How did a baby in central Minnesota contract the virus that causes polio, a crippling disease that was essentially wiped out in the United States a quarter century ago? That question has mystified state and federal health officials since tests confirmed the polio virus in an unidentified infant last week. The case is especially puzzling because the baby, who was born in this country, was somehow exposed to a strain of virus found in oral polio vaccines, which haven't been used in the United States for five years.
"It is not a public health concern for the general public," said Kris Ehresmann, chief of immunization at the Minnesota Health Department. "But it is definitely a situation that is of great scientific interest. It's a unique situation." Investigators now are testing relatives and others who have had close contact with the child to see whether anyone else may have been infected. They suspect that someone contracted the polio virus in another country and unwittingly passed it on.
The baby had no symptoms of polio, Ehresmann said. The virus was discovered during tests while the child was hospitalized for an unrelated immune condition. Officials declined to identify the child's gender or age, saying only that he or she is less than a year old. The Health Department was asked to run lab tests to find out whether a virus was making the child sick. When no routine viruses showed up, they started looking for obscure ones. And they found the polio virus. The child hadn't been vaccinated against polio, apparently because of underlying medical problems.
But health experts were astonished at the test results, to put it mildly. It's been 50 years since the polio vaccine was developed, in the midst of an epidemic that paralyzed as many as 21,000 Americans a year at its peak. By 1979, the disease had been wiped out as a natural threat in the United States. For the next 20 years, virtually the only cases reported in this country - an average of eight a year - were caused by the oral vaccine, which used a modified live virus. Five years ago, the U.S. discontinued the oral vaccine and now uses a shot made from a killed virus that doesn't cause illness.
Since then, federal officials say, no one had contracted polio in the United States. To make sure of their findings, state officials sent samples of the virus to the U.S. Centers for Disease Control and Prevention in Atlanta for more tests. Last week, the agency confirmed that it's polio. "It's an unusual thing in any country," said Dr. Jim Alexander, a vaccine specialist at the federal agency. "There are many more questions so far than we have answers."
But they learned something remarkable, Ehresmann said. Using genetic fingerprinting, the CDC experts discovered that the virus strain had been used in an oral vaccine two years ago. That means that someone got the oral vaccine elsewhere - it's still used in much of the world -and inadvertently transmitted the polio virus to someone else.
"You could have somebody who ... would appear completely healthy who could be unknowingly shedding virus," she said. It is transmitted by direct contact with stool (i.e. diapers). Typically, she said, people can only infect others for about a week. But people with immune problems may harbor it indefinitely. The baby is still hospitalized. "I really hope that we'll be able to figure things out," Ehresmann said. "But it certainly is a possibility that there will still be some missing pieces to this puzzle when all is said and done.