Chapter 3

Non-O157 STEC

Non-O157 Shiga Toxin-Producing E. coli can also cause food poisoning.

E. coli O157:H7 may be the most notorious serotype of Shiga toxin-producing E. coli (STEC), but there are at least 150 other serotypes of STEC that cause illness in humans – these are referred to as non-O157 STEC. For many years, most recognized STEC outbreaks were associated with E. coli O157:H7. However, that was mostly because E. coli O157 was easily detected by stool cultures ordered by health care providers and conducted by clinical laboratories. Conversely, stool cultures were not useful in identifying non-O157 STEC at clinical laboratories because cultures could not differentiate non-O157 STEC from the numerous non-disease-causing E. coli strains normally found in the human intestine. Therefore, for years, pathogenic non-O157 STEC infections went largely undiagnosed.

That all changed in the early 2000s when tests to detect Shiga toxin (Stx), rather than the bacteria itself, became available and were increasingly used by clinical laboratories. A rapid antibody/antigen test to differentiate between O157 and non-O157 could be done at that point, or the Stx-positive specimen could be sent to a public health department lab to determine the STEC serotype. As a result of this change in laboratory testing practices, detection of non-O157 STEC increased markedly, and we now know that, as a group, non-O157 STEC infections in humans are actually much more common than O157 infections.

About 80% or more of human non-O157 STEC infections in the United States are caused by six serogroups: O126, O111, O103, O121, O45, and O145. Whereas almost all O157 strains isolated from ill humans contain Stx2 (alone or in combination with Stx1), many non-O157 STEC strains possess only Stx1. Therefore, non-O157 STEC as a whole do not cause as severe a disease as O157. However, non-O157 STEC are very heterogeneous with respect to virulence, and some strains certainly can be as severe as O157, including the ability to cause hemolytic uremic syndrome (HUS) and death.

All pathogenic strains of non-O157 STEC can cause bloody diarrhea and hospitalization. However, only the strains that carry Stx2 (as opposed to only Stx 1) typically cause HUS. E. coli O157:H7 still causes vastly more HUS cases in the United States than non-O157 serogroups; about 95% of HUS cases with confirmed STEC infection have E. coli O157:H7, and 5% have a non-O157 serogroup. One percent or less of non-O157 STEC infections result in HUS, but when they do, the HUS can be fatal.

Non-O157 STEC generally have similar outbreak sources and risk factors as E. coli O157:H7. Transmission is primarily foodborne, but can occur from water, animal contact, and person-to-person transmission. A higher percentage of non-O157 STEC infections than O157 are acquired during foreign travel, although most are still acquired domestically in the United States. As with E. coli O157:H7, the highest percentage of non-O157 STEC cases occur during the summer and early fall months.