet Med Assoc. 1991; 199: 374-377.

Endometrial fibrosis

Endometrial changes including endometrial fibrosis alters the mileu of the fetoplacental unit. This affects both hemotrophic and histotrophic nutrition.

Endometrial/placental insufficiency

It is generally considered that a successful pregnancy required all of the available surface area of the endometrium to be successful. Twining is the an obvious situation with reduction of maternal fetal interface. Endometritis and endometrial fibrosis would limit surface area and microcotyledonary development, placentitis reduces surface area. Yet twins are born and mares with partial hysterectomy give birth to live foals.

Uterine neoplasia

Canisso et al (2013) reported on a mare with failure of pregnancy that had a uterine lymphoma.

Canisso IF1, Pinn TL, Gerdin JA, Ollivett TL, Buckles EL, Schweizer CM, Ainsworth DM. B-cell multicentric lymphoma as a probable cause of abortion in a Quarter horse broodmare. Can Vet J. 2013; 54: 288-291.

Hormonal failure

Allen and Wilsher (2012) reported on a mare with persistent endometrial cups who had prolonged eCG in her blood that dropped precipitously resulting in one abortion..

Allen WR, S. Wilsher S. Persistent endometrial cups in the same mare in two successive pregnancies. Equ Vet Edu 2012 24: 247-250


Paternal cause of Failure of Pregnancy

Rarely is there mention of the effect of a stallion on pregnancy outcome.

Platt (1973) listed abortion rates in mares based on stallions. Of 7 stallions that covered 875 mares, one stallion had an abortion rate of 27.8%, one was 19.7%, one was 12.9% and another 7.1%. The 3 remaining stallions combined had 9.8% rate.

Platt H. Aetiological aspects of abortion in the thoroughbred mare. J Comp Pathol. 1973; 83: 199-205.

Maternal infectious disease



Darenius (1992) reported on early fetal death in mares with endometritis

Darenius K. Early foetal death in the mare. Histological, bacteriological and cytological findings in the endometrium. Acta Vet Scand. 1992; 33: 147-160.

Placental disease


Serum amyloid A concentration in mares with placentitis is above normal and higher in those that abort than those that do not abort. Levels return to normal after parturition. This could be used as a prognostic tool in mares diagnosed with placentitis.


Coutinho da Silva MA, Canisso IF, MacPherson ML, Johnson AEM, Divers TJ(2013) Serum amyloid A concentration in healthy periparturient mares and mares with ascending placentitis (pages 619–624). Equine Vet J 2013; 45: 619-624.


Processionary caterpillars



Cawdell-Smith AJ, Todhunter KH, Anderson ST, Perkins NR, Bryden WL. Equine amnionitis and fetal loss: mare abortion following experimental exposure to Processionary caterpillars (Ochrogaster lunifer). Equine Vet J. 2012; 44: 282-288.

Cawdell-Smith AJ, Todhunter KH, Perkins NR, Bryden WL (2013) Exposure of mares to processionary caterpillars (Ochrogaster lunifer) in early pregnancy: An additional dimension to equine amnionitis and fetal loss. Equine Vet J 2013; 45: 755–760

McDowell KJ, Webb BA, Williams NM, Donahue JM, Newman KE, Lindemann MD, Horohov DW. Invited review: the role of caterpillars in mare reproductive loss syndrome: a model for environmental causes of abortion. J Anim Sci. 2010; 88: 1379-1387.

Sebastian MM, Bernard WV, Riddle TW, Latimer CR, Fitzgerald TD, Harrison LR. REVIEW paper: mare reproductive loss syndrome. Vet Pathol. 2008; 45: 710-722.

Todhunter KH, Perkins NR, Wylie RM, Chicken C, Blishen AJ, Racklyeft DJ, Muscatello G, Wilson MC, Adams PL, Gilkerson JR, Bryden WL, Begg AP. Equine amnionitis and fetal loss: the case definition for an unrecognised cause of abortion in mares. Aust Vet J. 2009; 87: 35-38.

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