The incidence of a smallpox-like disease—caused by the monkeypox virus—has increased 20-fold in the Demoncratic Republic of Congo (DRC) over the past 30 years, according to new research published online in the journal PNAS. The findings suggest that, as smallpox vaccination programmes ceased in the DRC in 1980, people are now immunologically ‘naïve’ to orthopoxviruses (including monkeypox and smallpox viruses) and could be at an increased risk of infection by this family of viruses.
The monkeypox virus can cause a serious zoonotic disease (potentially transmitted to humans from rodent species such as squirrels as well as primates such as monkeys) that is similar to smallpox. Smallpox vaccinations not only helped eradicate the smallpox virus, but also provided cross-protective immunity to monkeypox virus infection.
Anne Rimoin and colleagues assessed the burden of human monkeypox in the DRC by analyzing surveillance data for this disease from 2005–2007 in regions known to be endemic for the virus, a feat which had not been conducted since the early 1980s. In their up-to-date surveillance data across nine health zones in central DRC, 760 human monkeypox cases were confirmed by laboratory tests and the researchers found that the average annual cumulative incidence of monkeypox virus infection was 5.53 cases per 10,000 people across all zones. Men, children under the age of 15 years and people living in forested areas or who had not received a prior smallpox vaccination were at an increased risk of monkeypox infection whilst those who had been vaccinated against smallpox had a 5.2-fold lower risk of infection by the monkeypox virus than unvaccinated individuals. Rimoin et al. then compared the 1980s active surveillance data with their new 2000s findings from the same health zone and found that the incidence of human monkeypox in this zone increased substantially from 0.72 cases per 10,000 people in the 1980s to 14.42 cases per 10,000 people between 2005 and 2007.
The investigators add that “entire households are now mostly or completely [smallpox] vaccine naive”, which could result in increased human-human transmission between different generations within the same house. Furthermore, Rimoin and colleagues argue that because of several limitations (including poor access to remote areas) their study could have under-reported the true incidence of human monkeypox and the observed dramatic increase in disease incidence could in fact be a “conservative estimate”. Improved and continued disease surveillance will be needed to assess the true burden of monkeypox virus infection on public health in African populations, and could aid the development and implementation of strategies to reduce the risk of monkeypox virus infection.
Anne W. Rimoin, Prime M. Mulembakani, Sara C. Johnston, James O. Lloyd Smith, Neville K. Kisalu, Timothee L. Kinkela, Seth Blumberg, Henri A. Thomassen, Brian L. Pike, Joseph N. Fair, Nathan D. Wolfe, Robert L. Shongo, Barney S. Graham, Pierre Formenty, E, & Major (2010). Major increase in human monkeypox incidence
30 years after smallpox vaccination campaigns
cease in the Democratic Republic of Congo Proceedings of the National Academy of Sciences USA (Published ahead of print 30th August 2010) DOI: 10.1073/pnas.1005769107