Cases of monkeypox have been identified for the first time in North America, the Centers for Disease Control and Prevention (CDC) reports. The disease, which is similar to smallpox but less infectious and less deadly, had previously been found primarily in West and Central Africa. So far, the CDC is investigating 33 potential cases from three states--Wisconsin, Illinois and Indiana--and as of Monday afternoon, four cases had been confirmed through laboratory testing.

Researchers have tracked the outbreak to prairie dogs that were sold as pets from stores in the Milwaukee area and at a pet swap meet in northern Wisconsin. A Milwaukee animal distributor bought the infected animals, together with a sick Gambian giant rat, from a distributor in northern Illinois. According to the CDC, all of the people suspected of having monkeypox reported close contact with prairie dogs, either in a pet shop, a veterinary setting or in a household that had one of the animals as a pet.

Efforts are under way to trace other prairie dogs and rats that may be carrying the virus. "We're trying to track down these animals to eliminate the possibility that we will set up a persistent reservoir of the disease [in North America]," notes Stephen Ostroff of the CDC's National Center for Infectious Diseases. Investigators are particularly concerned that the prairie dogs could transmit the virus to other wild animals, including rodents and rabbits, that could entrench it further in the U.S. "At this point, we don't know how many animals or humans may be involved," Ostroff says. "And we don't know the extent and scope of the problem in terms of the number of affected areas."

The primary route of transmission is thought to be close contact between an infected prairie dog and humans. (Gambian giant rats are imported from Africa as exotic pets and may have transmitted the virus to prairie dogs, which are found throughout North America.) In some cases, a bite from an infected animal led to the development of illness. In others, handling of an animal with skin lesions was enough to transmit the virus. The CDC notes that the possibility of human-to-human transmission cannot be ruled out at this time, but this is thought to be unlikely. "We have to be a little cautious because we dont necessarily have all the information that we need at this point about how exactly the virus may be transmitted," Ostroff comments. "But it certainly looks to us like direct contact with one of these ill animals appears to be required."