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.
Also called: BKPyV, MCPyV
Quick facts
- Type
- Virus · Other DNA viruses
- Gram / shape
- Icosahedral
- Spread
- Primary infection in childhood
- Causes
- BK polyomavirus nephropathy · Ureteral stenosis
- Treatment
- BK nephropathy
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 →
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:
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
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.
Connections
← Back to the group: 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 (Viruses (MDPI), open access, 2025); Merkel Cell Carcinoma Treatment (PDQ®)–Health Professional Version (National Cancer Institute (NCI), 2025); Merkel Cell Carcinoma Treatment (PDQ®)–Patient Version (National Cancer Institute (NCI), 2025).
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Representative model, for educational purposes only.