The E. coli outbreak from romaine lettuce is finally over. But what caused it?

Good news, salad eaters, you can eat romaine lettuce without fear now.

On Thursday, the Centers for Disease Control and Prevention announced that the E. coli outbreak linked to romaine lettuce was over. According to a news release from the CDC, citing the Food and Drug Administration:

the last shipments of romaine lettuce from the Yuma growing region were harvested on April 16, 2018, and the harvest season has ended. Contaminated lettuce that made people sick in this outbreak should no longer be available.

Remind me...what was wrong with the lettuce?

Five people died and 210 got sick in the E. coli outbreak that affected people in 36 states. The CDC determined that this epidemic was caused by eating lettuce that originated in the Yuma growing region in Arizona.

E. coli is a type of bacteria. While most strains of E. coli are harmless to humans, some can cause diarrhea, urinary tract infections, respiratory illness, and pneumonia.

But what caused this?

Experts aren't sure, but contaminated canal water is a strong culprit.

To try to narrow down the source of the deadly bacteria, the FDA and CDC took samples from soil, water, and cow manure in the region where the infected lettuce had originated. According to a joint news release:

To date, CDC analysis of samples taken from canal water in the region has identified the presence of E. coli 0157:H7 with the same genetic finger print as the outbreak strain. We have identified additional strains of Shiga-toxin producing E. coli in water and soil samples, but at this time, the samples from the canal water are the only matches to the outbreak strain.

The FDA and CDC still need to run more tests before they can determine for certain if this contaminated water was used to water the lettuce crop, but it seems likely.

It wasn't the fault of just a single farm either, according to the same news release:

The traceback investigation indicates that the illnesses associated with this outbreak cannot be explained by a single grower, harvester, processor, or distributor. While traceback continues, the FDA will focus on trying to identify factors that contributed to contamination of romaine across multiple supply chains.  The agency is examining all possibilities, including that contamination may have occurred at any point along the growing, harvesting, packaging, and distribution chain before reaching consumers.