By Julie Steenhuysen CHICAGO (Reuters) – U.S. health officials have sent out a nationwide alert warning doctors to be on the lookout for symptoms of pediatric hepatitis, possibly linked with a cold virus, as part of a wider probe into unexplained cases of severe liver inflammation in young children. The warning follows investigations in the United States and Europe of clusters of hepatitis in young children. The U.S. Centers for Disease Control and Prevention (CDC) said it is working with counterparts in Europe to understand the cause of the infections. A common virus known as an adenovirus ha…