Poxvirus infections can affect many animals, including mammals, birds, reptiles, fish, and insects. Some poxviruses are zoonotic. In vertebrates, they often cause lesions of the skin and mucosae, which typically progress from macules to papules, vesicles, and pustules before encrusting and healing; however, respiratory and systemic disease is also common.
Histopathological lesions include virus-induced hyperplasia, hypertrophy and vesiculation, and intracytoplasmic inclusions, which represent sites of virus replication and accumulation in infected cells.
Poxviral infection is usually acquired either by inhalation or through the skin, or by both means (eg, sheeppox). In certain instances (eg, fowlpox, swinepox, myxomatosis), the virus is transmitted mechanically by biting arthropods.
Infection can be followed by generalized lesions (eg, sheeppox, cowpox) or remain localized (eg, pseudocowpox).
Poxviruses with lower virulence are used to immunize animals against some infections. The classic example is the global eradication of smallpox in humans by immunization with live vaccinia virus.
Poxviruses have large, double-stranded DNA genomes, comprising core genes involved in replication (in the cytoplasm) and a diversity of other genes responsible for host range and pathogenesis. These non–core genes often encode immunomodulatory proteins that protect infected cells from the host’s immune response, as well as growth factors that enhance viral replication but also cause characteristic hyperplastic and hypertrophic lesions in hosts.
Poxviruses are classified into genera according to their genome sequences; within each virus genus, the genomes, and therefore antigens, are very similar. Therefore, the viruses of smallpox, cowpox, and mpox (monkeypox) are in the genus Orthopoxvirus, along with vaccinia virus (the type species of the genus).
Because all orthopoxviruses share genetic similarities, vaccines against one virus can be used to provide cross-protection against other members of the genus. For example, smallpox and mpox vaccines are based on the vaccinia virus, cowpox virus was the basis for the original smallpox vaccine, and smallpox vaccines have shown effectiveness in preventing mpox infection.
The viruses that cause contagious ecthyma (orf), pseudocowpox, and bovine papular stomatitis form another genus (Parapoxvirus); the avian poxviruses are in the genus Avipoxvirus; and the viruses that cause sheeppox, goatpox, and lumpy skin disease in cattle are in the genus Capripoxvirus.
Some poxviruses have wide host ranges, and their names reflect historical associations rather than their reservoir hosts. For example, the mpox and cowpox viruses have rodent reservoir hosts.
Poxviruses can survive long periods—many years—in cool, dry conditions.
Orthopoxvirus infections can usually be isolated in cell culture, but other poxviruses can be more difficult to culture in the laboratory. Examination of clinical samples by negative-staining electron microscopy can be used to visualize virus particles; however, genus-specific PCR assay is increasingly the main diagnostic method, with additional PCR assay or genome sequencing used for further characterization of viruses to species level and for epidemiological studies.
