Cases of monkeypox, a disease caused by a DNA virus closely related to smallpox and cowpox, have increased dramatically in rural villages in the heart of the Democratic Republic of Congo (DRC), according to researchers working in the war-ravaged African country.

A monkeypox patient in Lomela, Congo.The patient, who was examined by epidemiologist Anne Rimoin, eventually died from monkeypox-related complications. Image courtesy of Anne Rimoin

Reporting their results online at PNAS, an international team of scientists found that within one area the average annual incidence of monkeypox between November 2006 and November 2007 increased by about 20 times compared to the average annual incidence recorded in the 5 years between 1981 and 1986, the last time scientists actively monitored the study population.

Though monkeypox is seldom fatal, the alarming increase in the DRC, where monitoring is sporadic at best, means that the disease has the potential to emerge as one that is more deadly and spreads faster, according to Anne Rimoin, the University of California, Los Angeles, epidemiologist who led the research team. "Each infection gives the virus the opportunity to evolve into a more virulent variant," Rimoin told The Scientist. "We're worried about what could happen. This study is a warning bell."

Anecdotal evidence has indicated for years that when global smallpox vaccination campaigns halted in the late 1970s, human monkeypox -- to which the smallpox vaccine lends cross protection -- was rare.

Rimoin's data strongly suggests that since smallpox vaccinations stopped, the incidence of monkeypox has been creeping back up, thanks in part to several species of wild animals serving as reservoirs. "The data do tell us that it appears that when smallpox immunization was common, monkeypox was acquired less often and transmitted less often," Don Burke, director of the University of Pittsburgh's Center for Vaccine Research who was not involved in the study, told The Scientist. Indeed, Rimoin's team found that people vaccinated for smallpox living in the study area had more than a 5-fold lower risk of monkeypox than unvaccinated individuals.

Monkeys are a common food source and are easily found in local villages. But they may harbor infectious diseases. Image courtesy of Anne Rimoin

Though the original animal host of the virus that causes monkeypox is unknown, African squirrel, rat, mice, shrew, dormouse, and primate species are reservoirs. Transmission from animal to human likely occurs when people are bitten or come into contact with the blood or other body fluids of an infected animal. The human populations that Rimoin and her colleagues studied live near the tropical forest habitats of these animals and frequently eat them. With more migration from the countryside into cities and more so-called "bushmeat" starting to appear in markets in larger urban centers, such as DRC's capital Kinshasa, the monkeypox virus could spread beyond rural populations, Rimoin said. "It's only a matter of time that infected rodents are sold and distributed in larger cities."

In 2003, almost 100 people in the US contracted monkeypox after prairie dogs sold as pets came in contact with an infected shipment of African rodents, according to the US Centers for Disease Control and Prevention. None of the infected Americans died, but the incident "showed that the virus is capable of spreading to new animal reservoirs outside of central Africa," Rimoin noted.

But infection spreading from animals to humans is only part of the problem. The more worrying aspect of the disease's spread is human-to-human, or secondary, transmission, according to Burke. Rimoin said that her team wasn't able to track the rise in secondary transmission, but that she plans to conduct these studies soon. "When we see an increase that is so great, it suggests that human to human transmission may have also increased."

Rimoin discusses the dangers of bushmeat hunting with local hunters in the Sankuru district. Image courtesy of Anne Rimoin

In addition, Rimoin said she plans sequence the virus samples she collected in DRC and compare them to samples collected in the 1980s to see if the monkeypox virus has been evolving in the last thirty years into become more transmissible or more virulent.

In the meantime, Rimoin suggested that the best way to stem the spread of the virus within the DRC is to launch educational campaigns that address proper handling of potential reservoir species and healthcare practices that prevent human-to-human transmission in the country. Though revisiting a widespread smallpox vaccination effort would almost certainly curb infection rates, she said that this would be virtually impossible. "At present the logistics of this are great and the expense would also be great," Rimoin said.

For now, monitoring of the situation is essential, agree Burke and Rimoin. "This is a perfect example of positioning ourselves to be able to predict and prevent at an early stage rather than waiting for a full blown epidemic," said Burke.

A. Rimoin, et al., "Major increase in human monkeypox incidence 30 years after smallpox vaccination campaigns cease in the Democratic Republic of Congo," PNAS, doi: 10.1073/pnas.1005769107, 2010.

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