Federal health officials have expanded the nation’s largest ongoing cyclosporiasis outbreak, confirming that cases linked to contaminated shredded iceberg lettuce have now spread to fifteen states.

The Centers for Disease Control and Prevention added six states — Arkansas, Iowa, Missouri, Nebraska, New Hampshire, and North Carolina — to the official investigation list on Wednesday. They join nine previously affected states: Illinois, Indiana, Kansas, Kentucky, Michigan, Ohio, Oklahoma, Pennsylvania, and West Virginia.

Health investigators have centered their focus on shredded iceberg lettuce sourced from Taylor Farms de Mexico, which voluntarily recalled product grown in central Mexico last month. The recalled lettuce was distributed nationwide to retailers and restaurant chains, including select Taco Bell locations and Marketside-brand packages sold at Walmart.

The intestinal parasite Cyclospora cayetanensis causes severe gastrointestinal symptoms, including watery diarrhea, severe stomach cramping, fatigue, and nausea. While the recalled product’s best-by dates have passed, health officials urge consumers and food service operators to remain vigilant and thoroughly wash any produce to prevent cross-contamination.