July 15, 2005
Friday

Researchers say they've solved a medical mystery arising from the outbreak of monkeypox that hit the Midwest two years ago.

Although the virus is reported to kill about 10 percent of those who are infected in Africa, the illness spread by exotic pet rodents to at least 72 people in the Midwest claimed no lives.

The researchers now believe the disease was milder because the rodents carried a weaker strain of the virus - something they hadn't known existed.

"We have at least two biological strains of monkeypox virus - one on the west coast of Africa and the other in the Congo basin," said Mark Buller, a researcher at St. Louis University and senior author of the study published online Friday in the journal Virology.

"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."

Monkeypox is a close relative of smallpox, and vaccination against that virus seems to offer some protection, but there's no proven, safe treatment for monkeypox once it starts.

Considered a rare disease that has occurred mainly in the rainforest countries of central and West Africa, the virus was first discovered in monkeys in 1958, and the first human cases were detected in 1970.

With the U.S. outbreak - the first-ever reported outside Africa - Buller and colleagues from the federal Centers for Disease Control and Prevention, the U.S. Army Research Institute for Infectious Diseases and several other universities took a closer look at the virus.

Other genetic analysis had shown that strains from the Congo and West Africa were genetically distinct. The new research showed that the Congo strain, which is responsible for about 90 percent of reported human cases, is more virulent.

After mapping the genes from several West African specimens and comparing them to the Congo strain, the researchers identified six specific genes that are likely most responsible for the greater death toll.

Buller said recent studies in Africa suggest that the incidence of monkeypox is increasing as more humans encroach into habitats where animals carry the disease. The CDC traced the 2003 U.S. outbreak to a shipment of exotic African rodents, including Gambian giant rats, to a Texas wholesaler.

During shipping, some rodents were held close to a group of prairie dogs that went to a Chicago pet distributor. The prairie dogs then went on to pet shops and exotic-animal fairs throughout the region.

From that April through June, monkeypox cases linked to handling or even just breathing the air near the prairie dogs were reported in Wisconsin, Illinois, Indiana, Kansas, Missouri and Ohio.

Although monkeypox typically produces a less severe illness than smallpox, the symptoms are similar - fever, swollen lymph nodes, respiratory problems and pus-filled blisters spread across the body. The disease usually lasts for about a week, and research in Africa has shown mortality rates ranging from 1 percent to 10 percent.

Since the outbreak, the CDC has banned shipments of rodents from Africa that may carry the disease, as well as prairie dogs.

On the Net:

www.cdc.gov/ncidod/monkeypox/

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