MADISON, Wis. - Doctors initially feared a smallpox outbreak when they began seeing cases of a mysterious disease that has spread to at least 19 people who came into contact with pet prairie dogs in the Midwest.

The symptoms were alarmingly similar - fever, chills, rashes and swollen lymph nodes, said Milwaukee's health commissioner, Dr. Seth Foldy. It was when the prairie dog connection surfaced that they knew it must be something else.

"We asked the question but discounted it very early," Foldy said Sunday. "Smallpox has never been known to affect another species."

The Centers for Disease Control and Prevention said Saturday pet prairie dogs from a suburban Chicago pet distributor likely are infected with monkeypox, a member of the same viral family as smallpox. The pet rodents may have gotten the disease from another animal at the distributor.

The virus can pass animal-to-animal and animal-to-human, and scientists believe it can pass human-to-human, as well, but it had never been documented in North America, Foldy said.

So far, at least 17 people in Wisconsin and one each in Illinois and Indiana have become sick since early May with symptoms consistent with monkeypox after coming in contact with prairie dogs.

Illinois health officials were investigating three more potential cases in the metropolitan Chicago area, said Jena Welliever, spokeswoman for the state Department of Public Health. The three had contact with a prairie dog and had developed a rash, she said Sunday.

"It eventually will clear up as you treat the symptoms," said Mark McLaughlin, a spokesman for Froedtert Memorial Lutheran Hospital in Milwaukee, which has treated several patients with the symptoms.

"We don't need people to go off the deep end. This is not an epidemic in the public's common perception of that," he said.

Of the people infected, two remained isolated at the hospital in satisfactory condition Sunday, McLaughlin said. He said doctors treating them are wearing masks as a precaution.

Both Wisconsin and Illinois banned the sale or importation of prairie dogs, and officials urged people not to release prairie dogs for fear of spreading the disease to other wildlife.

If the disease gets a foothold in indigenous North American species, it could become almost impossible to control, Foldy said.

"We don't want that happen," Foldy said. "It would have an unknown impact that I'd prefer not to find out."

The death rate from monkeypox in Africa ranges from 1 to 10 percent, he said. However, he said the mortality rate might not reach those levels in the United States, where people are typically better nourished and medical technology is more advanced.

"The person-to-person transmission in a rural Congolese setting is potentially very different than in a metropolitan American setting," Foldy said. "We have isolation, soap, running water, sterile dressing materials, we have washing machines. These are all things that have reduced the prevalence of germs that are spreadable by person-to-person contact."

Federal health officials believe the prairie dogs may have been infected with monkeypox by a Gambian rat at a Villa Park, Ill., pet distributor.

The owner of Phil's Pocket Pets has given Illinois officials a list of everyone who bought prairie dogs, Gambian rats or other exotic animals since April 15, according to the Illinois public health department. A telephone number for Phil's Pocket Pets could not be found Sunday.

South Milwaukee pet distributor SK Exotics purchased some of the prairie dogs and sold them to two pet stores in the Milwaukee area.

Mike Hoffer, owner of Hoffer TropicLife, said he got a shipment of 10 prairie dogs from SK Exotics on May 5 but didn't sell any after his staff noticed they were sniffling. Seven animals were euthanized Wednesday and the others died earlier of a respiratory ailment, he said.

He said he has sold 20 to 25 prairie dogs a year over the last decade.

"They're cute," Hoffer said. "They fall into place someplace between a guinea pig and a rabbit. I think people are maybe blowing it out of proportion."

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Wisconsin Department of Agriculture: http://datcp.state.wi.us/index.jsp