Human circovirus HK (HCirV) is a small, non‑enveloped circular single‑stranded DNA virus belonging to the family Circoviridae. It represents a novel circovirus species recently identified in patients with hepatitis in Hong Kong.
Genome and biological characteristics
Circoviruses have compact genomes of about 1.8‑2.0 kb that encode two major proteins: a replication‑associated protein (Rep) needed for rolling‑circle replication and a capsid protein (Cap). Human circovirus HK shares this genomic organisation but is genetically distinct from animal circoviruses such as porcine circovirus 2 and avian circovirus. Viral DNA replicates in the nucleus of infected cells, utilising host DNA polymerases to produce circular replicative intermediates. HCirV virions are icosahedral and approximately 17 nm in diameter. Sequence analyses place HCirV within the genus Circovirus and show that it is most closely related to human circovirus 1 and 2 strains detected in France and China. The natural reservoir and transmission routes are unknown, but the detection of viral DNA in blood suggests a systemic infection. Cell binding assays indicate that HCirV virus‑like particles attach to human hepatoma cells, consistent with hepatic tropism. No antiviral drugs or vaccines exist for circoviruses in humans.
Clinical observations and emerging significance
A study of 278 patients with hepatitis in Hong Kong found HCirV viremia and sequences in eight patients, whereas none of 184 asymptomatic controls were positive. In two HCirV‑infected patients alternative causes of hepatitis were evident, but in six others – four of whom were immunocompromised – the virus could not be ruled out as a contributor to liver injury. Persistent infection was documented in three patients, and one patient presented with an infectious mononucleosis‑like syndrome. Additional reports from Europe have described hepatitis associated with human circovirus infection in immunocompromised transplant recipients, suggesting that host immune status may influence disease severity. Because circoviruses are known to infect a variety of animals, the zoonotic origin of HCirV is under investigation. Diagnostic detection relies on PCR and sequencing; the prevalence in the general population remains unclear. HCirV illustrates the ongoing discovery of previously unrecognised human viruses. While its pathogenic role is not fully established, clinicians should be aware of HCirV when evaluating unexplained hepatitis, especially in immunocompromised patients. Ongoing surveillance and research into its epidemiology and reservoirs are essential. Related Terms: Cyclovirus VS5700009, Porcine Circovirus 2, Hepatitis E Virus, Hepatitis D Virus, Deltavirus