As of March 5, 2024, there have been 32 063 confirmed cases of mpox and 58 deaths in the United States. This outbreak is part of a larger 2022 global outbreak (94 274 cases) that is occurring in 118 locations (countries, territories, and areas), the vast majority of which did not historically report mpox infections; it is caused by Clade IIb.
Transmission continues to occur primarily among men who have sex with men (MSM), but any individual who has been in close, personal contact with someone who has mpox – regardless of age, sexual orientation, or gender identity – is at risk for contracting mpox.
Mpox virus infections in children and adolescents younger than 16 years of age have been extremely rare, representing 0.002% of all US cases; none of the cases resulted in critical illness or death. Children aged 0-12 years typically acquired mpox after skin-to-skin contact with an infected household member during caregiving activities, and adolescents aged 13 years and above were most frequently exposed through male-to-male sexual contact. As of January 10, 2024, 64 cases have been reported in children 0-15 years old and 699 cases have been reported in adolescents / young adults 16-20 years old in the United States.
Domestic animals, pets, and wildlife in close contact with an infected individual may also be at risk for contracting illness.
Immunocompromised patient considerations: Immunocompromised individuals, particularly people with advanced or inadequately treated HIV, are at risk for severe and prolonged illness and even death. An increasing proportion of cases have been identified among Black and Hispanic / Latino individuals, who are disproportionately affected by HIV.
Note: Internationally, a more virulent mpox outbreak in the Democratic Republic of the Congo (DRC) involving a Clade I virus began in 2023. Per the World Health Organization (WHO), children are the most affected group, but it has also involved some sexual spread, including heterosexual transmission. In 2023, 14 626 suspected cases were reported, with 654 deaths. In 2024, as of March 22, the DRC has reported 3941 suspected mpox cases, with 271 deaths. Two-thirds of the mpox cases have been reported in children, with a case-fatality rate of 10% in infants and young children.
The 2022 outbreak of mpox 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 mpox 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.
Clinical features:
The incubation period of mpox is approximately 12 days (7-14 day range usually, but can be 5-21 days).
The clinical presentation of cases in the 2022 outbreak 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 (usually fewer than 10; 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 mpox 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 ocular symptoms (including conjunctivitis, keratitis, blepharitis, and lesions on the eyelids and the conjunctival mucosa). Oral mucosal lesions can occur without any other mucocutaneous symptoms.
Transmission:
Human-to-human transmission occurs 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 mpox 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. Ocular symptoms may result from autoinoculation (ie, rubbing the eye after touching lesions elsewhere on the body).
All skin lesions may be infectious. Persons are thought to be infectious starting 1-4 days prior to the onset of symptoms (a UK study of more than 2700 people with confirmed mpox virus between May 6 and August 1, 2022, suggests that presymptomatic transmission [1-4 days before symptoms appear] occurred in around half of all cases [53%]). Patients should be considered to be infectious until crusts have fallen off and the underlying skin re-epithelialized.
About Mpox
Mpox is a rare zoonotic Orthopoxvirus infection that is clinically similar to smallpox.
There are 3 genomic variants of mpox, with differing mortality rates. The Central African (Congo Basin) clade is now referred to as Clade I and is both more contagious and more severe with a reported mortality rate of around 10.6%. The West African clade is now referred to as Clade IIa and is thought to be less severe with a mortality rate of about 3.6%. The virus responsible for the 2022 outbreak is a Clade IIb virus. Researchers have also confirmed the sexual transmission of Clade I mpox virus in Africa, demonstrating that sexual transmission extends beyond Clade IIb.
Clades I and IIa mpox begin 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. Systemic symptoms are more prominent and severe in Clade I disease. 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.
In Africa, the disease affects people who have hunted or eaten squirrels and other infected mammals. Animal species susceptible to mpox virus 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 mpox virus was needed.
The first documented outbreak of mpox in the Western Hemisphere preceded the 2022 outbreak by almost 20 years and 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.
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