JPC SYSTEMIC PATHOLOGY
ENDOCRINE SYSTEM
January 2022
R-V01

Signalment (JPC #4048653):  5-day-old male Quarterhorse  

HISTORY:  This neonatal 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 moderately expanded by acute hemorrhage, fibrin, and edema.  Within these areas and scattered throughout the remainder of the section, the tunica media and tunica adventitia of small vessels are mildly expanded by extravasated protein, fibrin, erythrocytes, and small amounts of cellular and karyorrhectic debris (vasculitis).  Diffusely, endothelium is hypertrophic (reactive), and multifocally and rarely, endothelial nuclei are expanded by a single, 2-4um, eosinophilic viral inclusion that peripheralizes the chromatin.  Diffusely, alveolar septa are expanded 2-10 times 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 affecting 20% of the section there are random, 50-400um diameter foci of hepatocellular necrosis and hepatocellular and architectural loss with replacement by hemorrhage, fibrin, edema, and cellular and karyorrhectic debris (lytic necrosis), and infiltration by low numbers of histiocytes. In these foci, hepatocytes are shrunken and fragmented with hypereosinophilic cytoplasm and karyorrhexis (necrosis).  Rarely, there are multinucleated hepatocytes (viral syncytia).  Within and at the periphery of necrosis, hepatocytes occasionally contain a single, 2-4um, eosinophilic, intranuclear viral inclusion that peripheralizes the chromatin.  There are low numbers of periportal lymphocytes.  Less affected hepatocytes often contain few or a single discrete, clear, intracytoplasmic vacuole (lipid-type vacuolar change).  There is mild periportal and capsular edema.

Adrenal gland:  Affecting 40% of 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, 2-4um, 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, Quarterhorse, equine.

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

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

 

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 abortions in horses and generalized perinatal foal infections and death; this virus less frequently causes neurological and respiratory disease (see P-V10) in horses
• EHV-1 is widespread; 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; more severe respiratory disease, more associated with abortion and neonatal disease, and the only type to cause neurologic disease
  • Subtype 2 is more common; causes mild respiratory disease, infrequently associated with abortion
• There are also two distinct genotypes due to a point mutation that is a major factor in neuropathogenicity (G2254 genotype more neuropathic than A2254 genotype); however, development of neurologic disease is cofactoral and not fully understood
• Secondary bacterial pulmonary infection, e.g. Streptococcus zooepidemicus or other streptococci, is common
• Equine herpesvirus-4 is less common, sporadic, and causes a similar disease to EHV-1; one reference states that equine herpesvirus-3 (equine coital exanthema) can also rarely cause a similar disease
  • Viremia not typically established with EHV-4, thus non-respiratory disease is uncommon

PATHOGENESIS:

• Despite high frequency of exposure, the level of immunity is generally low (like other herpesviruses)
• Time from virus exposure (recrudescence of latent infection or reinfection) to abortion ranges from 9 days to several months; latency in T lymphocytes and/or trigeminal ganglia
• Fetal death occurs at the onset of prompt abortion
• Transmission via inhalation, ingestion of infective nasal discharge or aborted fetal material, or via fomites à virus proliferates rapidly in nasal, pharyngeal, and tonsillar mucosa à infects mononuclear leukocytes (primarily T-lymphocytes) à leukocyte-associated viremia à endothelial infection in numerous sites including lungs, uterus, and CNS (MHC class 1 is the entry receptor) àvasculitis, thrombosis, 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)
• EHV-1 has also been detected in semen of infected stallions, but venereal transmission has not been reported.

TYPICAL CLINICAL FINDINGS:

• 95% of EHV-1 abortions occur in the last 3 months of gestation and no earlier than 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

TYPICAL GROSS FINDINGS:

• Aborted fetus may have characteristic and diagnostic lesions:
  • Aborted in fresh state
  • Severe pulmonary edema is the most consistent gross lesion; heavy, rubbery lungs with rib impressions, pitting response to pressure, interlobular septal edema, and may have tan to white foci of necrosis (2-4mm); may have fibrin casts in the bronchi and rarely the trachea
  • Few to numerous foci of random hepatic necrosis (5mm) in 50% of aborted fetuses, classic lesion as with most herpesviruses
  • Edema of subcutis and fascia, amber body cavity effusion
  • Slight icterus
  • Meconium staining of the eponychia and amnion
  • Petechial hemorrhage, especially in upper respiratory mucosa
  • +/- Thymic atrophy and edema
  • +/- 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) and syncytial cells
  • Liver: Foci of necrosis (less common than lung lesions, may be extremely small); low numbers of hepatocellular eosinophilic intranuclear inclusions (inconsistent finding); edema, leukocytes in necrotic foci and portal triads; less commonly diffuse hepatitis without focal necrosis
  • Lymphoid tissues (splenic follicles, thymus, lymph nodes): Germinal center/lymphoid necrosis, focal hemorrhage; loss of distinction between cortex and medulla in thymic lobules due to cortical cell loss; eosinophilic intranuclear viral inclusions may be present in these foci including 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

ULTRASTRUCTURAL FINDINGS:

• Enveloped, 150 nm diameter virions with a 100 nm icosahedral nucelocapsid

ADDITIONAL DIAGNOSTIC TESTS:

• Immunofluorescent antibody, virus isolation
• ELISA (differentiates EHV-1 from EHV-4)
• PCR used to differentiate between EHV-1 subtypes 1 and 2
• Serology is of limited use; nearly 100% of horses have seroconverted to both EHV-1 and EHV-4

DIFFERENTIAL DIAGNOSIS:

Equine Abortion: 

Viral:

• Equine viral arteritis (Arterivirus): 5-10 months gestation; fetal autolysis, occasionally myocardial arteritis

Bacterial:

• Most common: Streptococcus , 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 (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 (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: Chronic necrotizing placentitis especially around cervical star
• Candida: 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, Bordetella bronchiseptica, E. coli, Bacteroides , 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:

• Pneumocystis carinii: Interstitial pneumonia, usually in immunocompromised foals; presence of carinii in silver stained sections

COMPARATIVE PATHOLOGY:

EHV-1 in other species:

• Isolated from numerous equid and ungulate species including Persian onagers, Damara zebra, Grant’s zebra, Burchell’s zebra, and Thomson’s gazelle;
• Guinea Pigs: if co-housed with EHV-1 infected equids, develop neurologic signs, abortion and stillbirth
• Camels (new and old world): if co-housed with EHV-1 infected equids, develop neurologic signs with edema, neuronal necrosis, and perivascular/meningeal lymphocytic inflammation; abortion has not been reported
• Polar Bears: fatal neurologic disease from EHV-1 and EHV-9 transmitted via fomite in captive situations (no equid contact)

Other Significant Alphaherpesviruses:

• Gallid herpesvirus 1 (P-V11) – Avian infectious laryngotracheitis
• Gallid herpesvirus 2 (H-V01, I-V13, N-V08) – Marek's disease (used to be a gamma, lymphoproliferative)
• Anatid herpesvirus 1 (D-V10) – Duck virus enteritis/duck plague
• Psittacid herpesvirus (D-V13) – Pacheco’s disease
• Bovine herpesvirus 1 (R-V02) – Infectious bovine rhinotracheitis, infectious pustular vulvovaginitis & infectious balanophosthitis; may also cause abortion (less common)
• Bovine herpesvirus 2 – Bovine mammillitis/pseudo-lumpy skin disease
• Bovine herpesvirus 5 – Bovine herpes encephalitis
• Leporid herpesvirus 4 – Acute death in rabbits; pulmonary edema/hemorrhage
• Suid herpesvirus 1 (N-V07, E-V01) – Pseudorabies/Aujeszky’s Disease
  • Reduced fertility in boars/sows; fetal resorption, mummification, abortion, and stillbirth in sows; and fatal meningoencephalitis in pigs, sheep, cattle, and dogs
  • Multifocal coagulative necrosis of chorionic vili and focal coagulative necrosis in many fetal organs (liver, adrenal, spleen) with intranuclear inclusions
• Equid herpesvirus 3 – Equine coital exanthema; rarely abortion
• Equid herpesvirus 4 – Equine rhinopneumonitis; less often abortion
• Equid herpesvirus 9 – Gazelle herpesvirus (original name)
  • The most recently recognized alphaherpesvirus, zebras are considered the natural host, characteristic lesion is fatal encephalitis; associated with an outbreak of epizootic encephalitis in Thomson’s gazelles; recent report of fatal infection in a Grant’s zebra (Moeller 2018); recent study on the effect of mouse strain on EHV-9 infection concluded that C3H, C57BL, DBA, BALB/c-nu/nu, and ICR were the most susceptible while BALB/c was less susceptible (El-Nahass 2017).
• Asinine herpesvirus 3 & 4 – Donkey pneumonia
• Canine herpesvirus 1 – May also cause abortions
• Feline herpesvirus 1 (P-V09) – Feline viral rhinotracheitis
• Macacine herpesvirus 1 (B virus) – B virus of macaques; lethal in humans and NWP
• Saimirine herpesvirus 1 (D-V14) – Herpes tamarinus/Herpes T of squirrel monkeys; lethal in owl monkeys
• Human herpesvirus 1 & 2 – Herpes simplex; lethal in owl monkeys

REFERENCES:

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