Monkeypox has been found in the US and Spain following an initial outbreak in the UK, health authorities have confirmed, with further suspected cases being investigated in Canada.

The outbreak began in the UK, with the World Health Organization (WHO) notified of two confirmed and one probable case of monkeypox. Since then, further cases have been confirmed in the UK, believed to have been acquired in the country, taking the total to nine. Probable cases – and some confirmed – have now been identified in Spain, with 23 patients in the region of Madrid showing signs of the disease.

The Massachusetts Department of Public Health yesterday confirmed that a case had been found in the US: a man who recently traveled to Canada, where 13 suspected cases are being investigated. "The case poses no risk to the public, and the individual is hospitalized and in good condition," the department added in a statement.

Monkeypox is a rare viral infection first seen in monkeys in 1958. It made the jump to humans in 1970 in the Democratic Republic of Congo. The disease is similar to smallpox – although much milder. Patients show fever, muscle aches, chills, and exhaustion, plus more visible symptoms later on such as macules, papules, vesicles, pustules, and scabs.

Generally, the disease is low-risk, causing mild illness. However, monkeypox can be fatal, especially in younger patients. Though concerning, the risk to the population remains low, as the disease generally does not spread easily from person to person.

"Human-to-human transmission is thought to occur primarily through large respiratory droplets," the US Centers for Disease Control and Prevention (CDC) write on their website. "Respiratory droplets generally cannot travel more than a few feet, so prolonged face-to-face contact is required."

Contact with body fluids, material from lesions, or clothing contaminated with lesion material are also known methods of transmission. There has been speculation that the disease has been transmitted via sexual intercourse after a number of current patients are men who identify as gay or bisexual.

“Generally speaking, monkeypox is spread by respiratory transmission, but the characteristics of the 23 suspected cases point towards transmission through mucus during sexual relations,” Madrid’s regional health department said in a statement. 

The Massachusetts Department of Public Health repeated advice that clinicians should consider monkeypox for unexplained rashes if the patient has recently traveled to a country with suspected cases, if they have come in close contact with a suspected case, or if a male patient reports sexual contact with other men.

However, experts say it is too early to speculate that monkeypox is not spreading through the usual routes of transmission.

“I would urge some caution at this stage before concluding that monkeypox has morphed into a Sexually Transmitted Infection (STI). Monkeypox is not particularly transmissible and the number of cases to date where the route of transmission is known remains relatively small," Francois Balloux, Professor of Computational Systems Biology and Director of UCL Genetics Institute, University College London said in a statement.

“What is known is that monkeypox is transmitted between people in close contact through body fluids, respiratory droplets, lesions or even contaminated materials such as bedding. ‘Sexual contact’ very generally qualifies as ‘close contact’.

“Thus, the recent observations of apparent transmission through sexual contact in the UK do not necessarily imply any recent change in the virus’ route of transmission.”