Experts believe that because lettuce is perishable, the produce that was making people sick was eaten or thrown away long ago. (Photo Credit: Tony Hertz/Universal Images Group/Getty Images/File via CNN Newsource)

(CNN) — A large multistate outbreak of cyclosporiasis that has been tied tolettuce sold at restaurants such as Taco Bell and some grocery stores now accounts for cases across 15 states, according to federal health officials.

Six states – Arkansas, Iowa, Missouri, Nebraska, New Hampshire and North Carolina – were added to the outbreak after lab testing by the US Centers for Disease Control and Prevention showed that illnesses there were probably caused by the same strain of the tiny cyclospora parasite, which burrows into the lining of the small intestine and can cause weeks of explosive diarrhea if left untreated.

Among the more than 6,000 cases the CDC has tallied in the outbreak, nearly 300 have been hospitalized, and two people have died.

The update is confirmation of how widespread the outbreak already was, experts said, rather than a sign that its footprint is expanding.

“We are just counting people who were already sick. Now comes the linking to Taylor Farms or other products,” said Bill Marler, a Seattle-based attorney and food safety advocate.

The CDC has implemented a level 3 emergency response to the outbreak, according to a source briefed on the agency’s actions. Level 3, the lowest level of emergency response, means the agency is actively monitoring and working on a situation, although the risk to the general public is low. It gives the CDC the flexibility to repurpose staff to assist in the outbreak investigation.

In detailed interviews, many people who’ve been sickened reported eating lettuce, often shredded iceberg lettuce supplied to restaurants like Taco Bell by produce giant Taylor Farms.

Taylor Farms de Mexico recalled all iceberg lettuce grown in central Mexico, including in salad kits sold in grocery stores, on July 17. In a statement on its website, the company said it conducted a broad recall of its products “out of an abundance of caution.”

The company additionally says it is conducting a “top-to-bottom” review of all safety processes and protocols.

Taco Bell says it has also removed affected shredded iceberg lettuce supplied by Taylor Farms from its restaurants in certain states.

Experts believe that because lettuce is perishable, the stuff that was making people sick was eaten or thrown away long ago.

Lettuce is believed to be a prime suspect in the multistate outbreak but may not be the cause of illnesses in other states. Because some people who got sick didn’t report eating lettuce and state health officials have said they cannot rule out other contaminated sources as contributors.

In its latest update, the CDC identified more than 22,700 cases of domestic cyclosporiasis this season; 10,468 cases are confirmed, and more than 12,200 others are undergoing further laboratory testing and analysis to determine whether the illness was acquired in the US.

By Marler’s tally, states have reported nearly 25,000 cases of cyclosporiasis so far this season.

Because many people may skip seeing the doctor for diarrhea, officials believe that many more cases haven’t been counted. The current count “is a floor and not an estimate,” Marler said. The true number of Americans sickened by cyclosporiasis this summer may be as high as 1 million, according to a multiplier published by the CDC in 2011.

The US Food and Drug Administration has investigated seven clusters of related cases of cyclosporiasis this summer. In five of those outbreaks, the agency lists the cause as “not identified.”

First deaths reported in cyclosporiasis outbreak