Orthopoxviruses

Orthopoxviruses are a genus of large, enveloped double‑stranded DNA viruses in the family Poxviridae, subfamily Chordopoxvirinae. The genus includes Variola virus (smallpox), Vaccinia virus, Cowpox virus, Monkeypox virus, Camelpox virus, and Horsepox virus. They share brick‑shaped virions and large genomes of about 170–190 kilobases encoding roughly 200 proteins. Unlike many DNA viruses, orthopoxviruses replicate in the cytoplasm and carry their own transcription machinery.

Structure and life cycle: Virions are complex, brick‑shaped particles that enter cells and form cytoplasmic viral factories. Gene expression occurs in early, intermediate, and late phases. After replication, intracellular mature virions acquire an external envelope to become extracellular enveloped virions, enabling spread. Release occurs by cell lysis or exocytosis. The natural host range is broad, including many animals; humans can be infected by several orthopoxviruses with varying transmission efficiency.

Disease and public health: Variola virus caused smallpox, eradicated in 1980 after vaccination campaigns. Other orthopoxviruses—monkeypox, cowpox, camelpox—cause zoonotic infections in humans with diverse severity. Vaccination with vaccinia‑based vaccines provides cross‑protection against the genus. Lab handling of variola is restricted to high‑containment facilities. Public health measures emphasize surveillance, vaccination, and post‑exposure prophylaxis where indicated.

Diagnosis and treatment: Diagnosis relies on PCR and sequencing of viral DNA from lesions or blood; serology supports diagnosis in some cases. Most infections are managed symptomatically. Antivirals such as tecovirimat (ST‑246) have activity against orthopoxviruses and are approved for smallpox and used for mpox under regulatory provisions; cidofovir and brincidofovir have been explored. Vaccination remains the principal preventive measure, especially for at‑risk populations and outbreak control.