Stopping smallpox vaccinations has cleared the way for the emergence of monkeypox in Africa

It was 30 years ago this year that the last smallpox vaccination was given worldwide, after the disease had officially been declared eradicated. But the elimination of smallpox vaccinations has, ironically, led to the emergence of a related--albeit less infectious--disease known as monkeypox. New findings reported Monday indicate that the prevalence of monkeypox infections in the Democratic Republic of the Congo has increased 20-fold since the halt of the smallpox vaccination campaign.

The monkeypox virus was first identified in the 1950s and the first confirmed human case was reported in the early 1970s. It got its name because it was first seen in laboratory monkeys, but its primary reservoirs in the wild are thought to be squirrels and other rodents. The symptoms of a monkeypox infection are similar to those of smallpox, with the most distinctive being lesions or pox on the skin. The virus is thought to kill 1% to 10% of those it infects, and there is no treatment. The vaccinia vaccine used to prevent smallpox is estimated to protect against 80% to 85% of monkeypox infections because of the close similarity of the viruses.

Most cases have been seen in the Democratic Republic of the Congo, but recent outbreaks have also been reported in the neighboring Republic of the Congo and Sudan. A 2003 outbreak in the United States affected an estimated 93 people in the Midwest. The source of that outbreak was ground squirrels infected by a rodent imported from Africa.

The World Health Organization monitored monkeypox in the Congo in the early 1980s, but effectively lost interest when there seemed to be few cases. The agency recommended against further vaccination programs with the smallpox vaccine because it did not see a high likelihood of human-to-human transmission of monkeypox. Because of the lack of health infrastructure in the Congo, monkeypox “fell under the radar,” said epidemiologist Anne W. Rimoin of the UCLA School of Public Health. In 2007, Rimoin founded Congo BioMed to promote biomedical research, training and sustainable health programs in the Democratic Republic of the Congo. She specializes in studying the cross-species transmission of disease.

Rimoin and her colleagues in the U.S. and Congo studied 760 laboratory confirmed cases of monkeypox in nine regions of the country between November 2005 and November 2007. They reported in the Proceedings of the National Academy of Sciences that the average incidence during the period was 14.42 cases per 10,000 people, compared with an incidence of 0.72 cases per 10,000 determined by the WHO in the early 1980s.

Monkeypox occurred throughout the year with no seasonality. Zones with the greatest amount of forest cover had the largest number of cases. The average age of victims was 11.9 years and 92.1% of the cases were in people born after smallpox vaccination ended. Males were more likely to be infected than females. Only 3.8% of the monkeypox victims had been vaccinated against smallpox, compared with 26.4% of the overall population.

The true incidence may be substantially higher, the researchers said, because they could confirm only those cases where they visually observed the presence of pox. They did not have equipment to identify antibodies to the virus in people who said they had had it recently. The researchers were also not able to reach some reported cases because of their remoteness.

-- Thomas H. Maugh II / Los Angeles Times