Pox diseases are acute viral diseases that affect many animals, including people and birds. Some poxviruses also cause zoonoses. Typically, lesions of the skin and mucosae are widespread and progress from macules to papules, vesicles, and pustules before encrusting and healing. Most lesions contain multiple intracytoplasmic inclusions, which represent sites of virus replication in infected cells. In some poxvirus infections, vesiculation is not clinically evident, but microvesicles can be seen on histologic examination and, in some, proliferative lesions are characteristic.
Infection is acquired either by inhalation or through the skin (eg, sheeppox). In certain instances (eg, fowlpox, swinepox), the virus is transmitted mechanically by biting arthropods. Infection may be followed by generalized lesions (eg, sheeppox) or remain localized (eg, pseudocowpox). Strains of poxvirus with reduced virulence are used to immunize against some infections, the classic example being the global eradication of smallpox in people by immunization with strains of live vaccinia virus.
Poxviruses can be classified according to their physicochemical and biologic properties. Immunologically, the viruses of smallpox, cowpox, monkeypox, etc, are closely related to vaccinia virus and are classified within the genus Orthopox. The avian poxviruses, the myxoma viruses, and some of the other poxviruses (eg, swinepox) are species-specific. The viruses of orf, pseudocowpox, and bovine papular stomatitis are parapoxviruses. It has been recognized that several orthopoxvirus infections of domestic animals and people, notably cowpox and monkeypox, are acquired from rodent reservoir hosts. Many of these rodent hosts have not been unequivocally identified. Thus, although the use of adjectives such as "cowpox" and "monkeypox" to describe these viruses may be epidemiologically inaccurate and misnomers, their retained use reflects both historical association and, until a better nomenclature evolves, pragmatism (see Cowpox Virus Infections in Cats and Other Species).
Poxvirus infections can be confirmed in the laboratory using several diagnostic techniques. The orthopoxviruses can usually be isolated in cell culture and by inoculation of embryonated eggs. Examination of clinical samples by negative-staining electron microscopy is frequently used to visualize virus particles. PCR is widely used to further characterize virus isolates.