JPC SYSTEMIC PATHOLOGY

REPRODUCTIVE SYSTEM

January 2025

R-V01

 

Signalment (JPC #1761490): Equine fetus 

 

HISTORY: This 5-day-old male Quarter horse became depressed, weak, and stopped nursing by 36 hours of age. His condition progressively worsened and he presented to the veterinarian at 48 hours of age. Bloodwork revealed severe leukopenia, azotemia, hypoproteinemia, severe dehydration, hypoxemia, hypercapnia, and acidosis, then he developed thrombocytopenia. His condition continued to worsen despite medical therapy, he broke with severe diarrhea, and then died.

 

HISTOPATHOLOGIC DESCRIPTION: Lung: The pleura and interlobular septa are multifocally and moderately expanded by acute hemorrhage and edema. Within these areas, and scattered throughout the remainder of the section, small vessel walls are expanded mildly by extravasated protein, erythrocytes, and small amounts of cellular debris (vasculitis). Multifocally, endothelial nuclei rarely are expanded by a single, 2-4uµ, eosinophilic viral inclusion that peripheralizes the chromatin. Diffusely, alveolar septa are expanded by congestion, edema, hypertrophic endothelium, few macrophages, few degenerate neutrophils, and occasional cellular debris (necrosis). Alveolar lumina contain low numbers of alveolar macrophages, degenerate neutrophils, and small to moderate amounts of fibrin. Bronchiolar epithelium is autolytic, sloughed into the lumen in sheets.

 

Liver: Multifocally, there are random small foci of hepatocellular necrosis and loss that are infiltrated by low numbers of histiocytes and admixed with moderate hemorrhage replacing disrupted hepatic sinusoids. Within these areas, hepatocytes are shrunken and fragmented with hypereosinophilic cytoplasm and karyorrhexis (necrosis). Rarely, there are multinucleated hepatocytes (viral syncytia). Within and at the periphery of necrotic, hepatocytes occasionally contain a single, 2-4µm, eosinophilic, intranuclear viral inclusion that peripheralizes the chromatin. There are low numbers of periportal lymphocytes. There is mild periportal and capsular edema.

 

Adrenal gland: Within the cortex, predominantly within the zona reticularis and zona fasciculata, there are multifocal to coalescing areas of lytic necrosis characterized by loss of architecture with replacement by cellular and karyorrhectic debris admixed with abundant hemorrhage, fibrin, edema, and low numbers of degenerate neutrophils. In these foci, cortical cells are shrunken with hypereosinophilic cytoplasm and pyknotic or karyorrhectic nuclei. Scattered throughout and at the periphery of necrotic foci, cortical cells contain a single intranuclear, 2-4µm, eosinophilic viral inclusion that peripheralizes the chromatin. There are rare multinucleated viral syncytia with intranuclear viral inclusions as previously described.

 

MORPHOLOGIC DIAGNOSIS: 1. Adrenal gland, cortex: Adrenalitis, necrohemorrhagic, multifocal to coalescing, severe, with intranuclear viral inclusion bodies and viral syncytia, Quarterhorse, equine.

2. Liver: Hepatitis, necrotizing, multifocal and random, moderate, with intranuclear viral inclusion bodies and viral syncytia.

3. Lung: Pneumonia, interstitial, necrotizing, diffuse, mild, with mild vasculitis and rare intranuclear viral inclusion bodies and viral syncytia.

 

ETIOLOGIC DIAGNOSIS: Herpesviral pneumonia, hepatitis, and adrenalitis

 

CAUSE: Equine Herpesvirus-1 (EHV-1)

 

CONDITION: Equine Viral Abortion

 

GENERAL DISCUSSION:

• Equine herpesvirus-1 (EHV-1), an Alphaherpesvirus, genus Varicellovirus, is a worldwide important cause of failure of pregnancy, late term abortions (6-11 month) in horses and generalized perinatal foal infections and death; this virus less frequently causes neurological and respiratory disease (see P-V10) in horses
• The virus is very common, widespread, and most horses are exposed by 1 year of age
• Two recognized subtypes based on restriction endonuclease cleavage of viral DNA; both cause respiratory disease, abortion, and neonatal disease; 
  • Subtype 1 is less common but more severe and more associated with abortion, and probably the only one associated with neurologic disease
    • Neurological disease is associated with vasculitis and affects adults
    • A single polymorphism in nucleotide 2254 of DNA polymerase gene (ORF30) is linked with neurological disease but horses infected with non-neuropathogenic strains can also develop neurologic disease
    • Mice models have shown other genes and factors can play a role in developing neurological disease
    • Can cause bronchopneumonia in juvenile or immunocompromised horse. Mild upper respiratory disease, rhinitis, is most common
  • Subtype 2 is more common, causes mild respiratory disease, and is infrequently associated with abortion
• Latency in established within neuronal cells (e.g, trigeminal ganglion) and lymphocytes 
• Secondary bacterial pulmonary infection, e.g. Streptococcus zooepidemicus or other Streptococcus spp, is common
• Equine herpesvirus-4 is less common, sporadic, and causes a similar disease to EHV-1
• Alphaherpesviruses are enveloped, icosahedral, 150 nm diameter, dsDNA viruses with intranuclear replication

 

PATHOGENESIS:

• Despite high frequency of exposure, the level of immunity is generally low (like other herpesviruses)
• Viral shedding occurs during pyrexia; shed in nasal secretions 2-10 days after infection; shedding can be prolonged (up to 3 weeks post infection)
• Fetal death does not occur until the onset of uncomplicated abortion 
• Inhalation or ingestion of infective nasal discharge or aborted fetal material, or via fomites à virus proliferates rapidly in nasal, pharyngeal, and tonsillar mucosa à spreads to local lymphoreticular tissue à infects mononuclear leukocytes (primarily T-lymphocytes) à viremia à endothelial infection in numerous sites including lungs, uterus, and CNS àvasculitis, thrombosis, edema, ischemia, infarction, necrosis
• Abortion occurs after endometrial arteriolar endothelial necrosis à thrombosis, edema, hemorrhage, infarction à separation of maternal and fetal layers of the placenta à virus from endometrium allowed into the placenta then to the fetus à fetal infection (fetal endothelium and cells of most organs)

 

TYPICAL CLINICAL FINDINGS:

• 95% of EHV-1 abortions occur in the last 3 months of gestation and has not produced abortion before 5 months of gestation
• No premonitory signs noted in mare (typically no respiratory signs in mares that abort), abortion is usually rapid and uncomplicated; fetus is aborted fresh or foal is born alive at or near term, and many die within the first few days of life from severe interstitial pneumonia and secondary bacteremia
• Respiratory disease: Affects young horses, uncomplicated rhinitis  
  • Mares: Inapparent or mild upper respiratory tract infection
• Neonatal foals: Dyspnea, increased respiratory rate, fever, congestion of mucous membranes, serous to mucopurulent nasal discharge, +/- coughing

 

TYPICAL GROSS FINDINGS:

• Aborted fetus may have characteristic and diagnostic lesions: 
  • Aborted in fresh state
  • Severe diffuse pulmonary edema is the most consistent gross lesion; heavy, rubbery lungs with rib impressions, pitting response to pressure, interlobular septal edema, and may have foci of necrosis; may have fibrin casts in the bronchi and trachea (characteristic)
  • Few to numerous, up to 5mm, white foci of random hepatic necrosis in 50% of aborted fetuses, classic lesion as with most herpesviruses 
  • Edema of subcutis and fascia, body cavity effusion
  • Petechial hemorrhage especially in upper respiratory mucosa
  • Thymus, spleen may be small with edema; loss of distinction between cortical medullary areas 
  • +/- Renal cortex multifocal necrosis
  • The placenta is normal
• Foals born alive with early death
  • Multifocal hepatic necrosis is not present

 

TYPICAL LIGHT MICROSCOPIC FINDINGS:

• Eosinophilic intranuclear viral inclusions in bronchiolar and alveolar epithelium, hepatocytes, and lymphoid reticular cells
• Aborted fetus:
  • Lungs: Pulmonary and septal edema with mononuclear inflammation (diffuse pneumonia); fibrinous alveolar exudate; hemorrhage; multifocal epithelial necrosis with intranuclear acidophilic viral inclusion bodies in bronchial and alveolar epithelium (specific) +/- syncytial cells
  • Liver: Foci of necrosis (less common than lung lesions, may be extremely small); low numbers of hepatocellular eosinophilic intranuclear inclusions (inconsistent finding, present in necrotic areas); edema, leukocytes in necrotic foci and portal triads; less commonly diffuse hepatitis without focal necrosis
  • Lymphoid tissues (splenic follicles, thymus, lymph nodes): Lymphocytolysis, germinal center/lymphoid necrosis, focal hemorrhage; loss of distinction between cortex and medulla in thymic lobules due to cortical cell loss (loss of T-cell) and increased medullary area cellularity due to due to adipocyte infiltrate; eosinophilic intranuclear viral inclusions may be present in these foci and in the thymic medullary epithelium
  • Multiorgan microscopic foci of necrosis +/- syncytial cells
  • The placenta is normal
• Foals born alive with early death: Liver lesions are not present, but focal necrosis of intestinal crypt epithelium with hemorrhage is sometimes present; severe interstitial pneumonia and secondary bacteremia.

 

ULTRASTRUCTURAL FINDINGS: 

 

ADDITIONAL DIAGNOSTIC TESTS:

• Immunofluorescent antibody, virus isolation from cell cultures of nasal epithelium, lung, liver, spleen or thymus
• ELISA (differentiates EHV-1 from EHV-4)
• PCR and monoclonal antibodies used to differentiate between subtype 1 and 2
• Serology is of limited use; nearly 100% of horses have seroconverted to both EHV-1 and EHV-4 and abortion can happen long after exposure 

 

DIFFERENTIAL DIAGNOSIS:

Equine Abortion: 

Viral:

Bacterial:

• Most common: Streptococcus sp., Escherichia coli, Pseudomondas sp., Klebsiella sp., and Staphylococcus sp. 
• Streptococcus zooepidemicus: Any gestational stage; fibrinonecrotic placentitis around cervical star; fetal autolysis; variable degrees of inflammation in fetal tissues
• Leptospira sp. (L. pomona most common): 3.5-11 months gestation; fetal interstitial nephritis
• Nocardioform actinomycete: Late gestation abortion, still birth, and premature birth; focally extensive, well-demarcated, necrotizing placentitis involving base of horns; brown mucoid exudate; filamentous bacteria along chorionic surface; also implicated in cystic adenomatous hyperplasia of the allantois (see R-N11)
• Salmonella sp. (S. Typhimurium most common; S. Abortus-equi rarely): Late gestation; maternal septicemia; fibrinonecrotic placentitis; variable degrees of inflammation in fetal tissues

Fungal: Late gestation

• Aspergillus fumigatus & Mucor sp.: Chronic necrotizing placentitis especially around cervical star
• Candida sp.: Diffuse necrotizing and proliferative placentitis
• Histoplasma capsulatum: Multifocal granulomatous placentitis

Non-infectious:

• Twinning: Most common non-infectious cause of abortion; mid-gestation
• Torsion of umbilical cord: Excessively long, allowing torsion in utero
• Premature placental separation ("Red Bag")
• Uterine body pregnancy
• Fetal congenital abnormalities

Unknown:

• Mare Reproductive Loss Syndrome: Early and late gestation; often premature placental separation; associated with presence of eastern tent caterpillars (ETC) in the spring, fibrinous pericarditis, and severe unilateral uveitis
  • Two theories for reproductive loss: 1: an unidentified toxin related to the ETC; 2: mechanical trauma from the ETC setae allow secondary bacterial infection 

 

Neonatal Equine Pneumonia: 

Bacterial:

• Streptococcus zooepidemicus: Most common cause of bacterial pneumonia in foals and a common cause of pleuropneumonia in older horses; fibrinous bronchopneumonia 
• Rhodococcus equi: Pyogranulomatous pneumonia
• Embolic pneumonia: Streptococcus equi, Actinobacillus equuli, Pasteurella multocida, Bordatella bronchiseptica, E. coli, Bacteroides sp., Salmonella sp., and Chlamydia psittaci 

Viral:

• Equine influenza virus (Orthomyxovirus): Mild self-limiting bronchointerstitial pneumonia 
• Rhinovirus (Picornavirus): Typically affects upper respiratory system not lungs 
• Equine adenovirus: Necrotizing pneumonia; smudgy basophilic intranuclear inclusions in bronchial, bronchiolar, and alveolar epithelial cells; usually in SCID Arabian foals 
• Equine Viral Arteritis (Arterivirus): Interstitial pneumonia with arteritis and edema

Fungal:

 

COMPARATIVE PATHOLOGY:

• EHV-1: 
  • A cause of fatal meningoencephalitis with neuronal necrosis in non-equid species like black bears and Thomas gazelles (Mesquita, Vet Pathol, 2021)
  • Neurotropic EHV-1 strains in mice and hamsters include Ab4,A4/72, and A9/92 (Mequita, Vet Pathol, 2021)
  • C57BL/6J mice stain has been used to study EHV-1 infection, disease is primarily respiratory with few EHV-1 strain causing neurological disease (Mesquite, 2021)
  • Papular, crusted, and ulcerative dermatitis of the mucocutaneous junctions including the tongue, perilabial region, nostrils, penis, and prepuce have been associated with EHV-1 in 1 horse (Peters, Vet Pathol 2023)
• Zebra-born EHV-1: Isolated from a Grevy’s zebra fetus, a Persian onager fetus, Thomson’s gazelles, polar bears, black bears, guinea pigs, and an Indian rhinoceros in association with neurologic disease and abortion; suspect to be a subtype of EHV-1 (Saleh, J Comp Pathol, 2020)
• EHV-5: Gammaherpesvirus, causes multinodular pulmonary fibrosis in horses
  • 5-fold higher prevalence in horses with lymphoma compared to healthy horses; case associated with colonic T-cell–rich, large B-cell lymphoma
  • EHV-5 is commonly found in horses with lymphoma, equine multinodular pulmonary fibrosis (EMPF), and interface dermatitis (Acevedo, J Vet Diagn Invest 2023; Peters, Vet Pathol 2023)
• EHV-9: The most recently recognized alphaherpesvirus, closely related to EHV-1
  • Implicated in fatal neurological disease in non-equid species in zoos
  • EHV-9 has earlier and more severe neurological and respiratory disease and increased ORF-30 expression compared to EHV-1 (strain Ab4p) and zebra-born EHV-1 experimentally infected hamsters (Saleh, J Comp Pathol, 2020)
  • Zebras are considered the natural host, characteristic lesion is fatal encephalitis; associated with an outbreak of epizootic encephalitis in Thomson’s gazelles
  • Experimentally found to cause fulminant encephalitis and pneumonitis in wide range of species including hamsters, dogs, cats and cattle (Saleh. J Comp Pathol, 2020)

 

Other Significant Alphaherpesviruses:

• Gallid herpesvirus 1 – Avian infectious laryngotracheitis
• Gallid herpesvirus 2 – Marek's disease (used to be a gamma, lymphoproliferative)
• Anatid herpesvirus 1 – Duck virus enteritis/duck plague

• Psittacid herpesvirus – Pacheco’s disease

• Bovine herpesvirus 1 – Infectious bovine rhinotracheitis & infectious pustular vulvovaginitis
  1. May also cause abortion (less common)
• Bovine herpesvirus 2 – Bovine mammillitis/pseudo-lumpy skin disease
• Bovine herpesvirus 5 – Bovine herpes encephalitis
• Porcine herpesvirus 1 – Pseudorabies/Aujeszky’s Disease
  1. May also cause inflammation in the adrenal gland with necrosis in intranuclear inclusion bodies
  2. May also cause abortion, stillbirth, reduced fertility; in cases of abortion may see chorionic villar necrosis and necrosis in the liver, adrenal gland and spleen of fetus with possible intranuclear inclusion bodies
• Equid herpesvirus 3 – Equine coital exanthema
• Equid herpesvirus 4 – Equine rhinopneumonitis
• Equid herpesvirus 9 – Gazelle herpesvirus (original name)
• Asinine herpesvirus 3 & 4 – Donkey pneumonia
• Canine herpesvirus 1
  1. May also cause abortions
• Feline herpesvirus 1 – Feline viral rhinotracheitis 
• Cercopithecine herpesvirus 1 – B virus of macaques; lethal in humans
• Saimirine herpesvirus 1 – Herpes tamarinus/Herpes T of squirrel monkeys; lethal in owl monkeys
• Human herpesvirus 1 & 2 – Herpes simplex; neurotropic; lethal in owl monkeys

 

REFERENCES:

1. Acevedo HD, Hassebroek AM, Leventhal HR, Duhamel GE, Carvallo FR. Colonic T-cell-rich, large B-cell lymphoma associated with equid herpesvirus 5 infection and secondary trans-colonic fistula in a horse. J Vet Diagn Invest. 2023;35(3):272-277.
2. Caswell JL, Williams KJ. The respiratory system. In: Maxie MG, ed. Jubb, Kennedy, and Palmers Pathology of Domestic Animals. Vol 2. 6th ed. St. Louis, MO: Saunders Elsevier; 2016:568. 
3. Durham AC, Boes KM. Bone Marrow, Blood Cells, and the Lymphoid/Lymphatic System. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 7th ed. St. Louis, MO: Elsevier; 2022:881-882.
4. Foster RA, Premanandan C. Female Reproductive System and Mammae. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 7th ed. St. Louis, MO: Elsevier; 2022:1291.
5. Mesquita LP, Costa RC, Mesquita LLR, et al. Pathogenesis of Equid Alphaherpesvirus 1 Infection in the Central Nervous System of Mice. Vet Pathol. 2021;58(6):1075-1085.
6. Miller AD, Porter, BF. Nervous System. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 7th ed. St. Louis, MO: Elsevier; 2022:961-962.
7. Peters-Kennedy J, Löhr CV, Cossic B, Glaser AL, Duhamel GE. Association of equine gammaherpesvirus-5 with facial lymphohistiocytic interface dermatitis in seven adult horses from the United States. Vet Pathol. 2023;60(6):888-897.
8. Saleh AG, El-Habashi N, Abd-Ellatieff HA, et al. Comparative Study of the Pathogenesis of Rhinopneumonitis Induced by Intranasal Inoculation of Hamsters with Equine Herpesvirus-9, Equine Herpesvirus-1 strain Ab4p and Zebra-borne Equine Herpesvirus-1. J Comp Pathol. 2020;180:35-45
9. Rosol TJ, Gröne A. Endocrine glands. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 3. 6th ed. St. Louis, MO: Elsevier; 2016:340.
10. Schlafer DH, Foster RA. Female genital system. In: Maxie MG, ed. Jubb Kennedy and Palmer’s Pathology of Domestic Animals. Vol 3. 6th ed. Philadelphia, PA: Elsevier Saunders; 2016:399, 435-437.
11. Valli VEO, Kiupel M, Bienzle D. Hematopoietic system. In: Maxie MG, ed. Jubb, Kennedy and Palmer’s Pathology of Domestic Animals. Vol 3. 6th ed. Philadelphia, PA: Elsevier Saunders; 2016:145-146, 182-183, 198.

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