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Monkeypox is a rare viral infection caused by an orthopoxvirus, which is a relatively large DNA virus that is related to smallpox. Monkeypox received its name because it was discovered in a group of laboratory monkeys in 1958. However, rodents, and not monkeys, are the most likely animal reservoir. Occasionally, monkeypox causes zoonotic infections in which animal-to-human transmission occurs by bite or scratch, bush meat preparation, or direct contact with body fluids.

Human to human transmission is thought to spread primarily through respiratory droplets, direct contact with skin lesions or bodily fluids, or indirect contact via contaminated clothing or linens. The incubation period from infection to the appearance of symptoms ranges from 5 to 21 days.

Monkeypox cases are typically mild, but they can progress to severe illness. Symptoms include fever and chills, headache, muscle aches, swollen lymph nodes, and a rash like chickenpox. The rash develops on the face and then spreads to other parts of the body including the palms of the hand and soles of the feet. The rash progresses from flat to raised lesions, and eventually to fluid filled vesicles that rupture and scab over. The case-fatality rate of monkeypox in Africa ranges from 1% to 10%, with the highest risk of death among children.

Two distinct clades of monkeypox virus have been identified in the Congo Basin and in West Africa. The Congo clade is more transmissible and virulent. The Congo Basin clade has a case-fatality rate of 10% while the West African clade has a case-fatality rate of 1%. Nigeria had an outbreak involving 450 people in 2017.

In an average year, a few thousand cases occur in western and central Africa. But cases outside Africa have been limited to a handful that are associated with travel to Africa or with the importation of infected animals. The United States had an outbreak in 2003, when a shipment of rodents from Ghana spread the virus to pet prairie dogs in Illinois and infected more than 70 people.

Recently, public health experts have become concerned about simultaneous outbreaks of monkeypox in at least 11 countries that normally don’t have the disease. As of May 20, 2022, more than 120 cases have been confirmed or suspected in the United Kingdom, Portugal, Spain, Canada, Sweden, Italy, Germany, France, Belgium, Australia, and the United States. Public health agencies have reported that most cases have involved men between the ages of 20 and 50 who have had sexual contact with other men and had no travel history to countries where monkeypox was endemic.  Although monkeypox is not known to be sexually transmitted, sexual activity certainly constitutes close contact.

On May 19, Portuguese researchers uploaded the first draft of a monkeypox virus genome that had been identified in their country. The preliminary data suggests that the virus is related to the West African clade. Additional sequencing will reveal if the monkeypox outbreaks in the different countries are related and whether a mutation has occurred that increases transmissibility. Monkeypox outbreaks affecting people in different countries with no apparent connection to one another suggest the virus might be spreading silently.

Monkeypox may become more prevalent due to waning immunity in people who received the smallpox vaccine and no immunity in those who never received it because smallpox had been eradicated. The last smallpox vaccine was administered in the United States in 1972.

The smallpox vaccine has been used to prevent monkeypox infections in those exposed or at high risk, with a success rate of about 85%. Tecovirimat is an antiviral drug that was approved for monkeypox in 2022 but is not widely available.


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