Background
Poxviridae are a family of oval or brick-shaped, quite large, double-stranded DNA viruses that can infect both humans and animals. The genus Parapoxvirus is included among these viruses; these viruses measure 260 X 160-nm and possess a unique spiral coat that distinguishes them from the other poxviruses. Parapoxvirus species are enzootic to hoofed animals (ungulates) throughout the world. Three similar parapoxviruses (orf virus, [1] pseudocowpox virus, [2] and bovine papular stomatitis virus [3] ) commonly cause infection in humans; transmission is through direct or indirect contact with infected animals. The zoonotic hosts of these parapoxviruses are sheep and goats (orf, ie, ecthyma contagiosum virus) and cattle (pseudocowpox virus [ie, milker's nodule virus or paravaccinia virus] and bovine papular stomatitis virus). [4]
Other parapoxviruses have been recognized in New Zealand red deer, [5] Finnish reindeer, [6] Japanese serows, [7] European musk oxen, [8] red squirrels in the United Kingdom, [9] harbor seals in the North Sea, [10] and California sea lions. [11] A novel parapoxvirus from white-tailed deer in the United States has caused cases of human infection. [12]
Parapoxvirus infections manifest as pathologic lesions on the animal's oral mucosa (eg, lips, nostrils, eyes) or the moist hairless areas of the skin (eg, udders, groin). The virus may be contracted even in the absence of obvious lesions on the animal. [13]
Incidence of deer-associated parapoxvirus infections may rise as the deer population in the United States continues to increase. [12]
Clinical cutaneous manifestations of infection with the parapoxviruses are identical; therefore, some authors propose the term "farmyard pox" for any of the 3 common parapoxvirus infections. [14]
Go to Poxviruses, Orf, and Milker's Nodules for complete information on these topics.
Pathophysiology
Parapoxvirus infection results in solitary or multiple, relatively painless, cutaneous lesions that heal slowly, usually without complications. Occasionally, the lymphatic system is involved. Even in immunocompromised hosts, little evidence suggests spread of infection outside external surfaces.
Lasting immunity to parapoxviruses does not seem to occur, and reinfection has been reported. [15]
Epidemiology
Frequency
United States
Orf, milker's nodule, and bovine papular stomatitis are viral illnesses enzootic to sheep, goats, and cattle throughout the world. [3, 16] No reports contain data specific to the United States.
International
Data from England and Wales for 1990-1995 indicate an annual mean of 15 human cases of parapoxvirus infections, significantly less than the reported annual mean of 46 cases between 1978 and 1986. Sheep were a more frequent source of infection than other ungulates. [17]
Among high-risk populations, such as animal caretakers or meat handlers, [18] the typical clinical appearance and the benign nature of the infection may be well known. As a result, infected individuals may not seek medical attention and many authors believe that the infection is much more common than actually reported.
Mortality/Morbidity
Parapoxvirus lesions generally heal without treatment, albeit slowly. Scarring typically is absent. Immunocompromised patients and those with atopic dermatitis are at risk for progressive or disseminated disease. One case reported described blindness resulting from ocular involvement; no cases resulting in death have been reported. [19]
Race
Race is often not specified in the existing literature, but the infection occurs throughout the world.
Sex
Most cases occur in males, reflecting the male predominance in the occupations or activities of the infected patients, which include veterinarians, veterinary students, farmers, shepherds, and other animal caretakers. Women are susceptible to infection if they have close contact with animals.
Age
Most cases occur in young to middle-aged adults, although school-aged children also are infected. Parapoxviruses do not appear to have a predilection for any particular age group. Children may be at higher risk due to behavioral reasons that cause them to sustain more animal bites, to have poorer adherence to good hand hygiene and personal protective measures, and to engage in high-risk behaviors such as nuzzling a sick animal. [20] An analysis of an orf outbreak found that age younger than 20 years was an independent risk factor for infection. [21]
