Image copyright Science Photo Library Image caption The severity of this outbreak of enterovirus is unusual

Hundreds of children across the US have been treated for a rare respiratory virus and more cases are expected in the next few weeks, doctors have said.

The enterovirus, EV-D68, is believed to be the cause of the outbreak and can cause severe respiratory illness.

Twelve states in the US Midwest have reported cases over the past month, with dozens of children admitted into intensive care.

Frequent hand washing and good hygiene help protect against the virus.

Enteroviruses are common and usually do not require hospital care. The symptoms typically manifest as an intense summer cold, with the number of infections declining in September.

But EV-D68, which was first recorded in California in 1962, is less common in the US and can cause mild or severe respiratory illness.

Over the past month, doctors in a number of states have reported an unusually high number of cases where symptoms have developed into acute respiratory distress and where the patient has needed hospitalisation, and in some cases, intensive care.

Image copyright Thinkstock Image caption Children have been advised to wash their hands

In a cluster of cases in Kansas City, 19 out of 22 children tested positive for EV-D68. In a similar cluster in Chicago, 11 out of 14 cases tested positive for the virus.

"We believe the unusual increases in Kansas City and Chicago might be occurring in other places in weeks ahead," said Anne Schuchat from the US National Center for Immunization and Respiratory Diseases.

"We don't know as much as we would like to know, but we believe the virus is spread through respiratory secretions," she said.

Infants, children and teenagers are most at risk from the virus, said Dr Schuchat.

More than half of the children hospitalised in the outbreak already had a history of asthma or other breathing difficulties.

No fatalities have been reported.

Dr Schuchat said 12 states had contacted the Centers for Disease Control for help in investigating clusters of the virus.

These include Colorado, North Carolina, Georgia, Ohio, Iowa, Illinois, Missouri, Kansas, Oklahoma, and Kentucky.

Dr Schuchat urged parents who had children who were having difficulty breathing to contact a doctor.

She also urged medics to consider laboratory testing if the cause of a respiratory illness was not clear.

Frequent hand washing and good hygiene is believed to reduce the risk of infection, she said.

She also advised parents who had children with asthma to make sure they take their medicine regularly.