JPC SYSTEMIC PATHOLOGY

DIGESTIVE SYSTEM

October 2024

D-V02

 

Signalment (JPC #3167236): Tissue from a 3 month old puppy

 

HISTORY: The dog had two episodes of vomiting and diarrhea and presented with weight loss, depression, and dehydration.

 

HISTOPATHOLOGIC DESCRIPTION: Small intestine: Diffusely and circumferentially intestinal villi are markedly blunted and fused, and there is multifocal loss of mucosal architecture. Overlying Peyer’s patches are multifocal to coalescing mucosal erosion characterized by a loss of enterocytes and lamina propria with replacement by eosinophilic cellular and karyorrhectic debris (necrosis) admixed with fibrin, hemorrhage, edema, moderate numbers of macrophages, lymphocytes, plasma cells, and fewer intact and necrotic neutrophils. This inflammatory infiltrate extends into the lamina propria and through the muscularis mucosa into the submucosa. In the superficial lamina propria in areas of erosion, there are numerous small caliber blood vessels, plump fibroblasts, and variably mature collagen (granulation tissue). Necrotic debris, hemorrhage, fibrin, edema, and inflammatory cells multifocally replace or widely separate normal crypts in less affected areas. Remaining crypts often exhibit one of the following changes: single cell death characterized by shrunken, hypereosinophilic epithelial cells with pyknotic or karyorrhectic nuclei; marked, often atypical crypt hyperplasia characterized by piling up of disorganized enterocytes with cytoplasmic basophilia and large, crowded, vesiculate nuclei with prominent nucleoli and frequent mitoses; or dilated crypts lined by attenuated epithelium. Scattered nuclei of remaining crypt epithelial cells contain 4 x 6 µm, ovoid to polygonal, eosinophilic to amphophilic, intranuclear viral inclusion bodies that marginate the chromatin. In areas where mucosal architecture is still intact, the lamina propria is expanded up to 2 times normal by increased clear space (edema). Within Peyer’s patches, there is marked germinal center lymphoid depletion and necrosis characterized by loss of germinal center lymphocytes with replacement by abundant eosinophilic cellular and scant karyorrhectic debris, 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, necrosis and 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 is the main binding protein on cell membrane)
  • Subtypes CPV-2a and CPV-2b involved in parvoviral enteritis 
• There are additional antigenic subtypes within CPV-2 which can infect cats as well as other carnivores such as raccoons, skunks, and minks 
• Canine parvovirus-1 (CPV-1, canine minute virus) is a different viral entity; 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

• When disease does develop, results in severe enteric lesions and can be fatal

• In utero infection with CPV may be infrequently associated with cerebellar hypoplasia in dogs (more common in cats with feline parvovirus aka feline panleukopenia virus (FPV)
• Genetic phylogeny:

• Protoparovirus

• Carnivore protoparvovirus 1

• Feline panleukopenia virus

• Blue fox parvovirus
• Canine parvovirus
• Mink enteritis virus
• Raccoon parvovirus

• Rodent protoparvovirus 1

• H-1 parvovirus
• Kilham rat virus

• Parvovirus H3
• Parvovirus R1

• Minute virus of mice
• Mouse parvovirus 1

 

PATHOGENESIS:

• Parvoviral replication is dependent on host-cell DNA polymerases - predilection for tissue with high mitotic rate such as fetal tissues, hematopoietic tissue (bone marrow), lymphoid tissues & intestinal crypts
• Cells must be in the S phase of the cell cycle (aka actively dividing cells = radiomimetic, similar to radiation effects)
• Oronasal exposure to à viral uptake by epithelium over tonsils and Peyer's patches via host transferrin receptor à replication in draining lymphoid tissue (1-2 days) à dissemination of infected lymphoblasts to many tissues (3-4 days) à lymphocytolysis releases virus, causing viremia à neutralizing antibody appears in circulation, terminating viremia (5-7 days) à 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; influenced 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
• 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:
  1. Generalized: Rare form in neonatal pups less than 2 weeks of age; necrosis of various organs, death occurs by 10 days of age
  2. Cardiac form (C-V01, lymphocytic myocarditis): Pups infected early in life (in utero or 2-8wks old), infection of proliferating cardiac myocytes with CPV-2 results in nonsuppurative myocarditis and sequelae of acute or chronic heart failure; generally not seen due to maternal antibodies present  
  3. Leukopenia/enteritis: Common form in pups 15 days and older (once passive immunity has disappeared)
• 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 (C-V01):

• Pale, flabby myocardium with pale streaks
• Pulmonary edema with white, frothy fluid in trachea and bronchi

 

TYPICAL LIGHT MICROSCOPIC FINDINGS:

• Severe necrohemorrhagic gastroenteritis; crypt necrosis; villous blunting and 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
• Serology
• In-situ hybridization, IFA, PCR, or IHC 
• Strand exchange amplification (SEA) has been used to detect CPV-2 and feline panleukopenia virus in fecal samples

 

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:

Feline panleukopenia virus-derived viruses

• Feline panleukopenia virus: Intrauterine infection (late gestation) results in cerebellar hypoplasia; in postnatal kittens and cats the virus causes panleukopenia and enteritis
• 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

• Mink: 

• Mink enteritis virus (MEV): Related to feline parvovirus; panleukopenia, enteritis

• Asian small-clawed otters (Watanabe, J Vet Diagn Invest. 2020)

• Canine parvovirus-2c has caused clinical disease in a captive population of Asian small-clawed otters
• Coinfection with CPV-2b and C. difficile in Asian small-clawed otters resulted in severe necrotizing enteritis

• Raccoon:
  • Infections with feline parvovirus, canine parvovirus, and mink enteritis virus have been documented in raccoons, including CPV-2, CPV-2a, CPV-2b, and CPV-2c
  • Results in lymphoid depletion, lymphoid necrosis, and necrohemorrhagic enteritis with sloughing of intestinal crypt epithelium and mucosal collapse
  • CPV-2a-like virus may be responsible for cerebellar hypoplasia and dysplasia (similar to feline panleukopenia viral infection in pregnant felids) (Wunschmann, J Vet Diagn Invest. 2020)
  • CPV-2a-like virus caused nonsuppurative encephalitis, cerebellar Purkinje cell loss, and poliomyelitis and demyelination of spinal cord in a juvenile raccoon (Wunschmann, J Vet Diagn Invest. 2021)
  • Coinfection with CPV-2a and Salmonella enterica subsp. enterica responsible for acute death in juvenile raccoons in Massachusetts (Lin, J Vet Diagn Invest, 2021)
• Wild canids:
  • CPV infection has been documented in the coyote, red fox, crab eating fox, dingo, maned wolf, and raccoon dog
  • Disease is variable, with some species (e.g. red fox) shedding virus in feces but showing no clinical signs
  • Blue fox parvovirus and raccoon dog parvovirus are similar to CPV and named for the hosts from which the virus was isolated
• Taiwanese pangolin (Chang, Vet Pathol. 2021):
  • Non-carnivore species infected with CPV-2c
  • Serosal redness, mesenteric lymph node enlargement, mucosal erythema, thinned mucosa
  • Necrotic enteritis, crypt necrosis, crypt hyperplasia, +/- intranuclear inclusion bodies
  • Detection with IHC or PCR possible

 

Other selected parvoviruses

• 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 parvoviruses
  • 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 (Bocaparvovirus) has been isolated; unclear role in enteritis complex 
• Mink amdoparvovirus:

• Amdoparvovirus is a sibling genus to Protoparvovirus, within the subfamily Parvovirinae
• Aleutian mink disease virus (U-V05, P-V14): Plasmacytosis, hypergammaglobulinemia, glomerulonephritis, arteritis, hepatitis, anemia, death

• Slow loris parvovirus:

• Recently sequenced from a single specimen
• Marked splenomegaly, mottled coloration of liver, pancreatic and mediastinal lymphadenomegaly
• Animal had persistent viremia and developed histiocytic sarcoma
• Suspected directed oncogenic relationship between viral infection and tumorigenesis, but further research needed

• Macaque parvovirus (Simian Parvovirus [SPV]):

• Affects captive cynomolgus, rhesus, and pig-tailed macaques
• Causes anemia in immunocompromised individuals, with intranuclear viral inclusions in erythroid precursors in bone marrow

• Goose parvovirus: Important and fatal disease young goslings and Muscovy ducks; hepatitis, myocarditis; intranuclear inclusion bodies in liver, spleen, myocardium, thymus, thyroid gland, intestines

• Not seen in the USA

 

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