Four more people have died from tainted romaine lettuce, federal health officials said Friday, bringing the total to five deaths related to a virulent strain of E. coli whose source has still not been located.

Officials say the people who became ill ate the tainted romaine lettuce from the Yuma, Arizona growing region when it was likely still available in grocery stores and restaurants. Canadian health officials also recently identified E. coli cases in several provinces that could potentially be linked to the outbreak in the United States.

When a person becomes infected with the bacteria, it can take two to three weeks before a report to the CDC. The five dead were reportedly from Arkansas, New York, California and, with two cases, Minnesota. Illnesses started on dates ranging from March 13, 2018 to May 12, 2018.

"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.

According to the Mayo Clinic, O157 E.coli symptoms include diarrhea, which could be bloody, as well as abdominal cramping or pain, and in some people, nausea.

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It is the largest USA outbreak of E. coli since 200 people fell ill in 2006.

Almost half of those who became ill had to be hospitalized.

The CDC said that some of the affected people had not eaten lettuce, but had contact with others who had fallen ill.

Most people recover within a week, but some illnesses can last longer and be more severe. Young children and adults have a greater risk of developing a form of kidney failure called hemolytic uremic syndrome, which is life-threatening.