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Molluscum Contagiosum and Other Poxviruses, Excluding Smallpox Virus

Molluscum Contagiosum and Other Poxviruses, Excluding Smallpox Virus

Fred Wang

The poxviruses include 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 1. Systemic human disease can result from infection with smallpox (variola major) virus, a poxvirus that infects only humans, or from zoonotic infection with monkeypox virus. Other poxvirus infections cause primarily localized skin disease in humans. Molluscum contagiosum virus (MCV) is an obligate human pathogen that causes distinctive proliferative skin lesions; molluscum contagiosum is the most frequent human disease resulting from poxvirus infection. Exposure to animals infected with other poxviruses can also cause localized skin disease in humans.

MOLLUSCUM CONTAGIOSUM

Molluscum contagiosum is generally a benign disease consisting of pearly, flesh-colored, umbilicated skin lesions 2 to 5 mm in diameter with a characteristic dimple at the center. A relative lack of inflammation and necrosis distinguishes these proliferative lesions from other poxvirus lesions. The lesions occur singly or in clusters. MCV is a human poxvirus that is transmitted by close contact, including sexual intercourse. Swimming pools are a common vector for transmission. Atopy and compromise of skin integrity increase the risk of infection. Lesions may be found anywhere on the body except the palms and soles and may be associated with an eczematous rash. The incubation period ranges from 2 weeks to 6 months, with an average of 2 to 7 weeks. In most cases, the disease is self-limited and regresses spontaneously after 3 to 4 months in immunocompetent hosts. There are no systemic complications, but skin lesions may persist for 3 to 5 years. Molluscum contagiosum develops especially often in association with the advanced stages of HIV infection, with a prevalence of 5 to 18% among HIV-infected patients. The disease is often more generalized, severe, and persistent in AIDS patients than in other groups, frequently involving the face and upper body. Extensive molluscum contagiosum has also been reported in conjunction with other types of immunodeficiency.

The diagnosis of molluscum contagiosum is typically made by its clinical presentation and can be confirmed by histologic demonstration of the cytoplasmic eosinophilic inclusions, or molluscum bodies, that are characteristic of poxvirus replication. MCV 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. Molluscum contagiosum may respond to effective control of HIV infection with highly active antiretroviral therapy. Cidofovir displays in vitro activity against many poxviruses, including smallpox virus and MCV, and case reports suggest that parenteral or topical cidofovir may have some efficacy in the treatment of recalcitrant molluscum contagiosum in immunosuppressed hosts.

ZOONOTIC POXVIRUS INFECTIONS

Monkeypox virus naturally infects nonhuman primates in the tropical rain forests of western and central Africa and can infect humans who come into direct contact with infected animals. Human disease is rare and is characterized by a systemic illness and vesicular rash similar to those of variola. A large outbreak of monkeypox occurred between February 1996 and October 1997 in central Africa, with a case-fatality ratio of 3%. The prolonged period of active cases suggested a potential for sustained person-to-person transmission, and the higher proportion of younger case-patients suggested the possible consequences of discontinued smallpox vaccination. Clinical presentations were occasionally confused with the more common varicella-zoster virus infection. Compared with the lesions of this herpesvirus infection, monkeypox lesions tend to be more uniform (i.e., in the same stage of development at the same time), diffuse, and peripheral in distribution.

The first outbreak of monkeypox infection in the Western Hemisphere occurred in the midwestern United States during May and June 2003. Monkeypox virus infections were diagnosed in several people who had close contact with ill prairie dogs, a Gambian rat, and a rabbit purchased as pets from a common animal distributor. Patients presented most frequently with fever, respiratory symptoms, and lymphadenopathy ~12 days after exposure. The typical vesicular rash developed with or shortly (1 to 3 days) after the fever. The Centers for Disease Control and Prevention recommended smallpox vaccination for persons having close or intimate contact with a documented case of human or animal monkeypox infection in order to reduce the risk of spread. Vaccination can be given up to 14 days after exposure.

Orf virus and pseudocowpox virus are parapoxviruses that naturally infect sheep and cattle. Direct contact with infected animals can result in infections in humans, typically on the hands, with the development of a nodular, highly vascular proliferative lesion that may ulcerate. Human orf virus infection is also called ecthyma contagiosum, and human pseudocowpox virus infection causes “milker's nodules.” Zoonotic infection with cowpox virus, an orthopoxvirus, causes painful hemorrhagic lesions, mostly on the hands or face, with fever or flulike symptoms and lymphadenitis. Lesions generally resolve in 6 to 8 weeks. Human infection with tanapox virus occurs after contact with infected monkeys. In most cases, a febrile prodrome is followed by eruption of a single nodular lesion on the exposed area, but multiple lesions have also been reported. The lesions are relatively large, often break down to form an ulcer, and resolve in 5 to 6 weeks.

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