Circovirus

Circoviruses are small, non-enveloped, icosahedral viruses with circular single-stranded DNA genomes approximately 1.7 to 2.0 kilobases in length. Their genomes encode a replication-associated protein (Rep) and a capsid protein (Cap), with some members possessing additional small open reading frames. Replication occurs in the nucleus by a rolling-circle mechanism and relies heavily on host DNA polymerases due to the limited set of viral genes.

Taxonomy and hosts: Circoviridae contains two genera, Circovirus and Gyrovirus. Members of the genus Circovirus include beak and feather disease virus (BFDV) in parrots and various porcine circoviruses (PCV1–PCV4). Gyrovirus contains the chicken anemia virus (CAV). circoviruses primarily infect birds and pigs, with a range of disease associations depending on the genus and specific virus.

Clinical significance: Porcine circovirus type 2 (PCV2) is the major pathogen linked to porcine circovirus diseases such as postweaning multisystemic wasting syndrome (PMWS) and porcine dermatitis and nephropathy syndrome (PDNS); many PCV2 infections are subclinical. Avian circoviruses, notably BFDV, cause beak and feather disease and can result in immunosuppression and growth impairment. Newly identified porcine circoviruses (PCV3, PCV4) have been associated with various syndromes, including reproductive and respiratory signs, but their causal roles are still being investigated.

Transmission and pathogenesis: Virus is shed in feces and secretions, spreading via horizontal contact and, in some cases, vertical routes. Disease typically arises with co-factors such as stress, coinfections, or inadequate immunity, given the generally subclinical nature of many infections.

Diagnosis and control: Diagnosis relies on PCR-based detection, serology, and histopathology. Prevention emphasizes biosecurity and vaccination; PCV2 vaccines are widely used and have reduced disease incidence, while vaccines for PCV3/PCV4 or avian circoviruses are not broadly available.