Romaine lettuce from the Yuma growing region in Arizona is thought to be the source of the latest E. coli outbreak. That being said, the CDC continues to investigate the outbreak and warned that new cases from May could still come to light due to a three-week lag in reporting.
According to the agency, most people who become sick start experiencing symptoms three to four days after consuming produce tainted by Shiga-toxin producing E. coli O157:H7.
The Centers for Disease Control and Prevention (CDC) said 25 more people had been infected since its last report on 16 May. Two people from Minnesota, one person from Arkansas and one person from NY have died, according to the update.
"We are actively evaluating a number of theories about how romaine lettuce grown on multiple farms in the same growing region could have become contaminated around the same time", Dr. Scott Gottlieb and Dr. Stephen Ostroff wrote.
The CDC said that some of the affected people had not eaten lettuce, but had contact with others who had fallen ill.
The growing season in Yuma, Arizona ended six weeks ago, and it's unlikely any tainted lettuce is still in stores or people's homes, given its short shelf life.
The outbreak was first reported on March 13, reports CNN.
This is the largest outbreak of its kind since a deadly E.coli outbreak in 2006 that was linked to spinach, CNN reported.
Officials urge anyone who thinks they may be ill with an E. coli infection to see their doctor.
Of the total 187 patients for whom information was available, 89 (or 48 per cent) were hospitalised, including 26 who developed a type of kidney failure called hemolytic uremic syndrome.