DES MOINES, Iowa -- Health officials said Thursday that influenza and other respiratory viruses have killed about 20 young children in Iowa this winter.

Patricia Quinlisk is the state epidemiologist. She said Iowa has several child deaths from such viruses each year, but this winter seems to be a little worse than most years.

Quinlisk said the deaths have occurred in children younger than 4 years old from around the state. She's asking doctors to report any similar cases of death or severe illness to her.

Officials said most of the children had mild to severe respiratory symptoms prior to death. Multiple viruses were isolated in laboratory tests, including influenza A and B, adenoviruses and respiratory syncytial viruses (RSV).

Officials said it is important to note that it is unknown at this time if these viral infections were the principal cause of death.

The Iowa Department of Public Health and the Centers for Disease Control and Prevention are investigating the cases to see if they're linked. The deaths occurred between December 2007 and today.

Quinlisk said one of the three strains of flu going around isn't covered in this year's flu vaccine. But she said the vaccine still provides some protection.