CHICAGO (Reuters) - U.S. health authorities on Wednesday recommended smallpox vaccinations for anyone exposed to "monkeypox" either from infected pets or from the roughly 63 human cases, all but one in the U.S. Midwest.

The U.S. government also banned the importation or trade of African rodents, including Gambian rats believed to be the original source of the smallpox-like illness previously unseen in the Western Hemisphere, said David Fleming, deputy director of the Centers for Disease Control and Prevention in Atlanta.

"I'm confident that everything that can be done is being done to prevent the spread of this virus," Fleming told reporters, citing efforts to track down and euthanize infected animals.

The source of the illness was believed to be a shipment of Gambian rats from Africa to Texas that later infected prairie dogs native to the U.S. Plains. Both species are part of a growing trade in so-called exotic pets.

Some infected animals were sold to an Illinois distributor, and the virus has been spread to pet shops, pet brokers and pet owners via formal and informal sales in Wisconsin, Indiana and Illinois. Some states quickly declared bans on trade in the animals.

State health departments have reported 29 confirmed or suspected cases in Indiana, 21 in Wisconsin, 12 in Illinois and one sickened boy in New Jersey who apparently contracted the illness on a recent visit to the Midwest.

While monkeypox is not as harmful as smallpox, authorities fear a sickened animal, which displays cold-like symptoms, such as a runny nose and eyes plus a rash, might escape or be discarded into the wild and spread the virus to squirrels or other species. One pet rabbit exposed to a prairie dog was infected.

VACCINE CAN BE DANGEROUS

The smallpox vaccine has been found to be roughly 85 percent effective in warding off monkeypox in humans, which kills between 1 percent and 10 percent of its victims in the rain forests of central and western Africa, Fleming said.

The CDC recommended that local public health authorities vaccinate health investigators, veterinary staff, relatives of monkeypox victims, and others who believed they were exposed to the virus as long as two weeks ago.

"We need to be prepared for the fact that monkeypox can be a fatal disease," Fleming said. "We are recommending smallpox vaccine for a limited number of people -- we feel the risk is sufficient to warrant recommending it, (even) for pregnant women and children."

The vaccine, which has been stockpiled in recent months because of a perceived bioterror threat, is potentially dangerous for some recipients -- a few people in every million can be expected to die from the vaccine. The recent limited vaccination program has produced some unforeseen side effects, including 21 people who have suffered inflammation of the heart or a membrane around the heart.