[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Microbiology](https://durgical.com/en/microbiology/) › [Other DNA viruses](https://durgical.com/en/microbiology/groups/other-dna-viruses/) › BK and Merkel cell polyomaviruses

# BK and Merkel cell polyomaviruses

- Virus
- Non-enveloped (naked)
- Exam priority: medium

Two small DNA viruses most people catch as children. BK can damage a transplanted kidney or the bladder; Merkel cell virus can cause an aggressive skin cancer.

Updated Nov 13, 2026

Also called: BKPyV, MCPyV

## Quick facts

**Type**
Virus · [Other DNA viruses](https://durgical.com/en/microbiology/groups/other-dna-viruses/)

**Gram / shape**
Icosahedral

**Spread**
Primary infection in childhood

**Causes**
BK polyomavirus nephropathy · Ureteral stenosis

**Treatment**
BK nephropathy

On this page

- The basics Key points
- At a glance
- Morphology
- Transmission
- Memory aid
- What the colours mean

- Clinical Virulence factors
- Diseases it causes
- Clinical picture
- Diagnosis

- Treatment Treatment
- Prevention

- Study Test yourself

- Connections Connections

## Key points

- Polyomaviruses: naked, circular dsDNA; T antigens bind p53 and Rb
- BK: Bladder (haemorrhagic cystitis after stem cell transplant) and Kidney (nephropathy after kidney transplant)
- SV40 immunostain on biopsy separates BK nephropathy from rejection
- Treatment of BK nephropathy is reducing immunosuppression
- Merkel cell carcinoma: CK20-positive, mostly virus-driven, treated with checkpoint inhibitors when advanced

## At a glance

**Shape**
Icosahedral

**Genome**
Double-stranded, circular DNA

**Envelope**
Non-enveloped (naked)

**Family**
Polyomaviridae, genus Betapolyomavirus (BK, JC) and Alphapolyomavirus (Merkel cell)

**Size**
About 40 to 45 nm

## Morphology

- Naked icosahedral capsid with a small circular genome
- Early genes make the large and small T antigens, which bind host tumour suppressors (p53, Rb)
- Related to JC virus, the cause of progressive multifocal leukoencephalopathy

## Transmission

- Primary infection in childhood, probably respiratory or faecal-oral; most adults have antibodies
- BK stays latent in renal tubular and bladder epithelium; a donor kidney can bring the virus into the recipient
- Merkel cell virus is part of the normal skin virome and is shed from healthy skin

**Reservoir:** Humans

### Isolation precautions

What the care team adds to standard precautions.

- Standard

## Virulence factors

- BK reactivates when T-cell immunity is suppressed (transplant drugs), lysing tubular cells and causing interstitial nephritis
- In hematopoietic cell transplant recipients BK replicates in the bladder urothelium, causing haemorrhagic cystitis
- Merkel cell virus integrates clonally into the tumour genome; its T antigens are always switched on and drive growth
- About 70 to 80% of Merkel cell carcinomas in the United States are virus-driven; the rest are caused by UV damage

## Diseases it causes

- BK polyomavirus nephropathy in kidney transplant recipients, with risk of graft loss
- BK haemorrhagic cystitis after hematopoietic cell transplant
- Ureteral stenosis in kidney transplant recipients
- Merkel cell carcinoma, a fast-growing neuroendocrine skin cancer [Merkel cell carcinoma →](https://durgical.com/en/conditions/merkel-cell-carcinoma/)

## Clinical picture

- BK nephropathy: often silent, found as a rising creatinine in the first year after kidney transplant; it can mimic rejection
- BK haemorrhagic cystitis: haematuria, dysuria, urgency and suprapubic pain weeks after a stem cell transplant, sometimes with clots
- Merkel cell carcinoma: a firm, painless, red or violet nodule on sun-exposed skin of an older or immunosuppressed person, growing quickly

## Diagnosis

- Screening after kidney transplant with plasma BK PCR; sustained high viral loads predict nephropathy
- Urine PCR and urinary decoy cells (enlarged nuclei with inclusions) are sensitive but less specific
- Kidney biopsy with SV40 large T antigen immunostaining confirms nephropathy and separates it from rejection
- Merkel cell carcinoma: skin biopsy with cytokeratin 20 positivity, then staging with sentinel node biopsy and imaging

## Treatment

BK nephropathy: lowering immunosuppression is the cornerstone; no antiviral is proven, though cidofovir, leflunomide and immunoglobulin have been used. Merkel cell carcinoma: surgery and radiotherapy, with immune checkpoint inhibitors (anti-PD-1 or anti-PD-L1) first-line for advanced disease.

Drug class only; doses and regimens are set by the clinician from the current guideline.

In Pharmacology:

- [Cidofovir](https://durgical.com/en/pharmacology/cidofovir/)
- [Leflunomide](https://durgical.com/en/pharmacology/leflunomide/)
- [Immune checkpoint inhibitors](https://durgical.com/en/pharmacology/classes/immune-checkpoint-inhibitors/)
- [Pembrolizumab](https://durgical.com/en/pharmacology/pembrolizumab/)

## Prevention

- Regular plasma BK PCR screening after kidney transplant to adjust immunosuppression early
- Sun protection and skin checks in older and immunosuppressed people
- No vaccine exists

## Memory aid

"B-K = Bladder and Kidney"; and Merkel's virus "marks" the skin of the old and the immunosuppressed with a fast red-violet lump.

## Test yourself

Think of the answer, then tap the question to see it.

## A kidney transplant recipient has a rising creatinine and high plasma BK viral load. What is the main treatment?

Reducing immunosuppression; no antiviral has proven benefit.

## Which stain on a kidney biopsy confirms BK nephropathy?

Immunostaining for SV40 large T antigen, which cross-reacts with BK virus.

## Which patient typically gets BK haemorrhagic cystitis?

A hematopoietic cell (stem cell) transplant recipient, usually in the weeks after transplant.

## How does Merkel cell polyomavirus cause cancer?

It integrates clonally into the cell genome and constantly expresses T antigens that inactivate tumour suppressors.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/microbiology/bk-and-merkel-cell-polyomaviruses/)

## What the colours mean

**Gram-positive**
Thick peptidoglycan wall: stains purple.

**Gram-negative**
Thin wall with an outer membrane: stains pink.

**Acid-fast**
Lipid-rich wall (mycobacteria): Ziehl–Neelsen stain.

## Connections

## Related 4

- [JC polyomavirus](https://durgical.com/en/microbiology/jc-polyomavirus/)
- [Human papillomavirus](https://durgical.com/en/microbiology/human-papillomavirus/)
- [Cytomegalovirus](https://durgical.com/en/microbiology/cytomegalovirus/)
- [Adenovirus](https://durgical.com/en/microbiology/adenovirus/)

## Related conditions 1

- [Merkel cell carcinoma](https://durgical.com/en/conditions/merkel-cell-carcinoma/)

[← Back to the group: Other DNA viruses](https://durgical.com/en/microbiology/groups/other-dna-viruses/)

Educational content on microorganisms; not a treatment guide and no substitute for a health professional's advice. No doses are given.

Sources: [BK Polyomavirus-Associated Nephropathy and Hemorrhagic Cystitis in Transplant Recipients—What We Understand and What Remains Unclear](https://pmc.ncbi.nlm.nih.gov/articles/PMC12474149/) (Viruses (MDPI), open access, 2025); [Merkel Cell Carcinoma Treatment (PDQ®)–Health Professional Version](https://www.cancer.gov/types/skin/hp/merkel-cell-treatment-pdq) (National Cancer Institute (NCI), 2025); [Merkel Cell Carcinoma Treatment (PDQ®)–Patient Version](https://www.cancer.gov/types/skin/patient/merkel-cell-treatment-pdq) (National Cancer Institute (NCI), 2025).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/microbiology/bk-and-merkel-cell-polyomaviruses/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/microbiology/bk-and-merkel-cell-polyomaviruses/