Washington DC - More than half of US states are grappling with an outbreak of gastrointestinal illness caused by a foodborne parasite, America's top health agency reported Tuesday.

Nearly 7,000 confirmed or suspected cases of cyclosporiasis, an infection triggered by a microscopic parasite, have been recorded across 34 states, according to the Centers for Disease Control and Prevention.

The illness "spreads when people consume food or water contaminated with the parasite, typically fresh produce that has not been adequately washed or cooked," said Gwen Biggerstaff, deputy director of CDC's Division of Foodborne, Waterborne, and Environmental Diseases.

Symptoms include explosive diarrhea, loss of appetite, fever, and vomiting.

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Relapses are possible, with symptoms disappearing, then coming back, the American Medical Association warned.

Infections have surged since May, and investigations are underway to determine the source.

"This is much, much higher than what we've seen last year or the year before," Biggerstaff said.

Authorities in Michigan – the state where most of the cases have been recorded – suspect contamination via salads and lettuce.

The nationwide fast-food chain Taco Bell is also under investigation by health authorities, according to The Washington Post.

The surge in cyclosporiasis cases has revived criticism of mass layoffs carried out last year by the Trump administration in federal health agencies, which resulted in the loss of experts and the CDC scaling back one of its foodborne pathogen surveillance networks.