Introduction
Background
Poxviridae are a family of oval, quite large, double-stranded DNA viruses. The genus Parapoxvirus is included among these viruses, which measure 260 by 160 nm and possess a unique spiral coat that distinguishes them from the other poxviruses. Parapoxvirus species are endemic in hoofed animals (ungulates). Three similar, if not identical, parapoxviruses cause human infections, which relate to direct or indirect contact with infected animals. The zoonotic hosts of Parapoxvirus species are sheep and goats (orf, ie, ecthyma contagiosum virus)1,2 and cattle (bovine papular stomatitis virus in humans and pseudocowpox virus [ie, milker's nodule virus or paravaccinia virus]). Other parapoxviruses have been recognized in New Zealand red deer, Finnish reindeer, red squirrels in the United Kingdom, and harbor seals in the North Sea.
Clinical cutaneous manifestations of infection with the parapoxviruses are quite similar because of the nature of the animal reservoirs; therefore, some authors propose the term farmyard pox for any of the 3 infections.
Pathophysiology
The clinical picture of parapox infection for all 3 infections is essentially identical. In general, infection results in solitary or multiple, relatively painless, cutaneous lesions that heal relatively slowly, usually without complications. Even in immunocompromised hosts, little evidence suggests spread of infection outside external surfaces.
The age distribution of patients is wide. Immunity to paravaccinia virus does not occur; in fact, reinfection occurs.
Frequency
United States
Orf, bovine papular stomatitis, and milker's nodule are viral illnesses endemic to sheep, goats, and cows. No reports contain data specific to the United States.
International
The Parapoxvirus causes worldwide infection of cows, sheep, and goats. Data reported from England and Wales between 1990-1995 indicate an annual mean of 15 cases of Parapoxvirus infections in these animals, significantly less than the reported annual mean of 46 cases between 1978-1968. More cases occur in sheep than in other bovines.
- Human infections occur with higher frequency in the spring and autumn, coinciding with the seasonal slaughtering of lambs and calves; although other reports suggest a higher incidence during the winter, presumably from the use of gorse, a prickly animal feed that may cause trauma and, hence, lead to infection in an animal.
- Because of the relatively benign nature of the infection and its occurrence predominantly among animal caretakers, especially those involved with sheep, infected individuals may not be likely to seek medical attention. Therefore, many authors believe that the infection is more common than actually reported.
Mortality/Morbidity
Bovine pustular stomatitis, orf, and milker's nodule show similar gross lesions; nevertheless, lesions range from inconspicuous pustules to giant tumors, which may be incorrectly diagnosed as malignancy, necessitating amputation of the affected digits. Pain often is negligible.
- Erythema multiforme has been described temporally related to orf and milker's nodule infection.
- Gianotti-Crosti syndrome, also referred to as papular acrodermatitis of childhood, causes self-limited symmetrically distributed papules on the face, buttock and extremities. It has also been reported to be temporally linked to infection with milkers' nodules.
Race
The literature does not mention race as a factor influencing the type and distribution of paravaccinia virus in the population; in fact, the race of the patients in the literature reviewed is not specified.
Sex
Most cases occur in males rather than females. This most likely is caused by the male predominance in the occupations or activities of the infected patients, which include veterinarians, veterinary students, farmers, shepherds, animal caretakers, and farm workers. Nevertheless, women are susceptible to infection. In one series, 6 of 19 subjects were women, reflecting their close contact with animals as farm wives, farm workers, or manual cow milkers.
Age
Most cases occur in young to middle-aged adults, although school-aged children also are infected. Parapoxvirus may not have a predilection for any particular age group because the distribution is associated with patient contact with the affected animals.
Breed
Sheep and cattle parapox infections manifest as pathologic lesions on the animal's oral mucosa (eg, lips, nostrils, eyes) and, occasionally, the most hairless areas of the skin (eg, udders, groin). The virus may be isolated in the mucosal fluids, even without obvious lesions on the skin.
Clinical
History
Parapoxvirus species are resistant to heat, cold, and drying and may persist on fences, feeding troughs, and barn beams. Most patients report direct contact when feeding or treating animals and when visiting or working on farms. Reports rarely show human-to-human transmission.
Physical
- The incubation period lasts 3-7 days. After this time, the lesions usually begin as discrete erythematous macules on the fingers, hands, or forearms; although, several reports show involvement of the face, neck, ear, and periocular area. Lesions may be single or multiple and often progress. The lesions appear targetlike, with a red center, a white middle ring, and a red halo.
- The acute stage occurs by week 2-3. The lesion appears nodular and weeping.
- The regenerative stage occurs by week 3-4. The lesion becomes ulcerated and thin-crusted at its summit.
- By week 4-5, the lesion may become papillomatous over its surface.
- Regression with thick crusting and reduction in elevation occurs by week 6, during which time the lesion eventually disappears.
- An erythematous macule may last for several months but resolves with no residual scarring.
- The diagnosis of Parapoxvirus infection usually is clinical in nature, based on the character of the cutaneous lesion and the exposure history of the infected individual.
Causes
Direct contact with infected animals, either alive or dead, is most typical but is not required for infection; researchers also report transmission from contaminated inanimate objects.
More on Parapoxviruses |
 Overview: Parapoxviruses |
| Differential Diagnoses & Workup: Parapoxviruses |
| Treatment & Medication: Parapoxviruses |
| Follow-up: Parapoxviruses |
| References |
| Further Reading |
| Next Page » |
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Keywords
viral infection, virus, poxviruses, Parapoxvirus, Poxviridae, Poxvirus, orf, ecthyma contagiosum, bovine papular stomatitis, pseudocowpox, milker's nodule, paravaccinia, farmyard pox
