Small localized outbreaks have occurred regularly in Central and West Africa in recent years and have been studied and monitored. A global monkeypox outbreak began in May 2022.
In the early 1980s, following the worldwide eradication of smallpox, vaccination against smallpox was discontinued. Individuals who received the smallpox vaccine are partially protected against MPXV, since there is a degree of cross-protective immunity between the mpox and smallpox viruses.
For the first time, in July 2022, the World Health Organization (WHO) declared the mpox outbreak as a public health emergency of international concern (PHEIC), following its "extraordinary" spread through more than 75 non-endemic countries (regions where the disease does not usually circulate).
On August 14, 2024, WHO declared a second PHEIC in response to an upsurge of mpox in the Democratic Republic of the Congo and several neighboring countries and the emergence of a new, potentially more transmissible viral strain (clade Ib).
What are the causes?
MPXV is a double-stranded DNA virus (around 200 kilobases) in the Poxviridae family, belonging to the Orthopoxvirus genus. It is related to the virus that causes smallpox, a disease declared eradicated by vaccination in 1980.
MPXV was first isolated in 1958 in a monkey colony in Copenhagen, Denmark, when the monkeys developed skin lesions that resembled those caused by smallpox. The disease was therefore given the name "monkeypox."
Although the name "monkeypox" continues to be frequently used, the disease is not transmitted to humans via monkeys but by rodents (see "How does the disease spread?" below). Since late 2022, WHO has recommended using the name "mpox" instead of monkeypox.
- There are two main types of MPXV: clade I, the "historical" strain of the virus present in the Congo Basin in Central Africa and clade Ib which originated from clade I.
- Clade II, found in West Africa. The virus currently circulating in Europe, clade IIb, originated from clade II which caused an outbreak in Nigeria.
How does the disease spread?
Mpox is a zoonosis, in other words a disease transmitted from animals to humans.
It is transmitted to humans by rodents (e.g. rope squirrels or Gambian rats in Africa). But the animal reservoir has not yet been formally identified. According to a study published in 2021 by the Institut Pasteur about monkeypox in Central African Republic, the genomic history suggests multiple introductions from rainforest animal reservoirs.
MPXV is transmitted in humans:
- mainly by contact with skin lesions containing viral particles or the mucous membranes of infected individuals,
- by direct contact with infected animals,
- indirectly via contaminated materials (like bedding or surfaces),
- the disease may also be transmitted via the respiratory droplets of an infected individual.
What are the symptoms?
The clinical presentation of mpox is a mild form of smallpox that was declared to have been globally eradicated in 1980.
Mpox is less contagious than smallpox and causes milder symptoms.
In Africa, mpox has typically presented as follows:
- an incubation period of around 12 days before the onset of the first symptoms;
- generally a febrile prodrome (fever, chills, aches, headache, fatigue/exhaustion, etc.) for 1 to 4 days; the individual is contagious as soon as the first symptoms emerge (see the fact sheet for health professionals from the Department of Public Health within the Ministry for Health (in French));
- then a skin eruption phase, lasting 2 to 4 weeks, with a rash in the form of small marks (a maculopapular rash developing into pustular lesions, vesicles, and then scabbing), which affects the entire body, including the palms of hands and soles of feet, with swelling of the lymph nodes.
The outbreak linked to clade IIb that has been ongoing since May 2022 in Europe – and has spread to the rest of the world – causes a more localized skin rash, often in the genital or perianal regions (see the Mpox fact sheet on the Santé publique France website (in French)).
Symptoms last for 2 to 4 weeks and infected individuals generally recover spontaneously. Complications may occur, such as secondary skin infections, septicemia, encephalitis or corneal ulceration. They can lead to severe forms of the disease. On its website, WHO reports a case-fatality rate of approximately 3 to 6% in 2022 for the outbreaks in Africa; the rate appears to be higher with the Central African strain (clade I) and in an endemic context. It should be noted that the case-fatality rate is highly dependent on patients' age (it is high among children aged under 5 years, particularly those who are malnourished and/or dehydrated), immunodeficiency status (whether or not they are infected with HIV), and, in particular, the quality of hospital care. As such, the case-fatality rate was much lower during the global outbreak of 2022, at approximately 0.2%.
How is infection diagnosed?
Mpox is initially diagnosed clinically by specialist physicians (infectious disease specialists or dermatologists). Diagnosis is then confirmed by a laboratory-performed PCR test on an oropharyngeal swab and a swab of a skin lesion.
Differential diagnosis must consider other diseases that cause skin eruptions, especially chicken pox but also measles, bacterial skin infections, syphilis, herpes, etc.
What treatments are available?
An antiviral agent, Tecovirimat, initially designed for treating smallpox, was used to treat mpox during the 2022-2023 outbreak. Treatment is only recommended for severe forms of the disease and is administered orally, as early as possible, for 15 days. Its clinical efficacy needs to be robustly demonstrated.
On its website, the French National Authority for Health has published a series of answers to basic questions for healthcare professionals about MPXV infection and primary medical care.
Find out more with the recommendations issued by the French National Authority for Health (in French)
How can mpox be prevented?
In endemic regions (in Africa), the main prevention strategy for mpox is to restrict human-wildlife interactions, which involves informing populations about the risk factors for zoonotic transmission (transmission by animals) so as to reduce the risk of transmission from animals to humans. It is also important to reduce other factors that contribute to the emergence of outbreaks, such as poverty – which leads to a dependency on bushmeat as a source of protein and to cramped and crowded living conditions – and military conflicts resulting in population movements.
More generally, the prevention strategy to limit human-to-human transmission involves awareness and information:
- making populations aware of risk factors for transmission: avoiding skin contact with infected individuals or contaminated items (see above),
- informing at-risk populations and health professionals.
The development of rapid diagnostic tests will help improve diagnosis and prevent human-to-human transmission.
Several mpox vaccines are available.
The smallpox vaccines used in the smallpox eradication program in the 1970s offer some cross-protection against mpox. Other vaccines causing fewer adverse effects have been developed more recently.
Some countries offer vaccination for individuals who are likely to be at risk, such as laboratory staff, healthcare workers, etc. In France, the National Authority for Health issued an opinion on July 7, 2022 (in French) recommending that preventive vaccination should be offered to those who are most exposed to the virus: men who have sex with men, owners of sex venues and sex workers.
Who is affected?
Mpox is an emerging infectious disease that was first identified in humans in 1970 in the Democratic Republic of the Congo (DRC). Most subsequent cases were reported in isolated rural areas and tropical rainforest regions in Central and West Africa.
The frequency of outbreaks and the number of cases they cause in human populations have increased regularly in recent years. The geographical spread of mpox has extended beyond the forests of Central Africa to savanna regions and urban areas and even to other parts of the world, where cases have been imported.
This pattern of transmission can be partly explained by the global decline in immunity conferred by the smallpox vaccine following the discontinuation of the vaccination program in the 1980s (see the Institut Pasteur's retrospective analysis in July 2020).
But there are also other contributory factors which have intensified over the past 30 years, including major changes in land use, mass deforestation, growing urbanization, destruction of wildlife habitats and a loss of biodiversity. These ecosystem pressures caused by human activity are leading to an increase in human-wildlife interactions and are changing structures and dynamics among animal communities.
In recent years, there has been a change in the epidemiological profile of patients in Africa, where the virus is increasingly prevalent in urban areas:
- In Nigeria, the majority of these patients since 2017 have been male and of sexually active age, with a relatively high proportion of patients infected with HIV. The infectious strain, known as clade IIb, is the same variant subsequently detected during the global pandemic of 2022 which mainly affected men that have sex with men (MSM). With 87,972 cases and 147 deaths across 110 countries, this pandemic prompted WHO to declare its first public health emergency of international concern (PHEIC) on July 23, 2022. This first pandemic was halted through a combination of preventive measures promoted by the non-profit sector among at-risk populations and vaccinations, though the virus continues to circulate at a low level (52 cases reported in France in 2023 and 107 in the first half of 2024) (in French).
- In the DRC, the number of cases has been constantly rising in the past two years and a particularly severe outbreak is ongoing in the east of the country (Kivu region). The majority of cases are young adults, many of whom work in mining areas, and also sex workers, indicating active sexual transmission of the virus in these communities. Clade Ib, the variant transmitted in these cases, has since been detected in several East African countries that have previously been free of the virus (in particular Rwanda, Burundi, Uganda and Kenya). This active circulation of the virus in East Africa and the emergence of a new clade whose transmissibility and case-fatality rate are not yet known, led WHO to declare a second PHEIC on August 14, 2024.
Access information on mpox on the website of the French Health Insurance Fund (Assurance Maladie) (in French)
October 2024