Edema disease is caused by hemolytic E coli that produce F18 pili and Shiga toxin 2e (Stx2e, also known as verotoxin 2e or VT2e). The F18 pili have two major antigenic variants, F18ab and F18ac; F18ab is characteristic of edema disease strains, and F18ac is associated primarily with enterotoxigenic E coli. A new variant of F18 has been identified on strains of E coli that carry the Stx2e gene, but the role of these strains in edema disease is not known. The Shiga toxin–producing E coli implicated in edema disease most commonly belong to four specific serotypes: O138:K81:NM, O139:K12:H1, O141:K85ab:H4, and O141:K85ac:H4. However, other serotypes of E coli may be implicated, and strains of serogroup O147 have been dominant in parts of the USA. These O147 strains typically carry the H17 flagella, but some have H14 or H4. Some of these O147 strains are not typeable with O147 specific antiserum but can be identified by their O antigen genes.