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 believed to have 80% protection against MPXV, since there is a degree of cross-protective immunity between the monkeypox and smallpox viruses.
In July 2022, the World Health Organization (WHO) declared the monkeypox outbreak to be a public health emergency of international concern, following its "extraordinary" spread through more than 75 non-endemic countries (regions where the disease does not usually circulate).
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 eradicated by vaccination in 1977.
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 section "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, found in the Congo Basin in Central Africa;
- Clade II, found in West Africa.
The virus currently circulating in Europe is Clade II, which also recently caused an outbreak in Nigeria.
Find out more about the nomenclature of the monkeypox virus variants (WHO website)
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.
Human-to-human transmission of MPXV occurs in one of the following ways:
- by direct contact with infected animals,
- by contact with skin lesions or bodily fluids,
- indirectly via contaminated materials (like bedding or surfaces).
- The disease may also be transmitted via the respiratory droplets of an infected individual.
From May 2022 onwards, thousands of cases of mpox were recorded in non-endemic countries (regions where the disease does not usually circulate). Studies are under way to help shed light on the epidemiology, sources of infection and modes of transmission of the disease. As of September 1, 2022, tens of thousands of cases had been diagnosed.
Rapid identification of new cases and ongoing surveillance are crucial in reducing the risk of human-to-human transmission.
See also Monkeypox: "This circulation of the disease is completely new" – The Conversation.
What are the symptoms?
The clinical presentation of mpox is as a mild form of smallpox, an Orthopoxvirus infection 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 (aches, headache, fatigue, 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 French General Directorate 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 and then scabbing), which mainly affects the palms of the hands and the soles of the feet, and swollen lymph nodes.
The outbreak 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 MPXV fact sheet published by Santé publique France (in French)).
The symptoms last for 2 to 4 weeks and the illness generally goes away on its own. 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 is higher with the Central African strain and in an endemic context. The case-fatality rate for the 2022 global outbreak is approximately 0.03%.
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 of mpox 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 initially designed for treating smallpox has recently been approved for treating mpox. Treatment is only recommended for severe forms of the disease and must be administered orally, as early as possible, for 15 days.
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
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.
The smallpox vaccines used in the smallpox eradication program in the 1970s offer some cross-protection against mpox. Other vaccines have also been developed more recently, including one that has been approved for mpox prevention.
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 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 professionals.
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 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 shifted 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.
Socio-economic instability and civil war are also increasing human-wildlife interactions, aggravating the risk of the virus being transmitted from animals to humans.
Thousands of human cases of mpox are reported each year in several countries in West Africa (including Nigeria) and Central Africa. In the Congo Basin, two countries are particularly affected: the Democratic Republic of the Congo (DRC) and the Central African Republic (CAR); in these countries there has been an increase in the number of outbreaks over the past two decades.
Since 2018, human cases have been exported, mainly from Nigeria, to non-endemic countries – the United Kingdom, Israel, Singapore and the United States –, without giving rise to local clusters/outbreaks of human-to-human transmission.
A global outbreak began in May 2022, leading to the largest number of cases outside endemic countries and to the declaration of a "public health emergency of international concern" by WHO on July 23, 2022.
These cases are not the result of travel to endemic areas.
Since July 6, 2022, WHO has published a bi-weekly epidemiological situation report on the outbreak.
Between January 1, 2022 and June 19, 2023, 87,972 cases and 147 deaths were recorded worldwide.
See the WHO fact sheet
March 2024