WASHINGTON — U.S. health officials announced Friday the end of the largest cyclospora outbreak in the nation's history, with nearly 13,000 confirmed cases linked to iceberg lettuce grown in Mexico. But the mystery of how the parasite spread remains unsolved.

The Food and Drug Administration confirmed that all recalled lettuce connected to the outbreak has been removed from the market. The parasite spreads through human feces, yet investigators have not determined how sewage contaminated enough produce to sicken people across 21 states.

Since May 1, the government recorded nearly 20,000 lab-confirmed cyclospora cases—more than 16 times the number reported during the same period last year. An additional 6,100 suspected cases were also reported. Two deaths occurred in Michigan, which experienced the most severe impact.

The previous peak year for U.S. infections was 2019, when approximately 4,700 illnesses were reported nationally. This year's surge far exceeds that benchmark.

Federal investigators initially focused on lettuce served at Taco Bell restaurants before identifying Taylor Farms as the source of the contaminated product. Taylor Farms, which grows lettuce in central Mexico, issued a recall for thousands of packaged salad products distributed to major U.S. restaurant chains owned by Yum Brands, including Taco Bell, Pizza Hut and KFC.

Ellen Shumaker, director of outreach for North Carolina State University's Safe Plates food safety program, said contamination can occur if produce grows in soil or contacts tainted water. The FDA has ended its on-site inspections and sample collection at lettuce growers and a processing facility in Mexico, though samples from those inspections await analysis.

The outbreak has exposed a regulatory vulnerability: U.S. authorities are conducting fewer international inspections, making it harder to intercept contaminated produce before it reaches American consumers. Cyclosporiasis remains less common than foodborne illnesses caused by salmonella and E. coli, and many cases are never traced to a specific source.