Poxviruses
The poxviruses are among the largest and most complex of all viruses. They contain a linear dsDNA genome of 130 to 375kbp and replicate in the cytoplasm of the cell. The family
Poxviridae
contains the genera
Parapoxvirus
(Orf virus, pseudocowpox virus, bovine pustular stomatitis virus, seal parapoxvirus, and red deer parapoxvirus)
,
Orthopoxvirus
(varioloa virus, cowpox virus, monkeypox virus, vaccinia virus, and buffalopox virus),
Yatapoxvirus
(yabapoxvirus and tanapoxvirus), and
Molluscipoxvirus
(molluscum contagiosum virus).
Parapoxviruses
Three parapoxviruses of domestic animals commonly cause infections in humans: Orf virus (sheep), pseudocowpox virus and bovine pustular stomatitis virus (both cattle). Other parapoxviruses may also rarely cause human infections and include seal parapoxvirus and red deer parapoxvirus.
Orf is a common disease of sheep, causing papules and vesicles on the lips ('scabby mouth') and sometimes around the nostrils and eyes. Lesions can also develop on the udders when ewes suckle infected lambs. Lesions heal within about one month however infective virus can persist in dried scabs in the environment for long periods. Vaccination with a live, non-attenuated virus is practiced in some countries, including New Zealand.
Pseudocowpox occurs in dairy herds worldwide. In contrast to cowpox it can persist in relatively small herds. Infection occurs manually via milkers? hands or teat cups of milking machines through small abrasions. Bovine pustular stomatitis virus lesions on the muzzles of calves may produce lesions resembling pseudocowpox on their mother's teats.
Both Orf and milker's nodule are occupational diseases from contact with infected animals. Infection occurs through skin abrasions; localised lesions are usually found on the hands.
Laboratory diagnosis has traditionally relied on electron microscopy of material from lesions. Virus can be isolated in primary bovine or ovine embryo kidney or testis cells or primary human amnion cells, and then successfully sub-cultured in human embryonic fibroblasts and LLC-MK2 cells.