ABOVE: Transmission electron tomography of monkeypox virus CDC/Cynthia S. Goldsmith

 Yesterday (May 18), the Centers for Disease Control confirmed that a man in Massachusetts was the first person in the United States to test positive for monkeypox. He had recently traveled to Canada, where he presumably contracted the virus, the Massachusetts Department of Health says in a statement. The case adds to the spate of infections detected at least five European countries and Canada, regions which are outside the disease’s typical stomping grounds.

The outbreak is “rare and unusual,” UK Health and Safety Authority Chief Medical Advisor Susan Hopkins says in a statement released on Monday after two additional UK denizens tested positive for the disease, adding that “exactly where and how they acquired their infections remains under urgent investigation.”

Monkeypox is a rare but potentially serious illness caused by the monkeypox virus, a relative of the smallpox virus (both are Orthopoxviruses and cause similar symptoms). The infection is characterized by flu-like symptoms and enlarged lymph nodes, followed by a rash composed of fluid-filled bumps that can begin in a localized area and spread. The virus got its name when it was first found in laboratory monkeys in 1958 in a facility in Copenhagen, Denmark, though it remains unclear if monkeys are a natural reservoir. In Africa, where the virus is endemic, there are two strains, according to The Guardian: a Central African strain with a fatality rate of up to 10 percent and a West African strain with a much lower mortality rate of between 1 and 3 percent. The latter strain has been identified in the UK patients.

Monkeypox rash on hands
Example of monkeypox rash on a patient during an outbreak in the Democratic Republic of the Congo
CDC/Brian W.J. Mahy

Monkeypox is thought to spread through bites, wild game hunting, or other contact with animals that are infected with the virus, reports The Washington Post. Infections outside of Africa are rare and usually involve travelers with recent exposure to animals in areas where the virus is endemic. For instance, two cases—one in Texas and one in Maryland—were reported in the US last year in people who had recently traveled to Nigeria. 

Yet, over the past month, clusters of cases have been detected in several countries. Health officials in the UK confirmed nine cases there since May 6, while the Spanish Health Ministry confirmed seven cases through PCR testing, with another two dozen or so suspected cases in the Madrid area, according to El Pais. Meanwhile, Portuguese health authorities have confirmed 5 cases and are investigating 15 more potential infections, and Canadian authorities have identified at least 15 suspected cases in Montreal, according to The New York TimesCases have also been confirmed in Sweden and Italy, according to the BBC.

“The fact that it’s in the U.K. in multiple unrelated clusters, plus Spain, plus Portugal, is a surprise,” Tom Inglesby, director of the Johns Hopkins Center for Health Security at the Bloomberg School of Public Health, tells the Times.

The Massachusetts DOH statement notes that while monkeypox does resemble smallpox, it’s less virulent and more difficult to contract. Unlike SARS-CoV-2 and other pandemic-causing respiratory viruses, monkeypox symptoms are highly visible, making isolation of infected people easier, experts tell the Post. When human-to-human transmission occurs, it’s believed the virus spreads predominately via contact with bodily fluids, pus from sores, or items contaminated with those substances—though it can also be transmitted short distances in large aerosolized droplets, which is why the CDC is monitoring six Americans who sat near an infected British patient on a long flight, CDC official Jennifer McQuiston tells STAT

While the first case in the UK was in a man who had recently traveled to Nigeria, the UK Health and Safety Authority notes in their Monday statement that some of the cases could not be explained by travel, indicating community transmission may be occurring. Such transmission is considered extremely rare; according to the World Health Organization’s fact sheet on the infection, the longest known chain of transmission was six successive infections. Experts tell NPR that it’s possible that the unusually high case numbers stem from a more transmissible version of the virus, but more research is needed to confirm this.

Officials are also exploring a hypothesis that the disease is being spread through sexual contact, as four of the seven cases in the UK and 22 of the 23 cases in Madrid appear to have involved sexual contact between men. According to The Conversationwhile this would be the first time sexual transmission of monkeypox has been documented, this mode of spread fits with the virus’s pattern of infecting new hosts during close contact.  

US and European health officials say that doctors should consider monkeypox as a potential diagnosis in people with otherwise unexplained rashes that have either traveled to a country with a confirmed case or have had contact with an infected patient. 

While the current case count is higher than in an average year and questions about the virus remain, Inglesby tells the Post that in the past, outbreaks of the virus have generally been small. “So I think the risk to the general public at this point, from the information we have, is very, very low.” 

See “Texas Monkeypox Case Underscores Need for Better Surveillance