MINNEAPOLIS - A rare virus is sweeping the Midwest, landing dozens of children in the hospital with breathing difficulties.

Illnesses are often linked to the beginning of a school year, as close contact and cooler weather facilitate the sharing of germs among children. Health authorities say the virus emerging in the middle of the country is more severe than a typical illness, and is particularly hard on children who have asthma or previous respiratory troubles.

The U.S. Centers for Disease Control and Prevention was notified about the disease clusters last month when hospitals in Kansas City, Missouri and Chicago reported more children brought to the emergency room and admitted to the hospital than expected, Anne Schuchat, director of CDC's National Center for Immunization and Respiratory Diseases, said Monday. There have been no deaths, the agency said.

"The situation is evolving quickly," Schuchat said in a conference call with reporters. "The CDC and our colleagues are gathering information to better understand this virus, how widespread it may be, which populations may be affected and whether others are experiencing severe respiratory illnesses that may be related to this virus."

Tests found Enterovirus D68 in children ages 6 weeks to 16 years, she said. About a dozen states have contacted the agency, she said.

Agency tests found the virus known as EV-D68 in 30 of the 36 specimens examined, she said. Well over half of the children in these two clusters had a history of asthma or wheezing, which may predispose them to a serious infection, she said. No adults have been diagnosed with the illness, she said.

There is no vaccine available for the virus first seen in 1962 and no specific medicines approved to treat it. The currently circulating strain has been linked exclusively to respiratory illnesses. They encouraged the standard approach to good hygiene to reduce the risk of spreading the virus, including regular hand washing, limiting time with those who are ill and staying home when sick.

A children's hospital in Missouri saw more than 300 cases by the end of August, with 15 percent treated in the intensive care unit, according to an alert from the state's Department of Health & Senior Services. Most of the specimens sent to the CDC found EV-D68.

The strain is a standout among more than 100 Enteroviruses that infect 10 million to 15 million Americans each year. Most of the infections come with runny noses and mild cold symptoms - or none at all. Previous outbreaks of EV-D68 in the U.S., Asia and Europe have ranged from mild to severe, including the need for mechanical ventilation.

Children currently hospitalized have received oxygen and medicines to aid their breathing, while a few have needed more aggressive care, Schuchat said. While there haven't been any deaths, they may still occur, she said.

Parents should be on the lookout for signs their children are having trouble breathing or wheezing, which may be a warning, she said.

Researchers aren't exactly sure how easily or often the virus spreads since it hasn't been studied extensively. It is found in saliva and nasal mucus and is likely to be passed from one person to another via coughing, sneezing or contaminated surfaces, the CDC said. The agency is still trying to determine how big the outbreak is and how long it will persist.

"Respiratory virus can spread quite quickly across the U.S.," Schuchat said. "We really do think clinicians throughout the country need to be on the alert. The unusual increase in Kansas City and Chicago may be occurring elsewhere in the weeks ahead."

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