Updated September 13, 2022. Refer to US Centers for Disease Control and Prevention (CDC) (Information for Clinicians) for the most current information. See Diagnostic Pearls section for the CDC case definitions.
The World Health Organization (WHO) declared monkeypox (MPX) to be a public health emergency of international concern on July 23, 2022. MPX was declared a public health emergency in the United States on August 4, 2022.
As of September 12, 2022, the CDC reports nearly 58 000 confirmed cases of MPX globally in over 100 locations, the vast majority of which (> 90%) have not historically reported MPX infections, and nearly 22 000 confirmed cases across the United States, including documented pediatric cases and a fatality in a severely immunocompromised adult. Transmission continues to occur primarily among men who have sex with men (MSM), but cases are being reported in other populations. Individuals with advanced or inadequately treated HIV are at risk of life-threatening disease.
Researchers in France have also described a case of a domesticated dog with confirmed MPX virus infection that likely was acquired through human transmission.
The 2022 outbreak of MPX is unique in several ways.
- Many cases have no clear connection to the larger clusters of cases and no clear history of associated travel.
- In the 2022 outbreak, it appears MPX is spreading through specific social and sexual networks, particularly among persons who identify as gay, bisexual, or MSM, although it is in no way limited to any specific population.
Community transmission seems to be occurring through close contact, ie, direct contact with skin lesions or bodily fluids, or indirect contact via contaminated clothing or linens, or exposure to large respiratory droplets.
Anyone who has had close physical contact with someone with MPX is at risk of contracting the virus.
The incubation period of MPX is approximately 12 days (7-14 day range usually, but can be 5-21 days).
The clinical presentation is distinct from prior descriptions of the illness. Notably, anogenital lesions (in some cases painful, in others painless), often without a prodrome, are being observed.
- Many patients have had no associated or preceding febrile illness, fatigue, or other systemic symptoms.
- The eruption that many of these patients develop does not begin on the face, hands, and legs and may not be widespread, nor are the lesions initially numerous. Many patients have presented with a small number of lesions (in some cases 1 or 2) involving the genital or perianal region before the rash spreads to the extremities. These lesions can be, but are not always, quite painful and/or pruritic and may leave scarring.
- The classically described lymphadenopathy associated with MPX does not seem to be a requisite aspect of cases in this outbreak, with some patients having only a single swollen lymph node and some having no lymphadenopathy.
- Some patients present with proctitis or anorectal pain.
- Oropharyngeal symptoms have been reported (including pharyngitis, oral / tonsillar lesions, odynophagia, and epiglottitis) as have conjunctival mucosa lesions.
In the 2022 outbreak, MPX may present in a form that can be very subtle and easily mistaken for many other conditions such as primary and secondary syphilis, genital herpes simplex virus (HSV), and chancroid, among others. This presentation has been described across many parts of Europe, the United Kingdom, Canada, and the United States.
Although human-to-human transmission historically has been rare, the cases in 2022 suggest community transmission occurring through close contact, ie, large respiratory droplets, direct contact with skin lesions or bodily fluids, or indirect contact via contaminated clothing or linens. The WHO notes that anyone who has had close physical contact with someone with MPX is at risk of contracting the virus, and there is a high likelihood that further cases with unidentified chains of transmission will be identified. MSM may be at higher risk for infection.
All skin lesions may be infectious. Persons are thought to be infectious starting at the onset of symptoms. Patients should be considered to be infectious until crusts have fallen off and the underlying skin re-epithelialized.
MPX is a rare zoonotic Orthopoxvirus infection that is clinically similar to smallpox.
Human MPX had been limited to the rain forest areas of Central and West Africa (principally the Democratic Republic of the Congo and the Republic of the Congo) until June 2003, when cases were first reported in Wisconsin, Illinois, and northwestern Indiana. This 2003 outbreak was associated with exposure to pet prairie dogs that had been housed near imported small mammals from Ghana. Infection had not been observed in the Western Hemisphere until this time.
In Africa, the disease affects people who have hunted or eaten squirrels and other infected mammals. Animal species susceptible to Monkeypox virus (MPXV) may include nonhuman primates, lagomorphs (rabbits), and some rodents. Predominant person-to-person transmission and prolonged chains of transmission were suspected in 1996 when 71 cases emerged in Katako-Kombe Health Zone, Kasai-Oriental, and Democratic Republic of the Congo, and again in 2003 in the Likouala region of Republic of the Congo. In order to sustain the disease in the human population, it was believed that repeated animal reintroduction of MPXV was needed.
There are 2 clades of MPX, with differing mortality rates. The Central Africa (Congo Basin) clade is both more contagious and more severe with a reported mortality rate of around 10.6%. The West African clade is thought to be less severe with a mortality rate of about 3.6%. In the United States, there have been no fatalities.
Historically, MPX begins with a prodrome of fever, headache, malaise, backache, lymphadenopathy, chills, nonproductive cough, and arthralgias followed 1-10 days later (usually by day 3) by the development of a papular, vesicular, then pustular eruption on the face, trunk, and extremities. Some patients also experience myalgias, nausea and vomiting, lethargy, sore throat, dyspnea, and sweats. The clinical presentation is similar to but milder than that of smallpox, with the primary difference being that individuals with MPX develop lymphadenopathy, whereas those with smallpox do not. Illness typically lasts 2-4 weeks. Individuals who received smallpox vaccination were reported to develop milder cases.
Before the 2022 outbreak, cases in the United States were primarily limited to laboratory workers, pet shop workers, and veterinarians. There were 2 US cases in 2021 (July and November), both from travelers returning from Nigeria.
The first documented outbreak of MPX in the Western Hemisphere was attributed to a shipment of small mammals from Ghana to the United States in 2003. An infected Gambian giant rat from this shipment infected prairie dogs, which in turn transmitted the disease to humans. The prairie dogs were sold by a Milwaukee animal distributor to 2 pet shops in the Milwaukee area and during a pet “swap meet” (pets for sale or exchange) in northern Wisconsin. Patients from this outbreak reported direct or close contact with prairie dogs, most of which were sick. Illness in the prairie dogs was frequently reported as beginning with a blepharoconjunctivitis that was followed by the appearance of nodular lesions in some cases. Some prairie dogs died from the illness while others reportedly recovered. Lesions in the 2003 US outbreak differed from smallpox lesions in that they were not in the same stage of evolution at the same time. In some patients seen in the United States in 2003, early lesions became ulcerated, especially those at animal bite / scratch sites.