As the number of monkeypox cases continues to rise globally, health experts are investigating a 2017 outbreak in Nigeria to help them better understand the recent spread of the virus.

How big is the threat from monkeypox? Here's what experts say.

Where monkeypox cases currently stand

In recent weeks, monkeypox, which is rarely found outside of Central and Western Africa, has infected almost 600 people in more than 25 countries, the Wall Street Journal reports.

So far, the United States has reported 21 confirmed cases across 11 states, according to CDC. Notably, most infected individuals have no travel history to regions where monkeypox is endemic.

Last month, the Massachusetts Department of Public Health reported the first confirmed case this year of monkeypox in the United States in a man who had recently traveled to Canada. Since then, other cases have been reported across several different states, including California, Colorado, Florida, New York, Utah, Virginia, and Washington. No deaths have been reported from any of the confirmed cases.

On Thursday, Chicago, Philadelphia, and Los Angeles County all reported their first monkeypox cases.

In May, CDC issued an alert encouraging doctors and state health departments to remain vigilant for potential monkeypox cases. In particular, CDC said that clinicians "should be vigilant to the characteristic rash associated with monkeypox."

At the moment, experts say the general public's risk of monkeypox is relatively low—but it may not stay that way. According to the World Health Organization (WHO), "immediate action from countries is required to control further spread among groups at risk, prevent spread to the general population and avert the establishment of monkeypox as a clinical condition and public health problem in currently non-endemic countries."

"Currently, the overall public health risk at [a] global level is assessed as moderate considering this is the first time that monkeypox cases and clusters are reported concurrently in widely disparate WHO geographical areas," WHO said in a statement. In addition, the agency said that, given the wide geographic scope of cases, the virus "may have been circulating unrecognized for several weeks or longer."

Turning to Nigeria for clues

In September 2017, health officials at the Nigeria Center for Disease Control were alerted to a suspected case of monkeypox in an 11-year-old boy. This marked the first time a person had been diagnosed with the virus in Nigeria since 1978, the Journal reports. In the following weeks, Nigerian health officials identified dozens more suspected or confirmed cases around the country.

Previously, monkeypox had primarily been detected in Central Africa among individuals who were infected by the wild animals they hunted, handled, or ate.

However, in the Nigerian outbreak, monkeypox was spreading from person to person, infecting people in urban and suburban areas. The outbreak was eventually contained by Nigerian public health officials who identified cases and urged infected individuals to isolate—but the virus did not disappear. Almost every year since the 2017 outbreak, Nigeria has reported a few dozen cases.

In some instances, people traveling from Nigeria even carried the virus to other countries, including the United Kingdom, Israel, and Singapore.

In the current outbreak, only a few cases have a travel history to the region, and it is not clear whether they are linked to other cases.

Adesola Yinka-Ogunleye, an epidemiologist at the Nigeria Center for Disease Control, said she was puzzled by the virus' behavior in 2017. Many epidemiologists, including Ogunleye, theorized that monkeypox had been circulating among animals for a long time and emerged in 2017 after the smallpox vaccine triggered a drop in population immunity against monkeypox.

According to Ogunleye, health officials likely missed an opportunity to gather clues to explain why the virus is suddenly spreading around the world. Ogunleye noted that efforts to better understand the virus' behavior have been hindered by a lack of funding and technical capacity, as well as the Covid-19 pandemic.

"If more attention had been paid to the outbreak in Nigeria, perhaps we'd have been able to answer more of the questions that currently still remain unanswered," Ogunleye said.

Notably, disease experts have said their knowledge of monkeypox is limited in several ways. For example, Rosamund Lewis, WHO's technical lead for monkeypox, said it was unclear whether an asymptomatic individual can be infected and spread monkeypox.

Experts also have an incomplete understanding of the severity of the disease and its incubation period, Lewis added. While experts believe the virus spreads primarily in close contact through skin lesions, it is unclear whether it also spreads other ways.

Despite limitations, Nigerian researchers were able to gather some helpful information on the virus. At the onset of the 2017 outbreak, Nigeria did not have any genomic-sequencing capacity, meaning they had to send samples to laboratories in other areas for analysis.

Since then, the country has developed that capability and has been sequencing monkeypox samples. According to Ogunleye, she and her colleagues plan to share the data with researchers around the world, which should help scientists gain a better understanding of the virus' evolution in recent years.

"Investment in research in Africa has been limited," Lewis said. "That may change now." (Roland, Wall Street Journal, 6/2; Breslin, The Hill, 6/2)