Mpox

• DEFINITION: An infectious viral disease that affects animals and humans, causing a characteristic blistering "pox" rash.
• ANATOMY OR SYSTEM AFFECTED: Immune system, respiratory system, skin
• CAUSES: Viral infection transmitted by bite or direct contact with infected animal (primarily African rodents); also transmitted person-to-person through respiratory droplets, direct contact with body fluids (including sexual contact), or skin lesions
• SYMPTOMS: Rash, fever, headache, lymph node enlargement, coughing; may spread to brain, causing encephalitis
• DURATION: Two to four weeks
• TREATMENTS: Protection with smallpox or mpox vaccine before or after exposure; antiviral drugs in severe cases

Causes and Symptoms

Mpox, formerly known as monkeypox until it was renamed by the World Health Organization (WHO) in 2022, is a disease caused by the monkeypox virus, a poxvirus in the Orthopoxvirus genus. The genus also contains three other species affecting humans: variola (smallpox), vaccinia (used in the modern smallpox vaccine), and cowpox (used in the original smallpox vaccine). Since smallpox was declared eradicated in 1980, mpox is regarded as the most serious naturally occurring poxvirus infection.

Bites or direct contact with an infected animal may result in transmission, and the handling or consumption of contaminated meat has also been linked to mpox 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 can 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 commonly affected. The 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 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 mpox 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 rates as high as 10 percent have been reported from Africa, fatal cases are rare with modern health care.

Prevention and Treatment

First-generation smallpox vaccines are about 85 percent protective against mpox infection, and second- and third-generation vaccines have also come into use against mpox. Individuals at risk should be vaccinated. Exposed individuals should receive the vaccine within four days of exposure but may benefit up to two weeks after exposure. Approval and availability of specific vaccines varies from country to country. In September 2019, the US Food and Drug Administration (FDA) officially approved the smallpox vaccine MVA-BN (Jynneos) for use against mpox. It was subsequently approved in Canada and Europe as well.

Vaccinia immunoglobulin may be considered for treatment or prophylaxis, although its effectiveness is unproven. The antiviral agent cidofovir is active against mpox 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.

Perspective and Prospects

In 1958, mpox was first described in captive Cynomolgous monkeys in Copenhagen, Denmark. The first human case was identified in 1970 in the Democratic Republic of the Congo. African squirrels are believed to be the main natural of mpox virus, but it has been found in a number of other African rodents.

The early twenty-first century saw a growing number of significant mpox outbreaks, mostly in African countries where the virus can be found in wild animal populations. For example, Nigeria, which had not reported a confirmed mpox case in forty years, experienced a large outbreak in 2017. However, some outbreaks also occurred outside Africa. A shipment of eight hundred African rodents from Ghana to an animal distributor in Texas in April 2003 transmitted the virus to captive prairie dogs that were sold as pets. Eighty-one human cases of mpox were subsequently reported in six US states. All patients survived, but 25 percent required hospitalization and two children had severe disease.

In May 2022, a significant mpox outbreak was reported in the United Kingdom; health authorities later identified a British national who had traveled to Nigeria as the index case. By mid-May, more cases had been reported in the UK and the outbreak had spread to Canada, Portugal, Spain, and the United States. Since most of the infected individuals had not recently visited Africa, health experts warned that community transmission had begun to occur in some areas. As case numbers climbed worldwide, researchers identified sexual transmission as a likely route of infection in a significant number of cases, with men who have sex with men most at risk. Scientists later confirmed that route of transmission.

In July 2022 the WHO declared the outbreak a public health emergency of international concern. By late August the outbreak had spread to almost one hundred countries and resulted in nearly 43,000 confirmed cases, twelve of which were fatal. As concerns over the virus grew, many countries, including the US, made efforts to increase the availability of mpox vaccines and increase awareness for vulnerable populations. In November 2022 the WHO proposed changing the terminology around the disease from "monkeypox" to "mpox." This announcement came amid criticism that the name "monkeypox" stigmatized patients and had racist undertones.

By early December 2022, the outbreak had infected roughly 82,500 individuals worldwide, including nearly 30,000 people in the US. While these case numbers were unprecedented in non-endemic countries, the death rate remained low among individuals with access to modern medical care. There was also significant progress in suppressing the spread of the outbreak by that point. Health experts cited a number of factors, namely widespread vaccine availability and changing behavior among some individuals in vulnerable populations, as a major reason for the decline in cases. In May 2023, the WHO lifted its public health emergency of international concern.

In 2024, epidemiologists increasingly warned of the possibility of a mpox resurgence after a deadlier than normal strain emerged in the Democratic Republic of the Congo (DRC). Researchers identified the more virulent form as Clade I, while the milder form seen in the 2022 outbreak was classified as Clade II. As case spread from the DRC to surrounding countries, the WHO declared another public health emergency of international concern in August 2024.

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