The US Centers for Disease Control and Prevention said Thursday that confirmed cyclosporiasis cases had risen sharply across several states this summer, prompting an expanded multistate investigation into the parasitic intestinal illness linked to fresh produce.

Cyclospora cayetanensis is a microscopic parasite that causes prolonged watery diarrhoea, cramping, fatigue and weight loss, often lasting weeks without treatment. The CDC has repeatedly tied summer outbreaks in the United States to contaminated fresh produce, including basil, cilantro and pre-packaged salad mixes imported during warmer months.

The Food and Drug Administration is working alongside the CDC and state health departments to trace the source through purchase records and supplier data. Investigators said cases had been reported in the Midwest and along the East Coast, with additional illnesses under review.

Public health officials warned that cyclospora cannot be reliably removed by rinsing alone and does not respond to standard produce washes. The CDC recommended that people with persistent diarrhoea seek testing, because the infection requires a specific antibiotic, trimethoprim-sulfamethoxazole, rather than routine treatment.

A CDC spokesperson said the agency expected case numbers to keep rising as laboratories completed testing and clinicians reported new illnesses through the summer season.