Definition of Orthopox

Orthopox: The family of viruses to which belongs vaccinia, the virus that causes smallpox.

Other members of the orthopox family of viruses capable of infecting humans include:

• Cowpox -- Usually contracted by milking infected cows. Causes ulcerative lesions (sometimes called "milkers nodules") on the hands of dairy workers. It protects against smallpox and was used by Edward Jenner for this purpose.

• Pseudocowpox -- occurs worldwide and is a disease primarily of cattle. In humans it causes non-ulcerating "milker's nodes."

• Molluscum contagiosum -- Minor warty bumps on the skin with a central indentation (an "umbilicus"). Transferred by direct contact, sometimes as a venereal disease.

• Monkeypox -- A rare smallpox-like disease, usually encountered in villages in central and west Africa. Transmitted by monkeys and other primates as well as by rodents.

• Camelpox -- A serious disease of African and Asian camels. The sequence of the camelpox virus genome is most closely related to that of the variola virus, the cause of smallpox.

• ORF -- A global occupational disease associated with handling sheep and goats afflicted with "scabby mouth". In humans it manifests as a single painless, papulo-vesicular lesion on the hand, forearm or face.

Poxviruses are very large rectangular viruses the size of small bacteria. They have a complex internal structure with a large double-stranded DNA genome enclosed within a "core" that is flanked by 2 "lateral bodies". The surface of the virus particle is covered with filamentous protein components, so that the particles have the appearance of a "ball of knitting wool". The entire particle is enclosed in an envelope derived from the host cell membranes. Most poxviruses are host-species specific, but vaccinia is a remarkable exception. True pox viruses are antigenically rather similar, so that infection by one elicits immune protection against the others.