JPC SYSTEMIC PATHOLOGY

HEMOLYMPHATIC SYSTEM

April 2024

H-V09 (NP)

 

Signalment (JPC #1810441): A 7-week-old male cocker spaniel

 

HISTORY: This dog had diarrhea and died.  

 

HISTOPATHOLOGIC DESCRIPTION: 

Spleen: There is diffuse, moderate lymphoid depletion within the white pulp. Periarteriolar lymphoid sheaths contain moderately decreased numbers of mature lymphocytes and scattered large lymphocytes that are separated by abundant eosinophilic fibrillar to finely granular material (collapsed sinusoids/reticuloendothelial matrix and fibrin). There is an increase in tingible-body macrophages that often contain phagocytized eosinophilic cellular and karyorrhectic debris, and there are few karyolytic lymphocytes (lymphocytolysis). Within the red and white pulp there are few megakaryocytes admixed with rare erythroid and myeloid precursors (extramedullary hematopoiesis).

 

Liver: Hepatic cords are diffusely mildly attenuated due to mild hepatocyte atrophy, resulting in widened sinusoids.

 

MORPHOLOGIC DIAGNOSIS: 1. Spleen: Lymphoid depletion, diffuse, moderate, with lymphocytolysis and stromal collapse, cocker spaniel, canine. 

 

2. Liver: Hepatocellular atrophy, mild.

 

ETIOLOGIC DIAGNOSIS: Parvoviral splenic lymphoid depletion 

 

CAUSE: Canine parvovirus-2 (CPV-2)  

 

GENERAL DISCUSSION:

• Common, highly contagious, often fatal; can infect all canids and causes three distinct age-related clinical syndromes
• Small (25 nm diameter), non-enveloped, icosahedral, single-stranded DNA virus
• Several antigenically different serotypes:
  • CPV-1: Minute virus of canines – Causes myocarditis, respiratory diseases, and enterocyte hyperplasia with eosinophilic to amphophilic intranuclear inclusion bodies
  • CPV-2: Subtypes CPV-2a, and CPV-2b – Causes parvoviral enteritis
• Most dogs and cats do not develop clinical disease; when clinical disease develops, severe enteric damage is major cause of death
• In utero infection with CPV may be infrequently associated with cerebellar hypoplasia in dogs (more common in cats with feline parvovirus)
• Maternal antibody interference at the time of immunization can cause vaccine breaks

 

PATHOGENESIS:  

• Parvoviral replication is dependent on host-cell DNA polymerases produced in the S phase of the cell cycle; requires actively dividing cells (radiomimetic)- thus greatest effects occur in tissues with a high mitotic rate (fetal, hematopoietic/ lymphoid tissues, intestinal crypts, cardiomyocytes in neonates)
• Oronasal exposure to contaminated feces (direct contact or fomites) > viral uptake by epithelium over tonsils and Peyer's patches > replication in nasopharyngeal lymphoid tissue (1-2 days) > dissemination of infected lymphoblasts to many tissues (3-4 days) > lymphocytolysis releases virus, causing viremia > neutralizing antibody appears in circulation, terminating viremia (5-7 days) > infection of gastrointestinal crypt epithelium and Peyer's patches or other GI epithelium (5-9 days)> virus enters intestinal epithelial cells via basolateral areas of the crypt enterocytes nearest Peyer’s patches and M-cells > enterocytes killed as virus is released > malabsorption-maldigestion diarrhea
  • In dogs, viral capsid proteins bind target host cell transferrin receptors to initiate receptor-mediated endocytosis (in the cat, capsid proteins bind neuraminic acid receptors and transferrin); these receptors determine which cells and which species are infected by parvovirus strains
• Occurrence and severity of gastrointestinal signs is determined by the extent of damage to epithelium in intestinal crypts; this is influenced by: 
  • Availability of virus (depends on rate of lymphocyte proliferation/lysis)  
  • Rate of cellular proliferation in intestinal crypts (lots of cells entering mitosis will support more virus)
• If the animal survives, and there are undamaged stem cells, mucosa can regenerate 
• In utero: Virus infects/replicates in placental trophoblasts > cytotrophoblasts & mesenchymal stroma of fetal placenta > fetal vascular system > dividing cells 
• Cytolysis of proliferating cells in the bone marrow causes myeloid and erythroid hypoplasia; megakaryocytes are the least sensitive to lysis
• Circulating neutropenia is due to failure of recruitment of neutrophils from the damaged marrow and increased peripheral consumption by the intestine 
  • Transient neutropenia (2-3 days) is more common in cats with panleukopenia
• Lymphopenia results directly from viral lymphocytolysis in all infected lymphoid tissues (e.g. spleen, lymph nodes, MALT, thymus) and indirectly through bone marrow depletion and is more common in dogs than neutropenia; if the animal survives, lymphocytes can be replenished in 2-5 days
• Myocarditis and myocardial fibrosis in dogs up to two years old has been associated with CPV-2 and remains an important cause of myocardial damage despite widespread vaccination

 

TYPICAL CLINICAL FINDINGS:

• Three forms of disease:
  • Generalized: Rare form in neonatal pups less than 2 weeks of age; necrosis of various organs, death occurs by 10 days of age
  • Cardiac form (lymphocytic myocarditis): In utero or in pups 2-8 weeks old; passive immunity usually strong enough during the period of actively replicating myocardial cells (first 15 days of life) to prevent heart manifestations, but there is occasional sudden death from cardiac arrhythmias; survivors usually die by 5 months of age from chronic myocardial fibrosis and resultant cardiac arrhythmias
  • Leukopenia/enteritis:  Common form in pups 8 weeks and older once passive immunity has waned
• Vomiting, anorexia, pyrexia, dehydration, lethargy, malodorous diarrhea (due to reduced functional absorptive surface in small intestine)
• Bone marrow hypocellularity (especially myeloid depletion)> leukopenia, with lymphopenia, hypoproteinemia & anemia (if hemorrhaging into gut)
• Myocardial and enteric forms rarely occur together
• Rarely associated with erythema multiforme (mucus membrane ulceration/vesicles)

 

TYPICAL GROSS FINDINGS:  

• Leukopenic/enteric form: 

• Segmental to diffuse necrohemorrhagic gastroenteritis with 
• Peyer's patch necrosis and hemorrhage

• Mucoid or bloody intestinal contents with fibrinous, congested mucosa
• Enlarged, congested and edematous mesenteric lymph nodes
  • Semi-liquid, yellow-gray bone marrow 
  • Thymic atrophy is consistently present in young animals
  • Cerebellar hypoplasia (rare)

• Myocardial form:
  • Pale streaks in myocardium, heart is pale and flabby
• Porencephaly and periventricular encephalitis in a puppy (Marenzoni, J Comp Pathol. 2019)

 

TYPICAL LIGHT MICROSCOPIC FINDINGS:  

• Lymphocytolysis in follicles and paracortical tissue in lymphoid organs; marked lymphoid depletion; necrosis of Peyer’s patches; follicular hyalinosis; prominent histiocytes; erythrophagocytosis
  • Prominent regenerative lymphoid hyperplasia at 7-8 days postinfection
  • Depletion of virtually all proliferating bone marrow elements in severe cases; during later phases, marked hyperplasia of stem cells 
• Severe necrohemorrhagic gastroenteritis; crypt necrosis; blunting; fusion
• Basophilic intranuclear inclusion bodies in GI epithelium are variably evident, especially in cells adjacent to Peyer’s patches; inclusions are most prevalent during the late incubation and early clinical period, prior to exfoliation or lysis of cells (may not be seen at the time an animal is submitted for necropsy)
• Lymphocytic and/or necrotizing myocarditis +/- intranuclear inclusions

 

ADDITIONAL DIAGNOSTIC TESTS:  

• Fecal ELISA antigen tests, serology
• In-situ hybridization, IFA, PCR, or IHC on dorsal side of tongue, pharynx, esophagus, small intestinal mucosa, bone marrow, or spleen
• PCR (differentiates between field and vaccine CPV-2 strains)

 

DIFFERENTIAL DIAGNOSIS:

• Lymphoid/Peyer’s patch necrosis in puppies and young dogs:
  • Canine coronavirus: Usually a self-limiting enteritis without the prominent epithelial and lymphoid necrosis; no crypt necrosis (affects superficial villous epithelial cells only); no leukopenia; rarely fatal
  • Canine gastrointestinal hemorrhage enteritis (HGE): Associated with Clostridium perfringens infection
  • Rotavirus: Affects villus tips, mild watery to mucoid diarrhea 
  • Radiation poisoning: Classically has similar lesions; differentiate by history
  • Canine distemper (Morbillivirus): Crypt necrosis and involution of GALT
• Hemorrhagic enteritis in puppies and young dogs:
  • Heavy metal toxicity; warfarin toxicity; infectious canine hepatitis (canine adenovirus type-1); shock gut (DIC)

 

COMPARATIVE PATHOLOGY:  

• Taiwanese pangolins infected with CPV-2 (Chang,Vet Pathol. 2021)
• Raccoons:

• Encephalitis in juvenile raccoon caused by CPV (Wunschmann, J Vet Diagn Invest. 2021)
• Parvoviral enteritis and concurrent salmonellosis as cause of sudden death in raccoons (Lin, J Vet Diagn Invest. 2021)
• Cerebellar hypoplasia and dysplasia in a juvenile raccoon (Wunschmann, J Vet Diagn Invest. 2020)

• Minute virus of canines (CPV-1; most closely related to bovine parvovirus):

• Sporadic cause of respiratory disease, mild enteritis, or myocarditis in 1-3 week old pups; enterocyte hyperplasia with intranuclear inclusions in villus epithelium, no crypt lesions

• Feline parvoviral infection (panleukopenia): 

• Early lesions: Lymphoid depletion and thymic involution (due to bone marrow lymphocytolysis)
• Later: Segmental necrohemorrhagic enteritis/serositis
• Infection during late prenatal life causes anomalies of the central nervous system > external granular layer (but not Purkinje cells) of cerebellum are dividing cells in perinatal kittens > cell lysis > cerebellar hypoplasia

• Mink parvovirus (Aleutian mink disease, P-V14, U-V05):  

• Immune complex glomerulonephritis, vasculitis, hypergammaglobulinemia, and nonsuppurative meningoencephalitis 
• Neonatal mink: Acute interstitial pneumonia with interstitial edema, hyaline membranes, and hypertrophy and hyperplasia of the type II pneumocytes with intranuclear inclusions
• In ferrets AMD is a more protracted disease (2 years or more) characterized by hypergammaglobulinemia and immune complex glomerulonephritis; prominent plasmacytic infiltrates in numerous organs especially renal interstitium, hepatic portal areas and splenic red pulp where a nearly pure population of plasma cells expands the red pulp; also posterior ataxia and paresis

• Porcine parvovirus (PPV)

• Usually subclinical in adults; co-infection of PPV & porcine circovirus 2 is an important cause of postweaning multisystemic wasting syndrome (PMWS) and has been linked with reproductive failure and the SMEDI syndrome (stillborn, mummification, embryonic death, infertility)
• In fetus, lesions are most commonly present in kidney, liver, brain, and placenta of older fetuses

• Rodent parvoviruses - Most cause subclinical infection

• Rats (4 serotypes): Rat virus (or Kilham's rat virus (RV)), Toolan’s H1 virus, rat parvovirus (RPV), and rat minute virus (RMV); Kilham’s rat virus is the most pathogenic and may be the only strain to cause clinical disease under natural conditions:
  • Lesions in adult rats: Lymph node congestion, scrotal/testicular/epididymal and cerebral/cerebellar hemorrhage, focal hepatocellular necrosis; intranuclear inclusions within hepatocytes, endothelium, biliary epithelium; no intestinal mucosal lesions
  • Lesions in infant rats: Cerebellar hypoplasia, hepatitis, jaundice
• Mice: Two types of parvovirus with similar pathogenesis and tropism as other species’ variants
  • Minute virus of mice (MVM): More pathogenic for hematopoietic tissues than MPV
    • Intranuclear inclusions in mononuclear cells of spleen and bone marrow
    • Can have cerebellar hypoplasia, limited duration
  • Mouse parvovirus (MPV): More common, tropism for T lymphocytes resulting in immune modulation/suppression, usually persistent infection but no clinical signs
• Hamster parvovirus: Epizootic in weanling/suckling hamsters, high mortality; dental and facial deformities; testicular and cerebellar hypoplasia 

• Goose parvovirus: Lethal disease of young goslings and Muscovy ducks; hepatitis, myocarditis; intranuclear inclusion bodies in liver, spleen, myocardium, thymus, thyroid gland, intestines 
• Bovine parvovirus: status as an enteric pathogen is unclear; Isolated from diarrheic calves, minimal gross lesions, rarely causes death and abortion; associated with cerebellar hypoplasia
• Asian small-clawed otters: CPV should be considered in otters with diffusely red small intestines containing red-to-brown malodorous watery ingesta, loss of enterocytes, and crypt epithelial cell necrosis. (Watanabe, J Vet Diag Invest. 2020)

 

REFERENCS:

1. Barthold SW, Griffey SM, Percy DH. Pathology of Laboratory Rodents and Rabbits. 4th ed. Ames, IA: Wiley-Blackwell; 2016:17-19, 122-124, 175-176. 
2. Chang YC, Lin ZY, Lin YX, Lin KH, Chan FT, Hsiao ST, Liao JW, Chiou HY. Canine Parvovirus Infections in Taiwanese Pangolins (Manis pentadactyla pentadactyla). Vet Pathol. 2021;58(4):743-750.
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; 2022:885.
4. Gal A, Castillo-Alcala F. Cardiovascular System, Pericardial Cavity, and Lymphatic Vessels. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 7th ed. St. Louis, MO: Elsevier; 2022:691-692.
5. Lin CM, Hause B, Gualtieri D, Robinson N. Parvoviral enteritis and salmonellosis in raccoons with sudden death. J Vet Diagn Invest. 2021;33(6):1172-1175
6. Marenzoni ML, Calò P, Foiani G, Tossici S, Passantino G, Decaro N, Mandara MT. Porencephaly and Periventricular Encephalitis in a 4-month-old Puppy: Detection of Canine Parvovirus Type 2 and Potential Role in Brain Lesions. J Comp Pathol. 2019;169:20-24.
7. Mauldin EA, Peters-Kennedy J. Integumentary system. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 1. 6th ed. St. Louis, MO: Elsevier; 2016:628.
8. Schlafer DH, Foster RA. Female genital system. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 3. 6th ed. St. Louis, MO: Elsevier; 2016:429-430.
9. Stanton JB, Zachary JF. Mechanisms of Microbial Infections. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 7th ed. St. Louis, MO: Elsevier; 2022:262-263. 
10. Uzal FA, Plattner BL, Hostetter JM. Alimentary system. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 2. 6th ed. St. Louis, MO: Elsevier; 2016:153-158.
11. Watanabe TTN, Dubovi EJ, Evans DE, et. al. Outbreak of canine parvovirus 2b and Clostridium difficile infection in Asian small-clawed otters. J Vet Diagn Invest. 2020;32(2):226-229.
12. Wünschmann A, Lopez-Astacio R, Armien AG, Parrish CR. Cerebellar hypoplasia and dysplasia in a juvenile raccoon with parvoviral infection. J Vet Diagn Invest. 2020;32(3):463-466.
13. Wünschmann A, Lopez-Astacio R, Armién AG, Reed L, Parrish CR. Parvovirus-induced encephalitis in a juvenile raccoon. J Vet Diagn Invest. 2021;33(1):140-143.

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