A virus-caused respiratory illness has put hundreds of children into hospitals in the U.S. Southeast and Midwest, a number that's causing increasing concern, health officials admit.

Twelve U.S. states have reported cases of illness that may be linked to Enterovirus D68, a virus difficult to differentiate from a common cold but worrisome for the numbers of hospital admissions it has prompted, they say.

"It's worse [than a cold virus] in terms of scope of critically ill children who require intensive care," says Dr. Mary Anne Jackson, division director for infectious diseases at Children's Mercy Hospital in Kansas City, Missouri. "I would call it unprecedented. I've practiced for 30 years in pediatrics, and I've never seen anything quite like this."

Outbreaks of enterovirus cases have been reported in Alabama, Kentucky, Georgia, Colorado, Ohio, Michigan, Iowa, Missouri, Illinois, Kansas, Utah and Oklahoma.

While there isn't a vaccine for the EV-D68 virus, most cases turn out to be mild; still, the record numbers of cases in the Midwest and Southeast has put health officials in many states on alert.

"Respiratory infections like this can spread quite quickly across the U.S.," says Anne Schuchat, director of the CDC's National Center for Immunization and Respiratory Diseases. "We want people to be on the lookout."

The respiratory illness linked to EV-D68 can be particularly dangerous, officials say, with severe cold symptoms, a hacking cough and even difficulty in breathing.

Children with asthma or people with weakened immune systems are especially vulnerable, they say.

A first spike in cases in Kansas City was noticed around August 15, says Jackson.

"It could have taken off right after school started," she says. "Our students start back around August 17, and I think it blew up at that point."

Previously, fewer than 100 confirmed cases of EV-D68 had been reported since the enterovirus was first identified in California in the 1960s, but that could be because the Centers for Disease Control doesn't require tracking of such cases by state health, officials say.

"It's one that we don't know as much about as we would like," Schuchat says.

Parents are being cautioned to monitor their children for any evidence of difficulty in breathing or other symptoms including coughing, rash or fever.

Those looking for information on the virus can visit a CDC website, cdc.gov/non-polio-enterovirus.

EV-D68 was detected last year as well as now in the United States, and has also been seen in Japan, Netherland and the Philippines in the past, officials said.

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