JPC SYSTEMIC PATHOLOGY
DIGESTIVE SYSTEM
October 2018
D-V02

Signalment (B10-10951):  Tissue from a puppy

HISTORY: Unknown

HISTOPATHOLOGIC DESCRIPTION:  Small intestine: Diffusely and circumferentially intestinal villi are fused and blunted, with a multifocal loss of mucosal architecture. In areas where mucosal architecture is still intact, the lamina propria is expanded up to 1x normal by increased clear space (mucosal edema). There are multifocal ulcerations of the mucosa, characterized by a loss of enterocytes and replacement by eosinophilic cellular and karyorrhectic debris (necrosis) admixed with fibrin, hemorrhage, edema, moderate numbers of macrophages, lymphocytes and plasma cells and fewer viable and degenerate neutrophils. This inflammatory infiltrate extends into the lamina propria and through the muscularis mucosa into the submucosa. Similar necrotic debris, hemorrhage, fibrin and inflammatory cells multifocally replace or widely separate normal crypts.  Remaining crypts often exhibit one of the following changes: single cell necrosis, characterized by shrunken, hypereosinophilic epithelial cells with pyknotic or karyorrhectic nuclei, or pseudocarcinomatous crypt hyperplasia characterized by piling up of disorganized enterocytes with cytoplasmic basophilia and large, crowded, vesiculate nuclei with prominent nucleoli and frequent mitoses, or crypts that are dilated with flattened attenuated epithelium. Scattered remaining crypt epithelial cells contain 4x6um polygonal eosinophilic to amphophilic intranuclear viral inclusion bodies that marginate the chromatin. Within Peyer’s patches, there is marked germinal center lymphoid depletion and remaining lymphocytes are hypereosinophilic with pyknotic, karyorrhectic or karyolytic nuclei (lymphocytolysis) and multifocal infiltration by macrophages and occasional degenerate neutrophils.  There is transmural congestion and edema.  Multifocally, few basophilic bacilli are adhered to the apical villar surface.

MORPHOLOGIC DIAGNOSIS:  Small intestine:  Enteritis, necrotizing, diffuse, severe, with villar blunting and fusion, crypt hyperplasia, lymphoid depletion with lymphocytolysis, and intranuclear viral inclusions, breed unspecified, canine.

CAUSE:  Canine parvovirus - 2 (CPV 2)

ETIOLOGIC DIAGNOSIS:  Parvoviral enteritis

GENERAL DISCUSSION:

• Canine parvovirus-2 (CPV-2) is a strain of feline panleukopenia virus (FPV) that infects dogs; other strains of this virus include feline panleukopenia virus and mink enteritis virus
  • Virus receptor binding determines host susceptibility to parvovirus (transferrin receptor)
• There are additional antigenic subtypes within CPV-2 which can infect cats as well as other carnivores such as raccoons and skunks.
• Canine parvovirus-1 (CPV-1) is a different viral entity, canine minute virus; causes myocardial lesions as well as some enteric lesions
• Extremely environmentally stable & resistant to most common disinfectants
• Parvovirus virions are small (25 nm diameter), non-enveloped and have icosahedral symmetry; the genome is composed of negative-sense, single-stranded DNA
• The majority of infected cats and dogs do not develop clinical disease
• In utero infection with CPV may be infrequently associated with cerebellar hypoplasia in dogs (more common in cats with feline parvovirus)

PATHOGENESIS:

• Parvoviral replication is dependent on host-cell DNA polymerases - predilection for tissue with high mitotic rate such as fetal tissues, hematopoietic (bone marrow), lymphoid tissues & intestinal crypts
• Oronasal exposure to > viral uptake by epithelium over tonsils and Peyer's patches via transferrin receptor > replication in draining 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)
• Severity of gastrointestinal disease determined by the severity of damage to crypt epithelium; influence by:
  • Availability of virus (depends on rate of lymphocyte proliferation/lysis)
  • Rate of cellular proliferation in crypts of Lieberkühn (many cells entering mitosis will support more virus)
• If the animal survives, mucosa can regenerate if there are undamaged stem 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 from viral lymphocytolysis in all infected lymphoid tissues and is more common in dogs than neutropenia; if the animal survives, lymphocytes can be replenished in 2-5 days

TYPICAL CLINICAL FINDINGS:

• Three forms of disease:
  • Generalized: This is a 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): Pups infected in the first 15 days of life, when myocardial cells are still replicating; only occurs with naïve bitches, as maternal antibody should cover this period
  • Leukopenia/enteritis: Common form in pups 15 days and older
• Vomiting, anorexia, pyrexia, dehydration, lethargy, malodorous diarrhea (due to reduced functional absorptive surface in small intestine) – signs usually start 5-7 days post infection
• Bone marrow hypocellularity (especially myeloid depletion)> leukopenia, with relative or absolute 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 hemorrhagic gastroenteritis with mucoid/bloody intestinal contents and fibrinous serosal exudates
  • Peyer's patch necrosis
  • Enlarged, congested, edematous mesenteric lymph nodes
  • Semiliquid, yellow-gray bone marrow
  • Thymic atrophy
  • Has been associated with cerebellar hypoplasia
• Myocardial form:
  • Pale streaks in myocardium, pale, flabby
  • Pulmonary edema with white frothy fluid in trachea and bronchi

TYPICAL LIGHT MICROSCOPIC FINDINGS:

• 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
• Necrosis and depletion of lymphoid tissues
• Bone marrow depletion
• Myocardial necrosis with intranuclear basophilic inclusion bodies in cardiomyocytes

DIAGNOSIS:

• Fecal ELISA antigen tests, fecal PCR, in-situ hybridization, serology,
• IFA, PCR, or IHC on dorsal side of tongue, pharynx, esophagus, small intestinal mucosa, bone marrow, or spleen

DIFFERENTIAL DIAGNOSIS:

• Canine enteric disease:
  • Coronavirus: Usually self-limiting enteritis, no prominent epithelial or lymphoid necrosis, no leukopenia, affects villus tips
  • Rotavirus: Affects villus tips, mild watery to mucoid diarrhea
  • Morbillivirus (canine distemper virus): Severe vomiting and diarrhea, intranuclear and intracytoplasmic inclusion bodies, severe leukopenia
  • Clostridium perfringens enterotoxemia
  • Intoxication (heavy metals, anticoagulant rodenticides)

COMPARATIVE PATHOLOGY:

• 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 panleukopenia virus: Intrauterine infection (late prenatal) results in cerebellar hypoplasia; in postnatal kittens and cats the virus causes panleukopenia and enteritis
• Porcine parvovirus: Usually subclinical in adults
  • A cause of SMEDI (Stillbirth/Mummification/Embryonic death/Infertility)
  • Co-infection of porcine parvovirus & porcine circovirus 2 may be related to development of postweaning multisystemic wasting syndrome (PMWS)
• Bovine parvovirus: Has been isolated from diarrheic calves, minimal gross lesions, rarely causes death, disease severity may be increased by concurrent infection
• Rodent parvoviruses : Most cause subclinical infection
• Mouse – 2 viruses within Parvoviridae
  • Minute virus of mice (MVM) – subclinical disease in immunocompetent animals
  • Mouse parvovirus (MPV) – predominant type in positive animals; subclinical disease in immunocompetent animals
  • Strain-specific clinical disease can develop
• Rat – 4 distinct serotypes
  • Rat virus (RV; Kilham’s rat virus) – most pathogenic; possibly the only strain that produces natural disease; no enteric disease; multifocal hemorrhage believed to be the result of endothelial cell damage and damage to megakaryocytes
  • H-1 virus (Toolan’s H-1 virus) – subclinical disease in immunocompetent animals
  • Rat parvovirus (RPV) – subclinical disease in immunocompetent animals
  • Rat minute virus (RMV) – subclinical disease in immunocompetent animals
  • Subclinical infection is common
• Hamster – susceptible to experimental infections by MVM, RV, H-1, and MPV; but also have their own parvovirus
  • Hamster parvovirus (HaPV) – epizootic in breeding colony of Syrian hamsters
  • Clinical signs: enamel hypoplasia, periodontitis, hemorrhage in the dental pulp, multifocal cerebral malacia, testicular hypoplasia, thrombosis and transmural hemorrhage in the small intestine
  • May be due to interspecies transmission of mouse parvovirus-3 (MPV-3)
• Rabbit – lapine parvovirus has been isolated; unclear role in enteritis complex
• Mink parvoviruses:
• Mink enteritis virus (MEV): Related to feline parvovirus; panleukopenia, enteritis
• Aleutian mink disease virus: Plasmacytosis, hypergammaglobulinemia, glomerulonephritis, arteritis, hepatitis, anemia, death

REFERENCES:

1. Barthold SW, Griffey SM, Percy DH. Pathology of Laboratory Rodents and Rabbits. 4th ed. Ames, IA: John Wiley & Sons, Inc.; 2016:17-19, 122-124, 175-176, 259.
2. Boes KM, Durham AC. Bone Marrow, Blood Cells, and the Lymphoid/Lymphatic System. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 6^th Elsevier Mosby; 2017:801.
3. Caswell JL, Williams KJ. Respiratory System. In: Maxie MG, ed. Jubb, Kennedy and Palmer’s Pathology of Domestic Animals. Vol 2. 6^th St. Louis, MO: Elsevier Ltd; 2016:527.
4. Ford J, McEndaffer L, Renshaw R, Molesan A, Kelly K. Parvovirus infection is associated with myocarditis and myocardial fibrosis in young dogs. Vet Pathol; 2017;54(6):964-971.
5. Miller LM, Gal A. Cardiovascular System and Lymphatic Vessels. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 6^th Elsevier Mosby; 2017:609.
6. Schlafer DH, Foster RA. Female Genital System. In: Maxie MG, ed. Jubb, Kennedy and Palmer’s Pathology of Domestic Animals. Vol 3. 6^th St. Louis, MO: Elsevier Ltd; 2016:429.
7. 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 Ltd; 2016:153-158.
8. Zachary JF. Mechanisms of Microbial Infections. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 6th ed. Elsevier Mosby; 2017:205-206, 216-217, 226-227.

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