The cases would have been the first known U.S. human-to-human transmissions of the virus. Monkeypox, a disease related to smallpox but less lethal, appeared in the country for the first time when at least a dozen people had contact in recent weeks with infected pet prairie dogs.
A nurse in Milwaukee and a medical assistant in Marshfield showed symptoms similar to those from monkeypox after treating patients with the disease. The boyfriend of the medical assistant also showed symptoms.
Jeff Davis, Wisconsin's chief epidemiologist, said further examinations suggest the three do not have the disease, but the state was awaiting test results from the federal Centers for Disease Control and Prevention to rule it out.
"We're not saying anything final until we get those test results," Davis said.
The federal government has recommended smallpox shots for people exposed to monkeypox, and Davis said Friday that Wisconsin officials will begin offering the vaccine Saturday to health care workers and others who have had close contact with infected animals or humans.
The vaccine can prevent the disease up to two weeks after exposure to the virus and is most effective in the first four days. But some health officials are wary because the vaccine in rare cases can cause serious and even fatal side effects.
The New York Times reports in its Saturday editions that contact with one prairie dog may have accounted for 18 human cases of monkeypox in Wisconsin.
The people -- three of whom had confirmed cases, six with probable cases and nine with suspected cases -- were exposed to the prairie dog as it was moved from place to place, Davis told the Times. "The prairie dog was a super-transmitter if there ever was one," Davis said. "The prairie dog was moved quite a bit from a pet store to a household to one veterinary clinic and then to a second veterinary clinic before it died," he told the newspaper.
But acting state veterinarian Robert Ehlenfeldt was more cautious, telling The Associated Press the prairie dog probably was not more contagious than other prairie dogs with the disease; it just had more contact with humans.
The virus that causes monkeypox is usually transmitted by contact with lesions on the skin. But federal health officials have recommended that health care workers use protections against the possibility of airborne transmission, the Times says. Wisconsin officials said that in a small number of cases those who got the virus from an animal did not appear to have had direct contact with that animal, the Times notes.
One Wisconsin resident who apparently caught monkeypox from the prairie dog had "minimal" contact with the infected animal but slept in the same room as the caged animal, the Times quotes Davis as saying. Also, "a substantial number of individuals in one particular veterinary clinic became ill as a result of" direct contact, he said. "We are trying to understand what that all means, precisely what the contact was."
By Friday afternoon, state health officials around the country had confirmed 12 human cases of monkeypox: four each in Wisconsin, Indiana and Illinois. Also, 71 possible cases had been reported - 22 in Indiana, 30 in Wisconsin, 15 in Illinois, two in Ohio and one each in Arizona and Kentucky.
Another possible case, involving an 11-year-old New Jersey boy, also had been reported, but the CDC told state officials Friday that he actually had the common childhood ailment chickenpox.
No one has died of monkeypox in the United States, but at least 14 patients with symptoms have been hospitalized, including a child in Indiana with a confirmed case who also has encephalitis, or brain inflammation. Indiana State Department of Health spokesman Matthew McCardle said Friday the girl was recovering.
The human mortality rate from monkeypox in Africa has ranged from 1 percent to 10 percent, but the virus may be less lethal in the United States because people typically are better nourished and medical technology is more advanced, according to the CDC.
Monkeypox, a disease never before seen in the Western Hemisphere, causes pus-filled blisters, rashes, chills and fever.
Federal health officials have traced the outbreak to prairie dogs distributed by Phil's Pocket Pets of Villa Park, Ill. The prairie dogs, which may have been sold to buyers in 15 states, were apparently infected at the business by a Gambian giant rat, a native of Africa, officials said.
Davis said the prairie dogs appeared to have infected people through bites or when people rubbed their eyes or noses after touching discharges from the animals.
Medical facilities have been receiving calls from concerned owners of prairie dogs, which have become increasingly popular pets. But people generally have remained calm, said Mark McLaughlin, spokesman for Froedtert Memorial Lutheran Hospital in Milwaukee, which has treated seven patients with suspected monkeypox.
"I don't see this creating any sort of public panic and it shouldn't," McLaughlin said.