polyomaviruses

Polyomaviruses are a group of small, nonenveloped DNA viruses that belong to the family Polyomaviridae. They have icosahedral capsids and circular double-stranded genomes of about five to six kilobases. Members infect a wide range of hosts, including mammals, birds, and fishes, and are generally host-specific. Most infections are asymptomatic and acquired early in life, with the virus establishing lifelong latency in certain tissues.

The genome features early and late regions. The early region encodes regulatory proteins, notably the large T antigen and small t antigen, which drive viral replication and can influence cell cycle progression. The late region encodes the structural proteins VP1, VP2, and VP3 that form the viral capsid. Transcription and translation are regulated by alternative splicing and promoter activity, with viral gene expression largely coordinated by the host cell cycle and replication status.

Replication occurs in the nucleus of the infected cell and relies on host cellular machinery. Productive replication typically requires cells to enter or mimic S phase, often during periods of immune compromise. The virus then assembles in the nucleus and new virions are released, usually without causing immediate cytopathic effects in permissive tissues.

Clinical relevance varies by virus and host immune status. In humans, many infections remain latent and clinically silent. However, certain polyomaviruses can cause disease when immunity is reduced. JC polyomavirus is associated with progressive multifocal leukoencephalopathy (PML); BK polyomavirus can cause nephropathy and hemorrhagic cystitis in transplant recipients. Merkel cell polyomavirus is linked to a subset of Merkel cell carcinomas. Transmission routes are not completely defined but may include respiratory and oral pathways, with latency in kidney and lymphoid tissues.

Diagnostics typically use PCR to detect viral DNA in urine, blood, or cerebrospinal fluid, along with serology or tissue-based histology in specific contexts. There is no universally effective antiviral therapy; management focuses on restoring immune function and monitoring disease progression.