Jessica Tomei holds her 4-year-old daughter, Sofia Jarvis, during a news conference at Lucille Packard Children's Hospital at Stanford University on Monday, Feb. 24, 2014, in Palo Alto, Calif.

Is Polio Back in the United States?

Polio-like disease in California has doctors scratching their heads.

Jessica Tomei holds her 4-year-old daughter, Sofia Jarvis, during a news conference at Lucille Packard Children's Hospital at Stanford University on Monday, Feb. 24, 2014, in Palo Alto, Calif.

Jessica Tomei holds her 4-year-old daughter, Sofia Jarvis, who has been diagnosed with a rare polio-like syndrome that has left her arm paralyzed. 

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Has polio returned to the United States? Probably not, but news reports in the past couple of days out of California sure have folks at the Centers for Disease Control and several medical groups concerned.

Polio is a highly contagious disease that was wiped out from developed countries, including the United States, decades ago thanks to widespread use of successful polio vaccines.

[READ: Polio-like Disease Appears in California Children]

But there are other enteroviruses that can cause polio-like illnesses, and something akin to the disease may have shown up in up to 25 children in California since 2012, according to Keith Van Haren, a pediatric neurologist at Lucile Packard Children’s Hospital in Palo Alto, Calif., where these cases have been identified.

“Physicians around California have begun to observe a new type of acute flaccid paralysis, a disease in which patients quickly and permanently lose muscle function in an arm or leg,” Van Haren said in an interview about the cases on the hospital’s website. “In some cases, patients have had respiratory symptoms before the paralysis begins. The disease resembles but is not the same as polio.”

The first known case in 2012 involved a 2-year-old who spent days in the intensive care unit at Children’s Hospital in Oakland with breathing problems. As doctors treated her asthma-like symptoms, and she didn’t respond to treatments, they began to look for other causes. She was referred to Van Haren’s clinic at the hospital, where doctors began to deal with the onset of partial paralysis. Other, similar cases started showing up across the state during the next two years.

While troubling, the cases are going to be looked at closely by public health experts, who also don’t know yet what it truly represents.

“This disease is very rare,” Van Haren said. “About 20 cases have been identified in the U.S. so far, all in California, and all occurring in the past 18 months.”

Van Haren and other researchers will discuss the California cases in greater depth in April at medical meetings when more is likely to be known. Right now, the new polio-like syndrome has been found in as many as 25 children across California – but not in any single location, which would represent a more dangerous “cluster.” The most likely culprit is an unknown enterovirus that looks like polio.

As a precaution, the CDC and other public health officials are now looking to see if there are more cases like this, both inside California and outside the state.

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Van Haren said he suspects that the real culprit is an enterovirus – a family of viruses that includes polio but also the sorts of milder foot and mouth diseases that are common in infants and children – that hasn’t been identified yet.

He also believes that other, similar cases will be found outside California once people start to look for it. Since polio has been gone from the United States for so long, younger doctors may not have much experience with cases such as these.

“Doctors are not sure (about the exact cause), but suspect it may be caused by a virus,” Van Haren said. “Some of the children identified with this disease have had enterovirus-68, which is from the same family as the polio viruses. This virus is suspected in part because a similar type of paralysis caused by another enterovirus has been seen for many years in Asia and Australia. Another possibility is that the disease may be autoimmune.”

Van Haren also cautioned that parents shouldn’t be worried. “We want to temper the concern, because, at the moment, it does not appear to represent a major epidemic but only a very rare phenomenon,” he said.

Still, the CDC is taking it seriously, as are others, because the polio vaccine doesn’t appear to work against this new disease. There also doesn’t seem to be any link between the polio vaccine and this new, rare disease, so parents shouldn’t hesitate to vaccinate their children, he said.