Signalment:  

Third trimester fetus, male Arabian horse.The dam aborted the fetus approximately 2 months before the expected foaling date.

Gross Description:  

The liver is markedly friable, and the subcapsular surface is stippled yellow and red. Within the thymus is a moderate amount of white to yellow opaque material.

Histopathologic Description:

Scattered randomly throughout the liver are numerous small foci of coagulative necrosis infiltrated by numerous macrophages, lymphocytes, and plasma cells. Within hepatocytes adjacent to the areas of necrosis are variable numbers of large, eosinophilic, intranuclear inclusion bodies that marginate the chromatin to the periphery. Within the portal triads are numerous mononuclear cells consistent with hematopoietic precursor cells. Immunostaining for equine herpesvirus demonstrates marked positive immunoreactivity within inflammatory cells and necrotic cellular debris in the areas of hepatocellular necrosis.

Within the spleen, white pulp is markedly expanded by karyorrhectic cellular debris, and few intact lymphocytes remain.

The thymic medulla contains a moderate amount of necrotic debris and is infiltrated by numerous eosinophils. Rarely, thymic reticular cells contain a single, large, eosinophilic intranuclear inclusion body. Within the lung, alveolar septa contain variable amounts of necrotic cellular debris. (Thymus and lung were not submitted)

Morphologic Diagnosis:  

Liver: Severe, acute, random, necrotizing hepatitis with hepatocellular intranuclear inclusion bodies.
Spleen: Marked lymphoid necrosis.
Thymus (not submitted): Moderate to marked lymphoid necrosis.
Lung (not submitted): Mild, multifocal, necrotizing interstitial pneumonia.

Lab Results:  

Fluorescent antibody testing for equine herpesvirus-1on liver tissue was positive.
Additional ancillary test results were as follows:
bacterial culture of the stomach contents did not isolate organisms;
bacterial culture of the thymus isolated few Staphylococcus xylosus and Acinetobacter lwoffii, interpreted as contaminants;
serology for Leptospira spp. and equine viral arteritis virus was negative.

Condition:  

Equine herpesvirus 1

Contributor Comment:  

Herpesviral abortion occurs in horses, cattle, goats, pigs, dogs, and cats.(9) In horses, herpesviral abortion is most commonly attributed to equine herpesvirus-1, an alphaherpesvirus that causes abortion, neonatal disease, respiratory disease, or neurologic disease in horses.(7,9) Most horses are infected with equine herpesvirus as young animals, as the virus is widespread.(7) Equine herpesvirus 1 can cause abortion as a result of initial infection with the virus or as a result of recrudescence of latent virus within the dam.(7) Most equine herpesvirus abortions occur in the third trimester of pregnancy, as was the case in the submitted fetus.(5,7)

This fetus demonstrated the classic histologic lesions of equine herpesvirus abortion including multifocal necrotizing hepatitis, splenic and thymic necrosis, and interstitial pneumonia.(7) Adrenal gland necrosis is also reported to occur.(9) The florid hepatic necrosis with intranuclear inclusion bodies is somewhat atypical, as in most cases of equine herpesviral hepatitis, inclusion bodies are rare.(7) Inclusion bodies are most commonly identified in the bronchiolar and alveolar epithelium, reticular cells of the spleen and thymus, and, less often, in hepatocytes.

Gross examination of this fetus revealed a friable liver, and foci of hepatic necrosis were not seen grossly. In addition, white to yellow fluid seen grossly in the thymus likely corresponded to necrotic cellular debris identified microscopically. Additional gross lesions that may occur in equine herpesviral abortions include pulmonary edema, bronchiolar fibrin casts, splenic petechiation, and renal cortical hemorrhage.(7) These lesions were not identified in this fetus.

Upon infection with equine herpesvirus 1, primary viral infection occurs within the upper respiratory tract, resulting in viremia. Viremia enables spread of the virus to the gravid uterus with infection of uterine endothelial cells. In the uterus, the virus may induce vasculitis and infarction of microcotyledons with subsequent spread to the feto-placental unit.(5,9) Uterine endothelial cells are more susceptible to infection in late pregnancy, leading to the increased risk of abortion in the third trimester.(5)

Though this fetus was aborted, occasionally foals will be born infected with the virus. In these cases, foals typically die with an interstitial pneumonia, and they may develop focal necrosis of intestinal crypt epithelium with intranuclear inclusion bodies in crypt epithelium.(7)

In this case, histopathologic changes were highly suggestive of equine herpesvirus infection. Immunohistochemistry was performed for confirmation. In addition to histopathology and immunohistochemistry, other methods of equine herpesvirus 1 diagnosis include virus culture and isolation and polymerase chain reaction.(5,7)

JPC Diagnosis:  

1. Liver: Hepatitis, necrotizing, multifocal and random, moderate , wi t h lymphohistiocytic pericholangitis and hepatocellular, endothelial, and biliary viral syncytia and intranuclear inclusion bodies.
2. Spleen, white pulp: Lymphoid necrosis, diffuse, severe, with random red pulp necrosis, fibrin, and intrahistiocytic intranuclear viral inclusion bodies.

Conference Comment:  

Equine herpesvirus 1 (EHV1) proliferates rapidly in nasal, pharyngeal, and tonsillar mucosa, and infects primarily T-lymphocytes, resulting in viremia and subsequent endothelial infection in numerous sites including the lungs, uterus, and CNS. This results in vasculitis, thrombosis, and ischemic necrosis. With damage of maternal uterine endothelial cells following viral infection, there is thrombosis, inflammation, and perivascular edema which leads to infarction of the endometrium and separation of maternal and fetal placental layers. Viral infection of fetal endothelium and other cells in many organs leads to abortion or birth of weak foals that die soon after pneumonia. Infection in older foals, usually greater than 1 year-old, is typically a self-limiting upper respiratory disease.(1,2,3,4,5,7)

Viral replication occurs in the nucleus, and the viral envelope is acquired by budding through the inner layer of nuclear membrane. Positively charged residues in EHV1, glycoprotein B and glycoprotein C, bind to heparan sulfate moieties of host cells, and equine major histocompatibility complex (MHC) I acts as a functional receptor for glycoprotein D (gD), facilitating entry and infection of the cell.(6)

This case was atypical because portal hepatitis and edema of the capsule and the space of Disse were the prominent features instead of necrosis. Another unusual feature was the abundance of syncytial cells within hepatocytes, biliary epithelium, and endothelial cells.

Other causes of equine abortion include equine herpesvirus 4, which occurs sporadically and is considered a milder pathogen then EHV1 and which generally causes lesions only in the endometrium and not the fetus; equine viral arteritis, an Arterivirus that causes fetal autolysis and myocardial arteritis; Streptococcus zooepidemicus, which causes fetal autolysis and fibrinonecrotic placentitis around the cervical star; Leptospira sp., which also causes fetal interstitial nephritis; nocardioform abortion due to Crossiella equip occurring in late gestation, which causes necrotizing placentitis involving the base of the uterine horns; and Salmonella sp., most commonly S. typhimurium and rarely S. abortus-equi; causing maternal septicemia and fibrinonecrotic placentitis. Other bacteria such as E. coli, Streptococcus equi, Staphylococcus aureus, Pseudomonas aeruginosa, Actinobacillus equuli, and Klebsiella pneumoniae, as well as late gestational fungal infection by Aspergillus fumigates, Mucor sp., Candida sp., and Histoplasma capsulatum can cause abortion due to necrotizing placentitis.(7) Mare reproductive loss syndrome, associated with the eastern tent caterpillar (ETC) and possibly the result of non-beta-hemolytic Streptococcus and Actinobacillus spp. infection or a toxin related to the ETC, is another potential cause of equine abortion.(8)

References:

1. Caswell JL, Williams KJ. Respiratory system. In: Maxie MG, ed. Jubb, Kennedy, and Palmers Pathology of Domestic Animals. Vol. 2. 5th ed. Philadelphia, PA: Elsevier Saunders; 2007:640-4.
2. Del Piero F, Wilkins PA, Timoney PJ, et al. Fatal Nonneurological EHV-1 Infection in a Yearling Filly. Vet Pathol. 2001:38;474474.
3. Foster RA. Female reproductive system and mammary gland. In: McGavin MD, Zachery JF, eds. Pathologic Basis of Veterinary Disease. 5th ed. St. Louis, MO: Elsevier Mosby; 2012:1111.
4. Lopez A. Respiratory system, mediastinum, and pleurae. In: McGavin MD, Zachery JF, eds. Pathologic Basis of Veterinary Disease. 5th ed. St. Louis, MO: Elsevier Mosby; 2012:505-6.
5. Lunn DP, Davis-Poynter N, Flaminio MJBF, et al. Equine herpesvirus-1 consensus statement. J Vet Intern Med. 2009:23;250-61.
6. Sasaki M, Hasebe R, Makino Y. Equine major histocompatibility complex class I molecules act as entry receptors that bind to equine herpesvirus-1 glycoprotein D.Genes Cells. 2011;16(4):343-357.
7. Schlafer DH, Miller RB. Female genital system. In: Maxie MG, ed. Jubb, Kennedy, and Palmers Pathology of Domestic Animals. Vol. 3. 5th ed. St. Louis, MO: Elsevier Limited; 2007:495, 506-8, 522-3, 532-534.
8. Sebastian MM, et al. REVIEW paper: mare reproductive loss syndrome. Vet Pathol. 2008;45(5): 710-22.
9. Smith KC. Herpesviral abortion in domestic animals. Vet J. 1997;153:253-268.

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