Enlarge / A 2003 photo of the arms and legs of a 4-year-old girl infected with monkeypox in Liberia. (credit: Getty | BSIP )

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.