Infectious-disease researchers are hoping to return to the Democratic Republic of Congo soon to try to pick up the trail of an exotic infection that is alarming some public health experts: the largest outbreak ever seen in humans of a well-known virus called monkeypox. A first cousin of the once-dreaded smallpox, monkeypox causes nearly identical symptoms.
According to a Research News article in tomorrow's issue of Science,* between February 1996 and February 1997 at least 92 cases of the disease and three deaths were reported in a remote region of what was then Zaire. Only 37 documented cases were reported from 1981 to 1986. These findings have led to intensive discussions about what might be causing the outbreak--in particular, whether the virus is traveling from human to human more readily than before.
Medical epidemiologist Ali Khan of the U.S. Centers for Disease Control and Prevention (CDC) in Atlanta headed an international team, organized by the World Health Organization (WHO) and the Zairian Ministry of Health, that visited 12 villages at the center of the outbreak for 5 days in February, cutting its trip short because of civil unrest. As Khan and colleagues detailed in the CDC's Morbidity and Mortality Weekly Report (MMWR) on 11 April, they believe 73% of the 89 people they studied were infected by other people. That would be a large jump from the 30% "secondary contact" rate reported by Czechoslovakian epidemiologist and monkeypox expert Zdenek Jezek, who did the 1981 to 1986 studies in Zaire. The recent investigation also found one patient who appeared to have been the source of eight other infections, which is twice the highest chain of transmission observed previously. "This is not just an outbreak of some rare exotic disease in the middle of nowhere," says Khan. "I'm personally concerned about what would happen if this disease shows up in a major city."
One explanation for the outbreak, floated by Jezek and others, is that smallpox vaccinations--which also confer immunity to monkeypox--were so successful that they were stopped nearly 2 decades ago, creating an ever-growing pool of people who are susceptible to the animal virus. Immunity is further compromised because the Democratic Republic of Congo has high rates of infection with HIV, which cripples immune systems. The civil war may have contributed another factor: Faced with a rising threat of starvation because of the unrest, villagers may have increased their hunting of animals that carry monkeypox, which include monkeys, squirrels, and rats.
According to virologist Peter Jahrling of the U.S. Army Medical Research Institute of Infectious Diseases in Fort Detrick, Maryland, "the probability is low" that this outbreak will spread further. "But it's not zero," he adds. To get a closer look, WHO hopes to organize another mission in a month if the political situation calms down enough. Khan is anxious to return. "It's important to get back out there and ask what's the magnitude of disease and how does transmission occur," says Khan. "We're all waiting for better data."