Chicago ? Health officials investigating an outbreak of monkeypox that apparently spread from pet prairie dogs to people said Monday the number of reported cases had risen to at least 40, including four that were confirmed.

It is the first time monkeypox, a smallpox-related virus normally found in Africa, has ever appeared in the United States, according to the Centers for Disease Control and Prevention. Seven people have been hospitalized; no one has died.

Steve Ostroff, deputy director of the CDC’s National Center for Infectious Diseases, said he expected the numbers to rise as human and animal samples were tested.

But Ostroff said that only people who had direct contact with infected prairie dogs, or in one case a rabbit, had come down with the illness.

“For the average citizen, I would not necessarily be concerned at this point of being exposed to monkeypox,” he said.

State health officials have reported 19 suspected cases in Wisconsin, 13 in Indiana and four in Illinois. Four cases have been confirmed in Wisconsin and Illinois.

There have been no instances in this outbreak of the virus being spread from person to person, though that has happened in Africa in the past.

Investigators said the prairie dogs probably were infected with the virus by a Gambian giant rat, which is native to Africa, at a Chicago-area pet distributor.

The Illinois Department of Agriculture, along with state and federal health officials, is trying to track down 115 customers — both individuals and pet stores — that bought exotic animals from Phil’s Pocket Pets since April 15.

Monkeypox in humans is not usually fatal but causes rashes, fevers and chills. Doctors initially feared they might be facing smallpox, which causes similar symptoms.