Few human accomplishments match the eradication of smallpox virus in the late 1970s. It was, and still is, the only disease wiped from the face of the Earth. However, three decades later, a related virus, monkeypox, is on the rise in parts of central Africa – precisely because of the success of smallpox eradication.
As the proportion of the smallpox-vaccinated population has fallen in the Democratic Republic of Congo (DRC), monkeypox infections have risen, according to a study to be published this week in Proceedings of the National Academy of Sciences.
Monkeypox isn’t as severe as smallpox, but it can cause similar symptoms and the same telltale skin lesions. Monkeypox kills an estimated 1 to 10% of those who contract it, and the virus is transmitted primarily by contact with rodents, pets and primates, though human-to-human spread is also possible.
Scientists first discovered the virus in the 1950s, and the first human case was reported in the early 1970s. However, smallpox vaccination had a knock-on effect on monkeypox and offered immunity to it. “The immediate threat after smallpox eradication wasn’t clear,” says David Heymann, who heads the Centre on Global Health Security at Chatham House in London and was involved in smallpox eradication.
The World Health Organization kept tabs on the disease through the early 1980s, but monitoring has been patchy since, says Anne Rimoin, of the University of California Los Angeles School of Public Health School. “Monkeypox emerged under the radar due to the lack of surveillance.”
To help fill this gap, her team gathered data on monkeypox infections in nine health zones in the DRC and counted at least 760 confirmed cases that occurred between November 2005 and November 2007. The vast majority of infections were in people born since vaccination ceased in the DRC in 1980. Vaccinated people were 5.2 times less likely to develop monkeypox than those who did not receive the vaccine.
Most infections continue to come from contact with animals, but human-to-human transmission is likely to be on the rise, Rimoin says. “Monkeypox right now is a rural disease but the potential has increased for monkeypox to become an urban disease.”
The disease is also found in several other central African countries, such as Sudan and Gabon, while a 2003 outbreak in the United States transmitted by imported rodents shows that monkeypox can pop up elsewhere, says Rimoin. She calls for increased monkeypox surveillance in central Africa, paying particular attention to spread between people. Widespread vaccination would be difficult to implement in the rural areas where monkeypox is most common.
“This is something that’s changing in its epidemiology, and it needs to be watched,” Heymann says.
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