ATLANTA — The US Centers for Disease Control and Prevention on Friday expanded its investigation into a cyclosporiasis outbreak centred in Michigan that has grown to more than 10,000 confirmed cases. Investigators are working to identify a contaminated food source.

Cyclosporiasis is an intestinal illness caused by the parasite Cyclospora cayetanensis, typically spread through fresh produce such as leafy greens, herbs and berries. The CDC has estimated more than 18,000 confirmed or suspected cases nationally, with Michigan reporting the heaviest concentration of infections.

The Food and Drug Administration is conducting traceback analysis alongside state health departments to pinpoint the implicated produce. Symptoms include prolonged watery diarrhoea, cramping and fatigue, and can persist for weeks without treatment with antibiotics.

The Michigan Department of Health and Human Services urged residents to seek medical testing if symptoms lasted more than several days, and said it was coordinating with neighbouring states reporting linked infections. State health officials warned that case totals were likely to rise as laboratory confirmations continued.

The outbreak ranks among the largest cyclosporiasis clusters recorded in the United States in recent years. Public health authorities cautioned that the source may involve a widely distributed product still on shelves.