A parasitic intestinal illness is spreading rapidly across the United States, with federal health authorities confirming at least 1,645 laboratory-confirmed cases across 34 states since May 1, along with more than 5,100 additional reports still under review. The US Centers for Disease Control and Prevention (CDC) issued a formal health alert on Tuesday, warning that case numbers are expected to continue rising through August. No deaths have been reported, though 141 people have been hospitalised.

The illness, cyclosporiasis, is caused by cyclospora, a microscopic parasite that infects the intestinal tract after a person consumes raw fruits, vegetables or water contaminated with faecal matter. It typically produces watery diarrhoea, nausea and frequent — sometimes severe — bowel movements, and is treated with antibiotics. Outbreaks tend to peak in late spring and summer. Although historically underreported, cases have trended upward over the past decade, partly due to improved detection methods and, experts say, the influence of climate change. The worst previous year on record was 2019, with roughly 4,700 cases.

This year's outbreak is centred heavily in Michigan, which has reported more than 3,300 cases alone. Northwest Ohio has recorded over 1,100, New York City more than 400, and Illinois more than 200. Health officials believe cases in Michigan, Ohio, Kentucky and West Virginia may be linked to a single common source, though investigations elsewhere suggest multiple separate exposure events. In Illinois, for instance, more than half of patients reported recent international travel, indicating those infections may have been acquired abroad.

The source of contamination has not yet been definitively identified. Michigan health officials, after conducting more than 1,000 patient interviews, said lettuce and salad greens have emerged as a recurring common factor, and advised consumers to buy whole heads of lettuce, discard outer leaves and wash the remainder thoroughly, while avoiding bagged lettuce and pre-mixed salad kits. The US Food and Drug Administration confirmed it is conducting traceback investigations into multiple produce items, including lettuce, tracing the supply chain to the farm level and using genetic sequencing where needed. Taco Bell, the fast-food chain owned by Yum Brands, said it had voluntarily removed certain ingredients at select restaurants as a precautionary measure, though federal officials have not confirmed any link to a specific vendor.

Some public health experts have criticised the pace and clarity of the official response. Dr Michael Osterholm, an infectious disease expert at the University of Minnesota, described the federal reaction as falling