Missouri health officials say 32 people in and around St. Louis are now confirmed to have been infected by the same strain of pathogenic E. coli, but a team of experts has still not been able to pinpoint the source of the outbreak.

In a news update, the Missouri Department of Health and Senior Services (DHSS) said the latest case was based on a specimen collected Nov. 3.

Another specimen, collected Nov. 2, also is a genetic match to the outbreak strain, but that is likely a case of secondary transmission — someone who was exposed by another ill individual rather than to the source of the infection.

Secondary cases are common in foodborne illness outbreaks because pathogens like E. coli can be transmitted person-to-person through oral-fecal contact, usually as a result of inadequate hand washing — for example, changing soiled linens or diapers while caring for someone who is ill with foodborne disease.

Secondary cases, however, do not give investigators additional clues about the source of an outbreak, the Missouri health department points out. 

The team of local, federal and state public health experts conducting case interviews, collecting food samples and doing lab analysis have said that some of the outbreak patients remember eating salads from a St. Louis-based grocery store chain.

But none of the 55 food samples analyzed in the outbreak investigation has been found to be contaminated with E. coli.

When salads or similar fresh foods are implicated as the source of an outbreak, a major difficulty is that by the time case interviews are conducted and food samples are collected and tested, the suspect food items have already been consumed or discarded.