May 19, 2022

Monkeypox outbreak erupts; US, UK, Spain, Portugal, and more report cases

A 2003 photo of the arms and legs of a 4-year-old girl infected with monkeypox in Liberia.
Enlarge / A 2003 photo of the arms and legs of a 4-year-old girl infected with monkeypox in Liberia.

A growing outbreak of monkeypox cases has spread across several countries, including the US, suggesting that the animal-transmitted disease that occurs in forested areas of Central and West Africa has been quietly spreading undetected.

So far, the US has reported one case in a Massachusetts man who had recently traveled to Canada, which, as of Thursday, reported 17 suspected cases in Montreal. The United Kingdom has identified nine cases, one of which is connected to recent travel to Nigeria, where monkeypox is endemic. But the other cases appear to have been infected within the UK and are all not linked to the travel-related case by contact or timing. Portugal is investigating more than 20 cases, Spain is reportedly investigating 23 cases, and Italy and Sweden have each reported at least one case.

Disease origins

Monkeypox is a relative of smallpox and produces similar symptoms, but it causes a milder disease than that of the eradicated virus. There are two clades of monkeypox: the West African clade and the Congo Basin clade. The West African clade, which is what has been detected in the UK, is the milder of the two. It is usually a self-limiting infection, though it can cause severe disease in some cases. The case fatality rate has been estimated at about 1 percent. The Congo Basin clade, meanwhile, has an estimated fatality rate of as high as 10 percent. For both clades, children are among those at high risk of severe disease, and infection can be particularly dangerous during pregnancy, causing complications, congenital conditions, and stillbirth.

Monkeypox is not considered easily spreadable among humans. The World Health Organization notes that the longest documented chain of transmission in humans is just six generations—meaning that the last person infected in an outbreak was just six direct links away from the person initially infected. The virus first jumps to humans from animals, and cases are often found at the interface of tropical rainforests. Animals that can harbor the virus include squirrels, Gambian pouched rats, dormice, different species of monkeys, and others, according to the World Health Organization. The main reservoir of the virus is unknown, but it’s thought to be rodents.

From animals, the virus can jump to humans through contact with an infected animal’s blood, other body fluids, or skin lesions. Hunting wild game and bushmeat are the main risk factors. People can also become infected by eating undercooked meat or animal products.

Symptoms and transmission

Symptoms usually develop five to 13 days after exposure, but the incubation period can range from five to 21 days. The disease usually begins with fever and flu-like symptoms, specifically headache, fatigue, muscle aches, and swollen lymph nodes. One to three days later, the characteristic rash emerges. Lesions develop all over the body, but they tend to concentrate on the face and extremities, particularly the palms of the hands and soles of the feet. The rash starts out as lesions that are flat at the base and then become raised and filled with fluid. A crusty scab then forms over each lesion and later falls off. The number of lesions an infected person develops can range from a few to several thousand.

There are no specific treatments for monkeypox, and patient care focuses on alleviating symptoms. The smallpox (vaccinia) vaccine had been estimated to be about 85 percent effective at preventing monkeypox but is currently not widely used. A newer version of the vaccine is also approved to prevent monkeypox but is also not widely used.

Once in humans, the virus spreads from prolonged face-to-face contact via large respiratory droplets, plus direct contact with the skin lesions or contact with materials that have been contaminated by body fluids or lesion fluid, such as linens or clothes. Again, sustained human-to-human transmission is considered limited. It’s also not considered a sexually transmitted disease, but direct contact during sex can spread the virus—which appears relevant in the current eruption of cases.


Many of the cases identified in the current outbreak are in men who identify as gay or bisexual or other men who have sex with men (MSM). In fact, the UK has targeted its public health efforts at sexual health clinics to find many of their most recent cases. Portugal noted in its health advisory that their more than 20 suspected cases are all in men, mostly young men.

“Many of these global reports of monkeypox cases are occurring within sexual networks,” Inger Damon, a poxvirus expert with the US Centers for Disease Control and Prevention, said in a statement late Wednesday.

But although sexual networks appear to be a main pathway for transmission so far, health experts are cautious to emphasize that it’s not the only one. “Anyone, regardless of sexual orientation, can spread monkeypox,” the CDC warned. Inger, along with the agency, urged all clinicians to look out for the disease, regardless of travel or specific risk factors.

While the mysterious community transmission in a variety of countries is raising concern among global public health agencies, international monkeypox cases are not unheard of. Travel-related cases crop up periodically; there were two imported monkeypox cases in the US during 2021, for instance. In 2003, the US saw an outbreak of 47 cases across six states; the eruption was traced to pet prairie dogs that had been housed with monkeypox-infected rodents from Ghana. All 47 cases had direct contact with the infected prairie dogs.

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