Monkeypox (Mpox)

Monkeypox, like smallpox Smallpox Smallpox is a highly contagious disease caused by the smallpox virus, an orthopoxvirus. Case fatality rate is about 30%. Natural infection has been eradicated. The main concern for outbreaks... read more , is a member of the Orthopoxvirus group. There are 2 distinct clades (groups of similar organisms descended from a common ancestor) of monkeypox: the West African clade and the Congo Basin clade. Despite its name, nonhuman primates are not monkeypox virus reservoirs. Although the reservoir is unknown, the leading candidates are small rodents (eg, squirrels) in the rain forests of Africa, mostly in western and central Africa. In November 2022, the World Health Organization introduced the name “mpox” to replace "monkeypox" (see WHO recommends new name for monkeypox disease).

Historically, human disease was mainly limited to sporadic cases and occasional epidemics, mainly in Africa. Most reported cases have been in the Democratic Republic of the Congo. Since 2016, confirmed cases have also been reported in Sierra Leone, Liberia, Central African Republic, Republic of the Congo, and Nigeria. A dramatic increase in incidence in Africa since 2000 is thought to be due to the cessation of smallpox vaccination in 1980; people who have received smallpox vaccine, even > 25 years 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.

Prior to 2022, cases outside of Africa were directly linked to travel to western and central Africa or animals imported from the region. A notable outbreak of monkeypox occurred in the US 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 37 confirmed and 10 probable cases in 6 states, but there were no deaths (1 References Monkeypox is caused by the monkeypox virus, an orthopoxvirus structurally related to the smallpox virus. Patients present with a vesicular or pustular rash that may be painful and often... read more ).

Since May 2022, cases of monkeypox have been reported in approximately 70 countries where the disease is not endemic. Sustained person-to-person transmission outside of Africa has been demonstrated. The World Health Organization (WHO) declared the 2022 monkeypox outbreak a public health emergency of international concern (see WHO: Monkeypox Outbreak 2022). The majority of confirmed cases in nonendemic countries are in Europe and North America (see Centers for Disease Control and Prevention [CDC]: 2022 Monkeypox Outbreak Global Map). Over 29,000 cases have been reported in the US as of November 2022 (see CDC: 2022 US Map & Case Count). All cases associated with the 2022 global outbreak have been the West African clade. Cases have been reported primarily in men who have sex with men, but monkeypox should be considered in anyone who presents with a rash consistent with monkeypox (2 References Monkeypox is caused by the monkeypox virus, an orthopoxvirus structurally related to the smallpox virus. Patients present with a vesicular or pustular rash that may be painful and often... read more , 3 References Monkeypox is caused by the monkeypox virus, an orthopoxvirus structurally related to the smallpox virus. Patients present with a vesicular or pustular rash that may be painful and often... read more ).

In animal-to-person transmission, monkeypox is probably transmitted via body fluids, including salivary or respiratory droplets or contact with wound exudate. This can occur through bites or scratches from an animal or through preparation and consumption of infected animal meat.

Person-to-person transmission occurs through prolonged close contact. Routes of transmission include respiratory droplets via prolonged face-to-face or intimate physical contact, direct contact with infectious lesions or other bodily fluids, and fomites via contact with clothing or linens contaminated with lesion crusts or bodily fluids. Maternal-fetal transmission can occur via the placenta. It is currently unknown whether transmission can occur through semen or vaginal fluids. In the 2022 outbreak, many cases appear due to transmission during sexual or intimate encounters, but this is likely through direct contact with infectious lesions or respiratory secretions.

The incubation period is 1 to 2 weeks but can be as long as 3 weeks. People are infectious from the time symptoms start until all lesions have crusted and scabs have fallen off to reveal healthy skin. This typically takes 2 to 4 weeks.

Based on a transmission study in Africa, the overall secondary attack rate following contact with a known human source is 3%, and attack rates up to 50% have been reported in people living with a monkeypox-infected person (4 References Monkeypox is caused by the monkeypox virus, an orthopoxvirus structurally related to the smallpox virus. Patients present with a vesicular or pustular rash that may be painful and often... read more ). Transmission in hospital settings has been documented in an endemic setting. One case of transmission to a healthcare worker has been reported (5 References Monkeypox is caused by the monkeypox virus, an orthopoxvirus structurally related to the smallpox virus. Patients present with a vesicular or pustular rash that may be painful and often... read more ). Case fatality rate is ≤ 10% with the Congo Basin clade, but is < 1% with the West African clade.