Salmonella spp. and Escherichia coli O157:H7 prevalence and levels on lettuce: A systematic review and meta-analysis.

Lettuce (Lactuca sativa), one of the most consumed leafy vegetables in the world, is frequently implicated with foodborne disease (FBD) outbreaks, with Salmonella spp. and Escherichia coli O157:H7 being the most common bacteria to cause this illness. Estimates of prevalence and levels of these pathogens on lettuce are scarce in developed or in developing countries, which hinders risk assessment attempts. In here, we present a systematic review and meta-analysis of reported prevalence and levels of Salmonella spp. and E. coli O157:H7 on lettuce using the worldwide available data. Literature was reviewed and examined the results for inclusion of articles in the meta-analysis. Data (prevalence and/or concentration of Salmonella spp. and E. coli O157:H7 on lettuce, sample characteristic, country of origin, and Salmonella identified serovars) were extracted, and meta-analysis was performed using Open Meta-Analyst, Task Order # 2 software. Although only one work reported the presence of E coli O157:H7 on lettuce, several reports indicated the presence of other, distinct enterohemorrhagic E. coli (EHEC) strains, with a mean prevalence of 0.041 (95% CI: 0.005-0.078) and concentration varying from <3.0 MPN/g to >1100 MPN/g. Furthermore, the mean prevalence of Salmonella spp. on lettuce was 0.041 (95% CI: 0.030-0.052), with reported concentrations varying between 0.054 ± 0.058 CFU/g to 218.78 MPN/g. In addition, subgroup analysis of the presence of Salmonella spp. in lettuce revealed a mean prevalence of the bacteria of 0.028 (95% CI: 0.014-0.042) in developed nations and 0.064 (0.041-0.087) in developing nations, with reports varying from 0.001 in Japan to 0.5 in Burkina Faso. Despite a relatively low prevalence, consumption of lettuce is inherently risky because it usually is eaten raw, without thermal treatment to inactivate pathogens. This potential risk further supports performance of quantitative risk assessments to quantify the probability of FBD caused by Salmonella spp. and E. coli O157:H7 transmitted to lettuce.