NAVLEReproductive·⏱ 25 min read·📅 Mar 28, 2026·by NAVLE Exam Prep Team·👁 1
Equine Infectious Abortion Study Guide
Overview and Clinical Importance
Infectious abortion represents a significant cause of reproductive loss in mares with major economic implications for the equine breeding industry. The primary infectious agents include Equine Herpesvirus-1 (EHV-1), Equine Viral Arteritis (EVA), Leptospira spp., and various bacterial agents causing placentitis. Studies indicate that up to 40% of mares may experience pregnancy failure from fertilization to parturition.
Tissue/Finding
Characteristic Features
Liver
Enlarged with multiple small (1-2 mm) white-yellow foci of necrosis
Body Cavities
Excess serous pleural and peritoneal fluid; subcutaneous edema
Integument
Icterus (jaundice) of mucous membranes; meconium staining possible
Lungs
Interlobular pulmonary edema; rib impressions; firm and mottled
Spleen
Splenomegaly with prominent lymphoid follicles
Thymus
Necrotic and friable
Equine Herpesvirus-1 (EHV-1) Abortion
Equine herpesvirus-1 (EHV-1) is the most important viral cause of abortion in horses worldwide. It is a double-stranded DNA virus belonging to the Alphaherpesvirinae subfamily that establishes lifelong latency in up to 70% of infected horses.
Etiology and Epidemiology
EHV-1 and EHV-4 are ubiquitous pathogens. While EHV-4 primarily causes respiratory disease, EHV-1 causes respiratory disease, abortion, neonatal foal death, and equine herpesvirus myeloencephalopathy (EHM). Transmission occurs via direct contact with nasal secretions, reproductive tract discharge, aborted fetus, or placenta.
High-YieldEHV-1 abortion typically occurs in the LAST TRIMESTER (greater than 7 months gestation) without premonitory signs in the mare. The fetus is usually expelled FRESH and enclosed within its membranes.
Pathogenesis
Following respiratory infection, EHV-1 replicates in respiratory epithelium and lymph nodes. Cell-associated viremia allows dissemination to endothelial cells of the pregnant uterus. Vasculitis leads to hypoxia and placental separation. The incubation period from infection to abortion is 9-120 days, though most abortions occur 2-4 weeks post-infection.
Clinical Signs and Presentation
Mare: Usually NO premonitory signs; abortion occurs suddenly
Timing: Typically 7-11 months gestation (last trimester)
Fetus: Delivered FRESH (not autolyzed) within intact membranes
Neonates: If born alive, typically weak and succumb within 3-7 days
Abortion storms: Multiple mares in a herd may abort sequentially
Fetal and Placental Lesions
Histopathologic Lesions
Intranuclear eosinophilic inclusion bodies in hepatocytes and bronchiolar epithelium (HALLMARK)
Focal hepatic necrosis with hepatocellular degeneration
Bronchiolitis and interstitial pneumonitis
Severe necrosis of splenic white pulp
Adrenal gland necrosis
NAVLE TipFRESH fetus + white liver foci + intranuclear inclusion bodies + last trimester = EHV-1. Remember: 'Fresh Fetus, Focal Liver necrosis, Final trimester' = EHV-1
Diagnosis
Important: Mare serology is of NO VALUE for EHV-1 abortion diagnosis due to variable time between infection and abortion.
Prevention and Vaccination
Test
Sample
Notes
PCR (Preferred)
Lung, liver, thymus, placenta
Most sensitive; differentiates EHV-1 vs EHV-4
Virus Isolation
Fresh tissues in transport medium
Definitive; takes 3-7 days
Fluorescent Antibody
Frozen sections of liver, lung
Rapid results
Histopathology
Formalin-fixed tissues
Inclusion bodies highly suggestive
Equine Viral Arteritis (EVA) Abortion
Equine viral arteritis (EVA) is caused by equine arteritis virus (EAV), an RNA arterivirus. Most infections are subclinical, but EVA can cause abortion, respiratory illness, and a carrier state in stallions. EVA is reportable in many jurisdictions.
Venereal route: Semen from carrier stallions (incubation: 6-8 days)
Carrier state: 10-70% of infected stallions become persistent shedders
Breed susceptibility: Higher seroprevalence in Standardbreds and Warmbloods
High-YieldMost EAV infections are SUBCLINICAL. Stallions can shed virus in semen for MONTHS to YEARS without showing signs.
Clinical Signs
NAVLE TipUnlike EHV-1, EVA fetuses may be AUTOLYZED with minimal lesions. The mare often shows CLINICAL SIGNS (edema, fever) before abortion. Hallmark = VASCULITIS.
Protocol
Timing
Notes
Standard
Months 5, 7, and 9 of gestation
Use killed EHV-1 vaccine
High-Risk Farms
Months 3, 5, 7, and 9
For farms with EHV-1 history
Leptospirosis Abortion
Leptospirosis is caused by pathogenic Leptospira interrogans serovars. In North America, serovar Pomona type kennewicki causes most clinical disease including abortion, equine recurrent uveitis (ERU), and acute renal failure.
Horses: Incidental hosts; mares shed in urine for 2-3 months post-abortion
Transmission: Contact with contaminated urine, water, soil
Important serovars: Pomona (most common), Grippotyphosa, Bratislava
High-YieldLeptospirosis is ZOONOTIC. Most equine leptospiral infections are SUBCLINICAL. Most abortions occur after 6 months gestation (peak 6-9 months).
Fetal and Placental Lesions
NAVLE TipCYSTIC ADENOMATOUS HYPERPLASIA of allantois = leptospiral placentitis. Unlike ascending bacterial placentitis, leptospiral does NOT involve cervical star (hematogenous spread).
Diagnosis
MAT: Gold standard serology; high titers in aborting mare
PCR: Fetal kidney, placenta, urine - rapid and sensitive
Vaccination: LEPTO EQ INNOVATOR - L. interrogans serovar Pomona vaccine; safe for pregnant mares. Initial: 2 doses 3-4 weeks apart; then annual boosters.
System
Signs
General
Fever (up to 41C), lethargy, anorexia
Respiratory
Nasal discharge, conjunctivitis ('pinkeye')
Edema
Periorbital, limbs, scrotum/prepuce, mammary
Reproductive
Abortion (up to 60% in naive populations); 6-29 days post-infection
Bacterial Placentitis
Bacterial placentitis is the most commonly diagnosed cause of equine abortion. Four morphologic patterns exist.
Types of Placentitis
Tissue
Lesions
Placenta (Pathognomonic)
Thick, edematous, hemorrhagic; brown mucoid material; CYSTIC ADENOMATOUS HYPERPLASIA of allantois; NOT involving cervical star
Liver
Enlarged, pale to yellow, mottled; hepatitis
Kidney
Swollen, edematous, pale streaks; nephritis
Fetus
Icterus, generalized edema, emaciated
Type
Route
Lesion Location
Pathogens
Ascending (Most Common)
Via cervix
CERVICAL STAR
S. zooepidemicus, E. coli, Klebsiella
Nocardioform
Hematogenous
Horn BASE (not cervical star)
Crossiella equi, Amycolatopsis
Diffuse
Hematogenous
Entire placenta
Leptospira spp.
Want full NAVLE study guides and timed practice questions?
Premium subscribers get condition-by-condition study guides, species-filtered practice questions, timed exam simulations, and a week-by-week study roadmap built for the boards.
Test yourself before moving on. Click an answer to reveal the explanation.
Question 1
A 9-year-old Thoroughbred mare at 8 months gestation aborts without preceding clinical signs. The fetus is fresh and enclosed within membranes. Gross examination reveals clear yellow fluid in pleural and peritoneal cavities, and an enlarged liver with multiple 1-2 mm white-yellow foci. Histopathology shows focal hepatic necrosis with eosinophilic intranuclear inclusion bodies in hepatocytes. What is the most likely cause of abortion?
Explanation
Equine herpesvirus-1 (EHV-1) is correct. The classic presentation includes: (1) sudden abortion without mare illness, (2) last trimester timing, (3) fresh fetus in membranes, (4) excess serosal fluid, (5) hepatomegaly with white necrosis foci, and (6) intranuclear inclusion bodies - pathognomonic for EHV-1.
Option A (EVA) - Incorrect. Mare typically shows clinical signs before abortion; fetus is often autolyzed with vasculitis, not hepatic inclusions.
Option B (Leptospirosis) - Incorrect. Characteristic finding is cystic adenomatous hyperplasia of allantois and diffuse placentitis. NO inclusion bodies.
Option D (Ascending placentitis) - Incorrect. Mare shows premonitory signs; cervical star involvement; fetus often autolyzed.
Option E (Nocardioform) - Incorrect. Lesions at horn base, not hepatic necrosis with inclusions.
Exam Focus: FRESH FETUS + HEPATIC NECROSIS (white foci) + INTRANUCLEAR INCLUSIONS = EHV-1. Mare shows NO warning signs and fetus is NOT autolyzed.
Question 2
Regarding Abortion (equine herpesvirus 1, equine viral arteritis, leptospirosis, etc.) in Equine species, which of the following statements is most accurate?
Explanation
The correct answer reflects a key high-yield fact about Abortion (equine herpesvirus 1, equine viral arteritis, leptospirosis, etc.): EHV-1 abortion typically occurs in the LAST TRIMESTER (greater than 7 months gestation) without premonitory signs in the mare. The fetus is usually expelled FRESH and enclosed within its membranes.
Question 3
Regarding Abortion (equine herpesvirus 1, equine viral arteritis, leptospirosis, etc.) in Equine species, which of the following statements is most accurate?
Explanation
The correct answer reflects a key high-yield fact about Abortion (equine herpesvirus 1, equine viral arteritis, leptospirosis, etc.): Most EAV infections are SUBCLINICAL. Stallions can shed virus in semen for MONTHS to YEARS without showing signs.