ATLANTA - The U.S. government banned the sale of prairie dogs, prohibited the importation of African rodents and recommended smallpox shots Wednesday for people exposed to monkeypox, the exotic African disease that has spread from prairie dogs to people.

The smallpox vaccine can prevent monkeypox up to two weeks after exposure to the virus but is most effective in the first four days.

"We're optimistic we can deliver the vaccine to these people in time to do good," said Dr. David Fleming, deputy director for Public Health and Science at the Centers for Disease Control and Prevention.

The government's aggressive response to the disease came the same day that the federal investigation of the monkeypox outbreak was expanded to eight more states, bringing the total to 15.

This is the first outbreak of monkeypox in the Western Hemisphere.

"We must do everything we can to protect persons who are exposed to monkeypox in the course of investigating or responding to the outbreak," CDC director Dr. Julie Gerberding said.

The Department of Agriculture will be in charge of enforcing the prairie dog ban, which also prohibits transporting the animals. Gambian rats and five other types of large African rodents were banned because a Gambian rat is believed to have spread the virus to prairie dogs, which are native to the American Plains.

Fleming said the smallpox vaccine is 85 percent effective against monkeypox. The smallpox vaccine is widely available because states stocked up on it out of fear of bioterrorism.

"State health departments have been actively involved in planning and preparing for the possibility of a bioterrorist event," Health and Human Services Secretary Tommy Thompson said. "We are now seeing that this level of preparation can also assist in unexpected, natural outbreaks."

The CDC said health care workers, veterinarians and family members who have cared for or had close contact with infected people or animals should get vaccinations. The agency also warned veterinarians and doctors to be on the lookout for the symptoms, especially in owners of prairie dogs or exotic rodents from Africa.

Monkeypox-infected prairie dogs distributed from Phil's Pocket Pets of Villa Park, Ill., may have been sold to numerous buyers in 15 states since April 15, according to a Department of Agriculture emergency warning issued Wednesday.

The states where possibly infected prairie dogs were being sought were Kentucky, Florida, Tennessee, Mississippi, Massachusetts, Michigan, New Jersey, New York, Illinois, Indiana, Wisconsin, Pennsylvania, Texas, Ohio and South Carolina.

As of Wednesday, health officials had confirmed a total of nine human cases of the disease -- four in Wisconsin, four in Indiana and one in Illinois. Fifty possible cases had been reported -- 23 in Indiana, 20 in Wisconsin, six in Illinois and one in New Jersey, CDC spokesman Tom Skinner said.

No one has died of the disease.

Monkeypox, which produces pus-filled blisters, fever, rash, chills and aches, is a milder relative of smallpox. It has a mortality rate of 1 percent to 10 percent in Africa, but U.S. officials believe better nutrition and medical treatment here probably will prevent deaths.