Papillomaviruses (HPV)

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. Be aware that JC and BK are the initials of the first patients from whom these viruses were first identified; modern authors have argued that the patients' full names should remain anonymous for privacy reasons. 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. We can see the viral inclusions within the nucleus. These cells are larger than the epithelial cells also present in the urine. 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; notice the viral inclusions visible within the renal tubule cell nuclei. – Hemorrhagic cystitis, which is characterized by hemorrhaging in the bladder. 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. 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. However, 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.