Federal health officials now count 33 possible cases of monkeypox, an imported viral disease apparently passed from prairie dogs to people.

MONKEYPOX Fact Sheet Q&A CDC Info. WHO Info. Guidelines For Vets, Pet Owners

The Centers for Disease Control and Prevention also reports four confirmed cases of the disease, which is similar to smallpox. Seven people have been hospitalized, but no one has died.

A CDC official expects case numbers to rise as human and animal samples are tested. He said the people who were sickened had direct contact with infected prairie dogs, or in one case, a rabbit. Investigators say the prairie dogs likely were infected by a giant Gambian rat at a Chicago-area pet distributor.

State health officials have reported suspected cases in Wisconsin, Indiana and Illinois. Officials are investigating the source of the infected animals to determine if distributors in other states might be involved.

Although the CDC said close contact with prairie dogs seems to be the primary transmission route, they said human-to-human transmission is still a possibility.

The outbreak marks the disease's first appearance in the Western Hemisphere. It's been found mostly in west African nations.

The human mortality rate in Africa has ranged from 1 to 10 percent, but a Wisconsin health official said the virus may be less lethal in the United States because people are typically better nourished and medical technology is more advanced.

The monkeypox virus is in the same family of viruses as smallpox, and officials said the smallpox vaccine has been known to protect people from both viruses.

CDC experts warn the public to avoid any contact with prairie dogs or Gambian giant rats that appear to be ill -- such as those that are missing patches of fur, have a visible rash on the skin, or have a discharge from eyes or nose. And they said to wash your hands if you have any contact with prairie dogs or Gambian rats -- or any other ill animal.