The poxvirus family includes a large number of related DNA viruses that infect various vertebrate hosts. The poxviruses responsible for infections in humans, along with the main manifestations of these infections, are listed in Table 183-1. Infections with orthopoxviruses—e.g., smallpox (variola major) virus (Chap. 221) or the zoonotic monkeypox virus—can result in systemic, potentially lethal human disease. Other poxvirus infections cause primarily localized skin disease in humans.
Table 183-1 Poxviruses and Human Infections
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Table 183-1 Poxviruses and Human Infections
|Genus||Species||Geographic Location||Host Reservoir||Human Disease|
|Cowpox||Europe||Rodents||Local pox lesion, occasionally systemic|
|Buffalopox||Indian subcontinent||Water buffalo||Local pox lesion, mild illness|
|Cantagalo and Araçatuba||South America||Cattle||Local pox lesion, mild illness|
|Molluscipoxvirus||Molluscum contagiosum||Worldwide||Humans||Multiple cutaneous lesions (molluscum contagiosum)|
|Parapoxvirus||Orf||Worldwide||Sheep, goats||Local pox lesions (contagious pustular dermatitis)|
|Pseudocowpox (paravaccinia)||Worldwide||Cattle||Local pox lesions (milker's nodule)|
|Bovine papular stomatitis||Worldwide||Cattle||Local pox lesions|
|Deerpox||Deer herds||Deer||Local pox lesions|
|Sealpox||Seal colonies||Seals||Local pox lesions|
|Yatapoxvirus||Tanapox||Africa||Monkeys||Local pox lesions|
Molluscum contagiosum virus is an obligate human pathogen that causes distinctive proliferative skin lesions. These lesions measure 2–5 mm in diameter and are pearly, flesh-colored, and umbilicated, with a characteristic dimple at the center (Fig. 183-1). A relative lack of inflammation and necrosis distinguishes these proliferative lesions from other poxvirus lesions. Lesions may be found—singly or in clusters—anywhere on the body except on the palms and soles and may be associated with an eczematous rash.
Molluscum contagiosum is a cutaneous poxvirus infection characterized by multiple umbilicated flesh-colored or hypopigmented papules.
Molluscum contagiosum is highly prevalent in children and is the most common human disease resulting from poxvirus infection. Swimming pools are a common vector for transmission. Atopy and compromise of skin integrity increase the risk of infection. Genital lesions are more frequent in adults, to whom the virus may be transmitted by sexual contact. The incubation period ranges from2 weeks to 6 months, with an average of 2–7 weeks. In most cases, the disease is self-limited and regresses spontaneously after 3–4 months in immunocompetent hosts. There are no systemic complications, but skin lesions may persist for 3–5 years. Molluscum contagiosum can be associated with immunosuppression and is frequently seen among HIV-infected patients (Chap. 189). The disease can be more generalized, severe, and persistent in AIDS patients than in other groups. Moreover, molluscum contagiosum can be exacerbated in the immune reconstitution inflammatory syndrome (IRIS) associated with the initiation of antiretroviral therapy.
The diagnosis of molluscum contagiosum is typically based on its clinical presentation and can be confirmed by histologic demonstration of the cytoplasmic eosinophilic inclusions (molluscum bodies) that are characteristic of poxvirus replication. Molluscum contagiosum virus cannot be propagated in vitro, but electron microscopy and molecular studies can be used for its identification.
There is no specific systemic treatment for molluscum contagiosum, but a variety of techniques for physical ablation have been used. Cidofovir displays in vitro activity against many poxviruses, and case reports suggest that parenteral or topical cidofovir may have some efficacy in the treatment of recalcitrant molluscum contagiosum in immunosuppressed hosts.
Although monkeypox virus was named after the animal from which it was originally isolated, rodents are the primary viral reservoir. Human infections with monkeypox virus typically occur in Africa when humans come into direct contact with infected animals. Human-to-human propagation of monkeypox infection is rare. Human disease is characterized by a systemic illness and vesicular rash similar to those of variola. The clinical presentation of monkeypox can be confused with that of the more common varicella-zoster virus infection (Chap. 180). Compared with the lesions of this herpesvirus infection, monkeypox lesions tend to be more uniform (i.e., in the same stage of development), diffuse, and peripheral in distribution. Lymphadenopathy is a prominent feature of monkeypox infection.
The first outbreak of human monkeypox infection in the Western Hemisphere occurred during 2003, when more than 70 cases were reported in the midwestern United States. The outbreak was linked to contact with pet prairie dogs that had become infected while being housed with rodents imported from Ghana. Patients presented most frequently with fever, rash, and lymphadenopathy ∼12 days after exposure. Nine patients were hospitalized, but there were no deaths. Smallpox vaccination can provide cross-reactive immunity to monkeypox infection; studies of people exposed in the outbreak detected subclinical infection in a few vaccinated individuals—an observation suggesting the possibility of long-term vaccine protection. The risk of human disease from animal orthopoxvirus infections may increase as smallpox immunity wanes in the general population and the popularity of exotic animals as household pets grows.
Cowpox and buffalopox are rare zoonotic infections characterized by cutaneous poxlike lesions and mild systemic illness. Outbreaks of similar poxlike lesions among cattle and farm workers in Brazil have been due to Cantagalo and Araçatuba viruses, which are virtually identical to vaccinia virus and may have become established in cattle during smallpox vaccination programs.
Parapoxviruses are widely scattered among animal species, but only a few are known to cause human disease via direct contact with infected animals. Parapoxviruses are antigenically distinct from orthopoxviruses and share no cross-immunity. Tanapox virus belongs to a separate, antigenically distinct genus and usually causes a single nodular lesion on the exposed area after contact with infected monkeys.
Gur I: The epidemiology of molluscum contagiosum in HIV-seropositive patients: A unique entity or insignificant finding? Int J STD AIDS 19:503, 2008