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By Brenda L. Tesini, MD, Assistant Professor of Medicine and Pediatrics, Division of Infectious Diseases; Associate Hospital Epidemiology, University of Rochester School of Medicine and Dentistry; Strong Memorial Hospital and Golisano Children's Hospital, University of Rochester Medical Center

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Monkeypox, a rare disease, is caused by the monkeypox virus, which 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; people who have received smallpox vaccine, even > 25 yr prior, are at reduced risk of monkeypox. Cases of monkeypox in Africa are also increasing because people are encroaching more and more on the habitats of animals that carry the virus.

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 body fluids, including salivary or respiratory droplets or contact with wound exudate. Person-to-person transmission occurs inefficiently; attack rates of 3 to 11% (but up to 50%) have been reported in people living with a monkeypox-infected person (1). Transmission in hospital settings has also been documented. Most patients are children. 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 occurs in monkeypox but not in smallpox. Secondary bacterial infection of the skin and lungs may occur.

Clinical differentiation of monkeypox from smallpox and chickenpox (a herpesvirus, not a pox virus) may be difficult. Diagnosis of monkeypox is by culture, PCR, immunohistochemistry, or electron microscopy, depending on which tests are available.

Treatment of monkeypox is supportive. Potentially useful drugs include

  • The antiviral drug cidofovir

  • The investigational drugs brincidofovir (CMX001) and tecovirimat (ST-246)

All of these drugs have activity against monkeypox in vitro and in experimental models. However, none of these drugs has been studied or used in endemic areas to treat monkeypox.

Cases of monkeypox are reported to public health authorities.


  • 1. Nolen LD, Osadebe L, Katomba J, et al: Extended human-to-human transmission during a monkeypox outbreak in the Democratic Republic of the Congo. Emerg Infect Dis 22 (6):1014–1021, 2016. doi: 10.3201/eid2206.150579.

More Information

  • CDC: Monkeypox

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