Heindrich N. Snyman, Tanya Rossi
University of Guelph, Animal Health Laboratory
AHL Newsletter 2026;30(2):17.
A multiparous (4th parity) Percheron draught mare suddenly aborted one morning. The mare was in late gestation (~ 305 days) and did not exhibit any obvious premonitory clinical symptoms at the time or within the days leading up to the abortion. During pregnancy, the mare had both outside and inside stable access with an artificial total light time of approximately 16 hours per day. Core vaccines (tetanus, rabies, West Nile virus, and Eastern/Western equine encephalomyelitis virus) were up to date, and the mare received a formulated diet including dry hay, whole oats, mineral, omega 3, microbinder, and a salt additive.
The aborted fetus and whole placenta were submitted for postmortem evaluation and diagnostic testing at the Animal Health Laboratory. On postmortem examination, the crown-rump-length (116.0 cm), umbilical length (71.0 cm), and number of umbilical twists (three and a half loose twists) were all within normal limits, and the fetus was normally formed with adequate joint mobility and normal limb positioning. The placenta was diffusely congested and contained a few indistinct scattered petechial hemorrhages along the allantochorionic surface, with small amounts of enclosed blood-tinged fluid. A single 11.5 cm curvilinear tear was present within the region of the cervical star. Examination of the fetus revealed lungs that were uniformly dark pink-red and rubbery, and samples sank when placed in formalin. The spleen was congested and meaty, and the liver was diffusely dark brown and congested. There was a small volume of tan mucoid gastric and intestinal content, and large amounts of normal olive-green pasty meconium within the colonic loops. The external and internal umbilical arteries and vein all appeared unremarkable.
Although these changes were relatively non-specific, the presence of petechial hemorrhages within the placenta and congestive splenomegaly were suggestive of antigenic stimulation and immune activation, and a potential infectious cause was considered. The changes in the lung could at least partially be ascribed to non-aerated fetal lung, but the increased rubbery consistency also suggested at least some inflammatory infiltrate, although this could simply have been due to fetal stress and meconium aspiration syndrome. The fetus and placenta were routinely sampled for histological evaluation, while pooled fetal and placental tissues were submitted for viral PCR testing (equine adenovirus and equine herpesvirus-1&4, and EHV-2&5).
On histological examination, the thymus, and to a lesser extent the spleen, contained scattered foci of acute fibrinous necrosis and karyorrhectic lymphocytes with acute interstitial hemorrhage (Fig. 1). Consistent with meconium aspiration syndrome, the pulmonary alveolar spaces were filled with fibrillar eosinophilic fluid with small to moderate numbers of clustered free-floating macrophages, fewer neutrophils, and large clumps of aspirated keratinized squamous epithelial cells and meconium Alveolar septa were hypercellular and alveolar spaces also often contained sloughed pyknotic to often fragmented alveolar pneumocytes that rarely contained small bright eosinophilic intranuclear inclusions that subtly marginated the nuclear chromatin. Bronchial and bronchiolar epithelium was intact with retained cilia and similar eosinophilic intranuclear inclusions were present (Fig. 2). The liver contained few small foci of acute hepatocellular degeneration with loose infiltrates of macrophages, fewer lymphocytes, and rare neutrophils (Fig. 3). Small numbers of the same mixed inflammatory cell infiltrates were also present within the pericholangiolar portal tracts. Other than vascular congestion, the placenta appeared histologically unremarkable.
PCR testing of the pooled fetal tissues samples was positive for EHV-1&4 and negative for both EHV-2&5 and equine adenovirus. Additional PCR testing further differentiated the strain as a non-neuropathogenic EHV-1 strain.
While EHV-1 is often better known as a respiratory pathogen or as a cause of neurological and ocular disease in horses, it also represents a major cause of late-term pregnancy loss and abortion in mares. EHV-1 is an alphaherpesvirus and infects horses worldwide. The virus typically infects the respiratory epithelium, and following the establishment of a leukocyte-associated viremia, it localizes in the uterus and placenta, leading to fetal infection and death. Abortions usually affect mares in the last/third trimester and although mares may exhibit mild respiratory symptoms, nasal discharge, or fever, abortions often occur suddenly without any premonitory signs. Placental lesions are usually subtle or absent, and in addition to placental edema and congestion, some cases may exhibit overt placental necrosis. Fetal lesions may also be subtle but when more profound, characteristic lesions include multifocal petechial hemorrhages and necrosis throughout the lungs, liver, spleen, thymus, and lymph nodes, and fetal icterus. Histologically, foci of necrosis often contain epithelial cells with characteristic eosinophilic intranuclear herpesviral inclusions.
Over the past 10-year period of equine abortion submissions to the AHL, equine herpesvirus-1 was implicated as the cause for abortion in a total of 30 cases (Table 1). The annual incidence varied between 0 to 7 cases per year, with a peak in the 2020 and 2021 years and no detected cases in 2023. 27 cases were submitted as full diagnostic work-ups that included a combination of postmortem, histology, bacteriology, and PCR testing, while two cases were based on histological lesions and confirmatory immunohistochemical (IHC) staining for EHV-1. A single case was suspected based on characteristic postmortem lesions, but was not further confirmed by molecular testing or IHC. Both neuropathogenic and non-neuropathogenic EHV-1 strains were implicated, with non-neuropathogenic strains (16) outnumbering neuropathic strains (11).
Figure 1. Histological findings in an aborted fetus infected with non-neuropathogenic equine herpesvirus-1. Thymus with foci of acute fibrinous necrosis, lymphocyte karyorrhexis and acute interstitial hemorrhage. H&E stain.
Figure 2. Histological findings in an aborted fetus infected with non-neuropathogenic equine herpesvirus-1. A few bronchiolar epithelial cells contain an eosinophilic intranuclear viral inclusion body with marginated chromatin (arrows). H&E stain.
Figure 3. Histological findings in an aborted fetus infected with non-neuropathogenic equine herpesvirus-1. The liver contains few small foci of acute hepatocellular degeneration with loose infiltrates of macrophages, fewer lymphocytes, and rare neutrophils (arrow). H&E stain.
Table 1. Equine abortion submissions to the AHL over a 10-year period (Jan 2016- May 2026). 2017 and 2021 each contained a single case where EHV-1 abortion was diagnosed based on histology and IHC staining, and in 2019, a single case was diagnosed based on characteristic postmortem findings.
References
1. Soboll‐Hussey G, et al. Relationship between equine herpesvirus‐1 viremia and abortion or equine herpesvirus myeloencephalopathy in domesticated horses: A systematic review. J Vet Intern Med. 2023 Dec 9;38(3):1872–1891.
2. Pronost S, et al. Neuropathogenic and non-neuropathogenic variants of equine herpesvirus 1 in France. Vet Microbiol. 2010 Oct 26;145(3-4):329-33.