The parapox viruses are morphologically and antigenically different from the orthopox viruses. The viruses are pathogenic to a large group of animals. The known diseases include orf (a pathogen of sheep and goats), paravaccinia or "milker's nodule", bovine papular stomatitis and pseudocowpox (cattle), sealpox, infective ecthyma chamois and some others. Humans can be infected with these viruses through direct contact by inoculation into the skin (scratches, wounds). The lesions thus occur mostly on hands or arms. The incubation period for paravaccinia is approximately one week. One or more hard hemispherical pea-size nodules develop on the hands. The surrounding skin is normal. The lesion itself is often shiny and taut. Necrosis of the lesion can occur as a complication. The number of lesions is usually limited. The lesions can itch or be painful and linger for months. There is a marked local granulomatous reaction. If the patient is immunodeficient, the lesions may become larger and chronic. The virus can be detected by electron microscopy. Sometimes a small local trauma can occur to the fingers during milking, as a result of which a small hair enters the wound. This acts like a foreign body ("milker's granulation nodule"). The differential diagnosis also includes mycosis, cat scratch disease, anthrax, cowpox, tularaemia and tuberculosis cutis verrucosa (Mycobacterium bovis). Orf is primarily a disease of sheep and goats. The animals exhibit moist red lesions on the mouth and nose. The virus is not transmissible to cattle. Sometimes humans who have contact with sick animals, chiefly lambs, are infected. A chronically raised lesion on the fingers or back of the hand characterises the disease. The lesion usually heals spontaneously after five weeks.