The offending strain in both the current outbreak, and a major outbreak in June, is E. coli O157:H7, which is a less common strain typically associated with cattle or other livestock.[1] CDC analysis (also helpfully linked by craftyguy) of the previous outbreak of this strain, which hospitalized or killed people in 36 states, found that the source was contaminated canal water used for irrigation near Yuma AZ.[2] A large CAFO (Confined Animal Feeding Operation) exists upstream nearby.[3] You can see it from space in satellite imagery. It is interesting to view the orientation of this CAFO to Gila River, and to zoom in to the level of individual cattle to get a sense of it's scale and topographic orientation.
Determining the source of water contamination is outside of the scope of office of CDC. EPA is responsible for this, but I can find no evidence of investigation of the source canal contamination. Perhaps EPA is becoming more “business friendly.”
[1] https://en.wikipedia.org
/wiki/Escherichia_coli_O157:H7#Natural_habitat
[2] https://www.cdc.gov/ecoli/2018/o157h7-04-18/index.html
[3] https://www.google.com/maps/place/McElhaney+Cattle+Company/@...
Edit: FDA published an environmental assessment on Nov. 1. They also attributed contaminated canal water as a possible cause. That assessment included "limited" samples (I can't figure out what kind of samples) from the CAFO, which found no matching e coli strain. The authors state that the assessment occurred several weeks after the growing season ended.
"It is uncertain how the outbreak strain of E. coli O157:H7 was introduced into this 3.5-mile stretch of irrigation canal water. The first illnesses in this outbreak occurred in March 2018, and therefore the outbreak strain may have been present in the irrigation canal months before the EA team collected the positive samples, or the outbreak strain may have been repeatedly introduced into the irrigation canal. A large concentrated animal feeding operation (CAFO) is located adjacent to this stretch of the irrigation canal. The EA team did not identify an obvious route for contamination of the irrigation canal from this facility; in addition, the limited number of samples collected at the CAFO also did not yield the outbreak strain. Other possible explanations for how the irrigation canal became contaminated are possible, but the EA team found no evidence in support of alternative explanations.
Growers suggested weather events in the Yuma growing region may have contributed to crop contamination. The EA team considered the possibility that leaf freeze damage and dew on romaine leaves created conditions favorable for windborne contamination of the crop with dust carrying the outbreak strain of E. coli O157:H7. While this type of STEC contamination has been demonstrated (Berry et. al, 2015), it does not explain the presence of the outbreak strain in a free-flowing irrigation canal months later when there was little wind in the region."
https://www.fda.gov/Food/RecallsOutbreaksEmergencies/Outbrea...
