Monkeypox May Have Been Spreading Silently for Years, Mistaken for STIs

As the total number of monkeypox cases around the world rises to several hundred, scientists have said the disease may have been spreading undetected for weeks, months, or years before the first cases outside of Africa were reported in this year's outbreak.

Now, 780 confirmed cases of monkeypox have been reported since the outbreak began on May 13 this year (to June 2), with at least 27 World Health Organization (WHO) member states reporting cases, including the U.S. and Canada.

Investigations are ongoing to find out more about why monkeypox is spreading through countries where it does not normally occur—the disease is endemic to African countries such as the Democratic Republic of the Congo, Cameroon, and the Central African Republic, where thousands of cases are reported each year.

Lab worker
A stock photo shows a scientist working with specimen samples in a lab. Monkeypox may have been spreading undetected before the current outbreak, it has been suggested. dusanpetkovic/Getty

However, in an outbreak update issued last week, the WHO said the "sudden and unexpected" spread of monkeypox in several non-endemic countries suggested that "there might have been undetected transmission for some unknown duration of time followed by recent amplifier events."

Rosamund Lewis, WHO technical lead for monkeypox, said during a briefing on Wednesday last week that this undetected transmission may have been going on for "weeks, months, or possibly a couple of years".

Amesh A. Adalja, a senior scholar at the Johns Hopkins Center for Health Security, told NBC News last weekend: "What's likely happened is an endemic infectious disease from Africa found its way into a social and sexual network and then was greatly aided by major amplification events, like raves in Belgium, to disseminate around the world."

Adalja added that the skin marks associated with monkeypox are being confused with other sexually transmitted infections which could be delaying diagnosis.

Meanwhile, scientists are working to better understand whether there has been some genetic change to the virus that may have enabled it to start spreading outside of Africa more effectively than before.

Monkeypox is a disease caused by a virus of the same name that was first found in monkeys kept for animal testing in 1958. The first human case was detected in 1970.

Symptoms of the disease are similar to but milder than smallpox. The incubation period—the interval between infection and symptoms—is usually from six to 13 days.

Symptoms usually start with fever, headache, other body aches, swelling of the lymph nodes and lack of energy, according to the WHO. Within a day or a few days after the fever, a skin rash appears that can affect various parts of the body.

Symptoms usually last from two to four weeks. Historically, the case fatality ratio of monkeypox has ranged from zero to as high as 11 percent in the general population, though in recent times it's been around 3 to 6 percent.

Monkeypox can spread between people through direct contact with infectious sores, scabs, or bodily fluids. It can also be spread through prolonged face-to-face contact as well as during sex, according to the U.S. Centers for Disease Control and Prevention (CDC).

It is not known if it can spread through semen or vaginal fluids.

The disease can also spread from animals to humans, such as through a bite or scratch or by handling wild game. More information on transmission can be found on the CDC's website.

The current and ongoing outbreak is the first time that many monkeypox cases have been reported at the same time in non-endemic countries around the world. Cases so far have mainly, but not exclusively, involved men who have sex with men.