What is monkeypox?
Monkeypox is a poxvirus in the Orthopoxvirus genus, which contains three other species affecting humans: variola (smallpox), vaccinia (the current smallpox vaccine), and cowpox (the original smallpox vaccine). The virion is large and brick-shaped, containing double-stranded deoxyribonucleic acid (DNA) that has been decoded and comprises 196,858 base pairs. Since smallpox was declared eradicated in 1979, monkeypox is regarded as the most serious naturally occurring poxvirus infection.
Bites or direct contact with an infected animal may result in transmission. Person-to-person spread also occurs through respiratory droplets, requiring close contact with an infected host. Direct contact with body fluids or skin lesions may also transmit the virus. Less commonly, virus-contaminated objects from infected humans, pets, or laboratory animals may spread the disease indirectly.
The disease can affect persons of any age, but children are more common. The incubation period varies from about six to sixteen days, and the illness commences with a fever that may be accompanied by headache and enlarged lymph nodes. One to three days later, a maculopapular rash develops. The rash primarily involves the periphery (head and extremities) and resembles smallpox more than chickenpox, which is more centrally (trunk) located. However, lymphadenopathy is not usually seen in smallpox. The rash of monkeypox may involve the palms and soles. The skin lesions progress through a vesiculopustular stage before finally crusting. Secondary infection and scarring may occur. Lesions may also be seen in the mouth and upper respiratory tract, producing a cough and occasionally respiratory distress. Spread of the infection to the brain, causing encephalitis, is a rare but serious complication. The illness typically lasts two to four weeks, and while mortality rates as high as 10 percent have been reported from Africa, fatal cases are rare with modern health care.
Smallpox vaccine is about 85 percent protective against monkeypox infection, and individuals at risk should receive the vaccine. Exposed individuals should receive the vaccine within four days of exposure but may benefit up to two weeks after exposure. Vaccinia immunoglobulin may be considered for treatment or prophylaxis, although its effectiveness is unproven. The antiviral agent cidofovir is active against monkeypox in vitro and in animals. Unfortunately, the efficacy of cidofovir for human cases is unknown. Because cidofovir is a toxic drug, it should be considered for treatment only in severe cases and should not be used for prophylaxis.
In 1958, monkeypox was first described in captive Cynomolgous monkeys in Copenhagen, Denmark. The first human case was identified in 1970 in the Democratic Republic of Congo. African squirrels are the main reservoir of monkeypox, but the virus has been found in a number of other African rodents.
A shipment of eight hundred African rodents from Ghana to an animal distributor in Texas during April 2003, resulted in the infection of coinhabiting captive prairie dogs. Many of these animals were sold as pets, and the result was eighty-one human cases of monkeypox in six states. All patients survived, but 25 percent required hospitalization and two children had severe disease.
Low herd immunity, along with repeated introduction of monkeypox from the African wild reservoir, will likely produce more human illness, both in Africa and at distant sites. The use of smallpox vaccine in high-risk and exposed persons, along with antiviral agents such as cidofovir for severe cases, should limit disease and improve outcomes.
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