The Centers for Disease Control and Prevention announced the measures Wednesday, the same day federal investigators searched for infected prairie dogs in eight more states, bringing the total to 15.
The smallpox vaccine can prevent monkeypox — an exotic African disease that has spread from prairie dogs to humans — 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, the CDC's deputy director for Public Health and Science.
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-four possible cases had been reported — 25 in Indiana, 17 in Wisconsin, 11 in Illinois and one in New Jersey, CDC spokesman Tom Skinner said. No one has died of the disease.
"In Africa, monkeypox is fatal in as many as 10 percent of people who get the disease," the CDC said on its Web site Thursday. "The case fatality ratio for smallpox was about 30 percent before the disease was eradicated."
Fleming said he is confident the outbreak will be controlled.
"Monkeypox is a disease that is potentially transmissible from person to person but at a fairly low level," he said. "I don't anticipate the same kind of problem that we anticipate from SARS."
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, rodents 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. More than 37,000 health workers in the United States have been vaccinated.
"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.
CDC officials didn't know how many people would have to be vaccinated, but Fleming said he expected the number to be modest.
About 40 out of every million people given the smallpox vaccine for the first time will face a life-threatening injury, and one or two will die.
Still, the CDC is recommending vaccinations even for pregnant women, children and people with eczema — for whom the vaccine is usually discouraged — who have been exposed to infected prairie dogs.
"Because of the real risk here ... we're recommending a somewhat aggressive approach of who should get the vaccine," Fleming said.
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.
Health officials from Kentucky and New York said Wednesday they have found no infected prairie dogs.
Later Wednesday, health officials in Mississippi said they had ruled out a possible threat, saying two animals shipped to the state from the Illinois pet shop turned out to be a pair of healthy flying squirrels.
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.
Investigators are seeking people who have bought or swapped exotic pets distributed since April by Pocket Pets, where a shipment of prairie dogs is believed to have been infected by a Gambian giant rat imported from Africa.
Peter Jahrling, scientific adviser at the Army Medical Research Institute for Infectious Diseases, said exotic animals may in the future have to be put in quarantine and examined thoroughly for diseases. That has worked for imports of primates, which imported yellow fever in the 1930s and suffered from Ebola in 1989.