Merck Manual

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Brenda L. Tesini

, MD, University of Rochester School of Medicine and Dentistry

Last full review/revision Oct 2019| Content last modified Oct 2019
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Monkeypox is caused by the monkeypox virus, which is related to the smallpox virus and causes a similar, but usually milder illness.

Where monkeypox originates is unknown, but it is thought to be spread by small rodents and squirrels in the rain forests of Africa, mostly in western and central Africa. Most cases have occurred in the Democratic Republic of the Congo. Despite its name, the monkeypox virus does not live in monkeys.

Monkeypox is rare. But recently, the number of cases has been increasing in Africa. Reasons may include the following:

  • People are no longer being vaccinated with the smallpox vaccine, which helped protect them from monkeypox.

  • People are moving into areas where the animals that carry the virus live.

In the United States, an outbreak of monkeypox occurred in 2003, when infected rodents were imported as pets from Africa. The rodents spread the virus to pet prairie dogs, which then infected people in the Midwest.

Monkeypox is probably spread when people come into contact with body fluids from infected animals—for example, if an infected animal bites a person or if a person breathes in airborne droplets containing the virus. Spread from person to person is less common.

Usually, monkeypox occurs in children.

Monkeypox causes symptoms that are similar to those of smallpox. The illness begins with a fever, chills, headache, muscle aches, backache, extreme fatigue, and, unlike smallpox, prominent swollen lymph nodes. About 1 to 3 days after the fever appears, a rash develops. It often begins on the face, then spreads to other parts of the body, including the palms and soles.

As in smallpox, the monkeypox rash begins as flat, red spots. The spots then turn into blisters, which fill with pus (forming pustules). After several days, the pustules crust over.

Monkeypox can make people more likely to develop other infections. Some people with monkeypox develop bacterial infections in the skin and lungs.

The illness is usually milder than smallpox, but it can cause death. No deaths occurred during the 2003 outbreak in the United States.

Monkeypox typically lasts for 2 to 4 weeks.

Diagnosis of monkeypox may involve

  • Sending samples of infected tissue to a laboratory for the virus to be grown (cultured) and analyzed

  • Blood tests for antibodies to the monkeypox virus

  • Detection of the virus's genetic material (DNA) in infected tissue

  • Examining a sample of infected tissue under a microscope

Treatment of monkeypox is mostly directed at relieving symptoms. The antiviral drug cidofovir may be helpful, but it has not been studied as a treatment for monkeypox.

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