Polyomaviruses

• Formerly classified in the papovavirus family with papillomaviruses.
• Small, naked icosahedral capsids that resist inactivation.
• They have double-stranded, circular DNA.
• In permissive cells, they cause lytic infections; in non-permissive cells, they produce latent infections.
• JC polyomavirus (JCV) and BK polyomavirus (BKV) are common, but not usually problematic.

• First, indicate that the viruses likely enter the human host via the respiratory tract; it has been suggested that there may be additional portals of entry, as well.
• Once inside the host, replication of the viruses occurs, followed by primary viremia.
• Viremia delivers the virus to the kidneys, were replication continues.
• In immunocompetent individuals, latent infection is established.
• In immunosuppressed individuals, active infection occurs; for example, patients with T-cell suppression, including AIDS patients, transplant recipients, and some pregnant women.

• The virus can replicate throughout the urinary tract: the kidneys, ureters, and bladder.

Decoy cells* are present in the urine of a patients with active BKV infections. – Note the viral inclusions within the nucleus. – These cells are larger than the epithelial cells also present in the urine. Be aware that they are called "decoy cells" because they can be mistaken for cancerous cells.

• Active BKV infection can have serious consequences for immunosuppressed individuals.
• The following manifestations are associated with kidney and hematopoietic stem cell transplant recipients:

– Ureteric stenosis, in which infection causes narrowing of the ureters. – Nephropathy; in the image, highlight the viral inclusions visible within the renal tubule cell nuclei. – Hemorrhagic cystitis; in the images of the urinary bladder, indicate the visible hemorrhaging in both the histological sample and the gross anatomical picture.

• Viremia carries the virus to the central nervous system, which can lead to Progressive Multifocal Leukoencephalopathy (PML).
• Up to 10% of AIDS patients develop PML, which is a fatal demyelinating disease.
• In the histologic and brain scan images, we can see areas of demyelination.
• In the gross anatomical image, we can see an area of granularity.
• PML, as its name suggests, produces demyelinated areas throughout the brain; symptoms include impaired speech, vision, and coordination, followed by paralysis of the arms and legs, and, ultimately, death.

Infection is common and asymptomatic*

• Many adults shed the virus from their skin.

MCV infection can lead to Merkel Cell Carcinoma* – Elderly or other immunosuppressed individuals – Highly aggressive form of skin cancer with a high mortality rate – The head and neck are common sites of initial lesions because exposure to sunlight and UV radiation increases the risk of developing MCV. – In the image, we see a small reddish nodule on an elderly woman's ear; these small bumps are characteristic of early MCV lesions.