cycloviruses

Cycloviruses are a genus of small, nonenveloped, icosahedral DNA viruses in the family Circoviridae. They possess circular single-stranded DNA genomes and are typically detected in a wide range of animal hosts as well as in environmental samples. Virions are among the smallest known circular ssDNA viruses, and the genomes are highly compact, generally encoding two major open reading frames for replication-associated protein (Rep) and capsid protein (Cap).

Genome organization is characteristic of circoviruses, with a compact, circular genome around 1.7 to 2.9 kilobases. The two primary coding regions encode Rep, essential for rolling-circle replication, and Cap, which forms the virion shell. Replicative origin regions commonly contain conserved noncoding motifs and stem-loop structures. Cycloviruses replicate by rolling-circle replication and rely on host cellular machinery to complete genome synthesis.

Taxonomy and diversity: Cycloviruses comprise multiple species described from diverse hosts, including mammals, birds, and invertebrates, as well as from environmental sources such as sewage and water. The genus is defined by sequence relationships, particularly in the Rep gene, and by genome organization consistent with Circoviridae. New cyclovirus genomes are regularly reported through metagenomic surveys, highlighting substantial genetic diversity and a broad ecological footprint.

Ecology and disease: The detection of cycloviruses across many hosts does not consistently associate with specific disease. For many isolates, pathogenic roles remain unproven or unclear, and they are often identified incidentally in virome studies. Ongoing research seeks to clarify host range, transmission dynamics, and any potential links to illness.

Diagnostics and research: Detection typically relies on PCR assays targeting Rep or Cap genes or on genome sequencing. Serology is less common. Cycloviruses continue to be of interest for understanding circovirus diversity, evolution, and potential cross-species transmission.