Some strains of monkeypox are more virulent than others, depending on where in Africa the virus came from

An outbreak of 72 cases of monkeypox in the United States during the summer of 2003 didn't produce a single fatality, even though the disease usually kills 10 percent of those infected.

Why did none of the patients die? New research from Saint Louis University Health Sciences Center and several partner institutions may provide an answer.

In this month's issue of Virology, researcher and senior author Mark Buller, Ph.D., from Saint Louis University Health Sciences Center and colleagues conclude that some strains of monkeypox are more virulent than others, depending on where in Africa the virus came from.

"We have at least two biological strains of monkeypox virus - one on the west coast of Africa, and the other in the Congo basin," Buller said. "The 2003 outbreak in the United States was from West Africa. If it had come from Congo, we might have had a bigger problem on our hands and very well might have seen patient deaths."

Researchers from the University of Victoria, Washington University School of Medicine, U.S. Army Research Institute of Infectious Diseases, the University of Alabama, East Carolina University and the Centers for Disease Control and Prevention are co-authors of the research.

Buller said that recent studies suggest the incidence of monkeypox is increasing due to encroachment of human into habitats of animal reservoirs. Monkeypox is classified as a "zoonosis," which means that it is a disease of animals that can be transmitted to humans under natural conditions. The first cases of monkeypox reported in humans involved contact between humans and animals in Africa.

The first outbreak of monkeypox in the Western hemisphere occurred in the U.S. Midwest from April to June of 2003. The virus entered the U.S. in a shipment of African rodents from Ghana in West Africa destined for the pet trade. At a pet distribution center, prairie dogs became infected and were responsible for 72 confirmed or suspected cases of human monkeypox.

"Unlike African outbreaks, the U.S. outbreak resulted in no fatalities and there was no documented human-to-human transmission," Buller said.

Monkeypox is part of a family of viruses that cause human smallpox, cowpox, and camelpox as well as monkeypox. Monkeypox usually produces a less severe illness with fewer fatalities than smallpox. But its symptoms are similar: fever, pus-filled blisters all over the body, and respiratory problems.

"Our finding may explain the lack of case-fatalities in the 2003 monkeypox outbreak in the United States, which was caused by a West African virus," Buller said.