Ontario's top doctor has issued a public alert concerning two E. coli cases thought to be linked to iceberg lettuce distributed in the province.  Dr. David Williams issued the warning Friday night saying two E. coli cases in the Chatham-Kent area are linked to the illnesses identified in 38 cases in the U.S.  All of the cases are thought to come from shredded iceberg lettuce distributed by Aunt Mid's Produce Company.  The release says the lettuce, which is distributed in five pound industrial bags, may have been used at institutions such as hospitals and long-term care homes, as well as restaurants in southwestern Ontario.