Nearly 500 children have been treated at one hospital alone — Children's Mercy in Kansas City, Missouri — and some required intensive care, according to authorities.
The suspected germ, enterovirus 68, is an uncommon strain of a very common family of viruses that typically hit from summertime through autumn.
The virus can cause mild coldlike symptoms including runny noses, coughing and wheezing but Mark Pallansch, director of the viral diseases division at the federal Centers for Disease Control and Prevention, said this summer's cases are unusually severe and include serious breathing problems.
"It's not highly unusual but we're trying to understand what happened this year in terms of these noticeable and much larger clusters of severe respiratory disease," Pallansch said Monday.
The virus typically causes illness lasting about a week and most children recover with no lasting problems.
Cases have been confirmed in Missouri and Illinois. CDC said it is testing to see if the virus caused respiratory illnesses reported in children in Alabama, Colorado, Georgia, Iowa, Kansas, Kentucky, Michigan, Ohio, Oklahoma and Utah. The states' tally changes as specimens are confirmed or test negative. A spokeswoman for Iowa's public health department said CDC tests confirmed the virus in samples from patients in central Iowa and a Colorado hospital said it has confirmed cases.
The CDC's Dr. Anne Schuchat said at a Monday news briefing that there are other viruses making kids sick.
"Most of the runny noses out there are not going to be turning into this," she said.
Children with asthma and other health problems are especially at risk for the enterovirus, but reported cases include children without asthma who have developed asthmalike breathing problems, Pallansch said. He said no deaths have been reported in the outbreak.
Dr. Mary Anne Jackson, director of infectious diseases at Children's Mercy, said local cases began appearing in mid-August and they appear to have peaked in her area.
Schuchat said the strain involved also appeared in the United States last year and in specimens from other countries. She said the CDC learned it had reappeared in this country last month when authorities in Chicago and Kansas City notified the agency about severe illnesses in children who had to be hospitalized. She said the virus was found in 11 of 14 specimens from Chicago and in 19 of 22 specimens from Missouri.
In the Denver area, more than 900 children were treated for severe respiratory illnesses at Children's Hospital Colorado and its urgent care locations and 86 were hospitalized in recent weeks. Spokeswoman Melissa Vizcarra said Monday that CDC had confirmed the virus in 19 of 25 samples from her hospital.
The University of Chicago's Comer Children's Hospital has treated several cases, including extremely sick children requiring life-support machines, said Dr. Rachel Wolfson, an intensive care unit physician.
Affected children are "as small as infants all the way up to teenagers," Wolfson said.
The virus can spread through sneezing and experts say good hand-washing practices are important to curb transmission.
"The take-home point is wash your hands and keep your hands away from your face," Wolfson said.
AP Medical Writer Lindsey Tanner can be reached at http://www.twitter.com/LindseyTanner