Monkeypox virus is structurally related to the smallpox virus and causes similar, but usually milder illness.
Monkeypox, like smallpox, is a member of the Orthopoxvirus group. Although the reservoir is unknown, the leading candidates are small rodents and squirrels in the rain forests of Africa, mostly in western and central Africa. Human disease occurs in Africa sporadically and in occasional epidemics. Most reported cases have been in the Democratic Republic of the Congo; a recent 20-fold increase in incidence is thought to be due to the cessation of smallpox vaccination in 1980.
In the US, an outbreak of monkeypox occurred in 2003, when infected rodents imported as pets from Africa spread the virus to pet prairie dogs, which then infected people in the Midwest. The outbreak involved 35 confirmed, 13 probable, and 22 suspected cases in 6 states, but there were no deaths.
Monkeypox is probably transmitted from animals via wounds or mucous membranes. Person-to-person transmission occurs inefficiently, with an attack rate of 8 to 9%. Most patients are children. People who have received smallpox vaccine are at reduced risk. In Africa, mortality rate ranges from 4 to 22%.
Clinically, monkeypox is similar to smallpox; however, skin lesions occur more often in crops, and lymphadenopathy may be more common.
Clinical differentiation of monkeypox from smallpox and chickenpox may be impossible. Diagnosis is by culture, PCR, immunohistochemistry, or electron microscopy, depending on which tests are available.
Treatment is supportive. Cases are reported to public health authorities.
Last full review/revision May 2012 by Mary T. Caserta, MD