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BK virus

Reviewers: Monika Roychowdhury, M.D., University of Minnesota Medical Center - Fairview (see Reviewers page)
Revised: 14 June 2011, last major update June 2011
Copyright: (c) 2003-2011, PathologyOutlines.com, Inc.


● Cystitis due to infection with human polyoma BK virus


● BK is a type of polyoma virus - others are JC virus and simian virus 40
● Each encodes a highly homologous form of a large T antigen, a transcriptional and replicational regulatory protein


● Associated with hemorrhagic cystitis in blood and marrow transplant recipients (Bone Marrow Transplant 2008;41:11)
● Also associated with ureteral stenosis and interstitial nephritis in renal transplant patients (Hum Pathol 2006;37:684)


● Hemorrhagic cystitis may be due to BK or JC viruses (J Med Virol 2010;82:138)

Clinical features

● Real time PCR can detect BK virus DNA in urine, although high BK viruria is not necessarily associated with kidney or bladder damage (New Microbiol 2007;30:275)
● Polyoma virus infection may be a risk factor for bladder carcinoma in immunocompetent patients (Diagn Cytopathol 2006;34:201)

Case reports

● 38 year old female transplant recipient with coexisting DeCoy and malignant cells in urine (Diagn Cytopathol 2010 Nov 9 [Epub ahead of print])


● Possibly intravenous cidofovir (Clin Infect Dis 2009;48:e86) or hyperbaric oxygen (Leuk Res 2009;33:556)

Cytology description

● Decoy cells are epithelial cells containing intranuclear viral inclusions in urine (Adv Exp Med Biol 2006;577:201)
● Decoy cells are easily identified in routine Papanicolaou stained urine cytology specimens, and can also be identified in unstained sediment by phase-contrast microscopy
● In Papanicolaou stained cytology smears, Decoy cells are comet-shaped cells with large basophilic, amorphous, homogenous, ground-glass like intranuclear inclusion bodies and a condensed rim of chromatin (Diagn Cytopathol 2010 Nov 9 [Epub ahead of print])

The intranuclear inclusion can have different phenotypes:
1. Classic: basophilic ground-glass inclusion surrounded by chromatin (most common)
2. Cytomegalovirus like intranuclear inclusion surrounded by a clear halo
3. Multinucleated DeCoy cells
4. Vesicular nuclei with clumped chromatin and nucleoli (Urine Cytology Findings of Polyomavirus Infections

● Urine cytology appears to be more sensitive than histology in detecting cells with polyoma virus (Am J Surg Pathol 2006;30:345)
● The negative predictive value of urine cytology is 99%
● Note: pancreatic enzymes can degrade decoy cells and granulocytes (Clin Transplant 2008;22:833)
● When DeCoy cells are detected in high numbers, i.e. >5 DC cells/10 high power field or >10DC cells per cytospin preparation, they have a positive predictive value to indicate BKV associated nephropathy of 27%

Cytology images

Decoy cells with enlarged nuclei and homogenized chromatin due to viral inclusions

Various types of Decoy cells

Renal tubular cells:
#1; #2; #3

Positive stains

● SV40

Electron microscopy description

● Densely packed viral particles with a diameter of approximately 40 nanometers

Electron microscopy images

Densely packed viral particles

End of Bladder > Cystitis > BK Virus

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