A massive cyclosporiasis outbreak linked to shredded iceberg lettuce has expanded to an additional six states, according to a source familiar with the matter.

The outbreak, caused by a microscopic parasite found in human feces, is responsible for hundreds of hospitalizations and two deaths since early May.

The newly linked states — Arkansas, Iowa, Missouri, Nebraska, New Hampshire and North Carolina — join Illinois, Indiana, Kansas, Kentucky, Michigan, Ohio, Oklahoma, Pennsylvania and West Virginia, the source said, who spoke on the condition of anonymity because they were not authorized to speak publicly. The Missouri Department of Health and Senior Services has reported 1,095 cases as of Sunday — more than double the number of cases it reported last week.

The agency’s update marks a significant expansion in the outbreak of cyclosporiasis, a foodborne illness that often leads to watery, sometimes explosive, diarrhea.

The additional states, however, do not necessarily mean the outbreak is continuing to spread, said Dr. Stephen Ostroff, a former acting FDA commissioner.

The majority of the cases have been traced to lettuce at Taco Bell that was supplied by Taylor Farms and sourced from central Mexico. Last month, the produce giant issued a voluntary recall of all iceberg lettuce supplied from central Mexico, which the company said was distributed to restaurants and retail stores in 27 states.

The Food and Drug Administration hasn’t yet identified other potential sources beyond Taylor Farms.

The newly linked cases, Ostroff said, may involve people who ate the recalled lettuce weeks ago but were only recently diagnosed and reported to health officials.

“With an incubation period up to two weeks, there should no longer be people newly getting sick from that product,” Ostroff said. “But it takes a while for people to seek health care after onset, get tested and diagnosed, reported to state health departments and then to CDC.”

“Think of it like solving a jigsaw puzzle,” he added. “You have all these pieces sitting there and the longer you work on it, the more likely it is you’ll figure out where each piece fits into the puzzle.”

More than 22,000 people have confirmed or suspected cyclosporiasis and 517 people have been hospitalized in the multistate outbreak, the Centers for Disease Control and Prevention said late Tuesday.

On Monday, health officials in Michigan — which has the most cyclosporiasis cases — confirmed the first deaths linked to this outbreak in the United States. Both people had underlying health conditions that may have been affected by the virus and dehydration, the officials said.

Ostroff said the key question about the newly added states is when their patients became sick.

The recalled Taylor Farms lettuce should no longer be available in the U.S., and its use-by dates ended Aug. 3, Ostroff said. If people were exposed after that, investigators may need to look for another source.

“We may have solved the Taylor Farms shredded lettuce piece but that’s unlikely to explain everything we’re seeing,” he said.