US hit by largest-ever outbreak of parasite-linked 'explosive diarrhoea'

The United States is experiencing the largest outbreak of its kind in the country's history from *Cyclospora cayetanensis*, a microscopic parasite that causes prolonged, watery diarrhoea, Nature reported on 16 July 2026 (https://www.nature.com/articles/d41586-026-02225-w). Since 1 May, 1,645 infections have been confirmed and more than 5,100 further cases are under investigation, according to Joel Barratt, a molecular parasitologist at Emory School of Medicine in Atlanta.

More than 100 people have been hospitalized, although no deaths have been reported. The figures — accumulated in just a few weeks — have already surpassed the total number of *Cyclospora* cases typically seen in a full year, Barratt told Nature.

Health officials have yet to pinpoint a single source, but the state of Michigan, which has recorded the highest number of infections, said on 13 July that lettuce or salad greens might be the culprit (https://www.michigan.gov/mdhhs/inside-mdhhs/newsroom/2026/07/13/cyclo-3). An update noted that investigators have since linked the outbreak to shredded lettuce served at the fast-food chain Taco Bell in five US states (https://www.washingtonpost.com/health/2026/07/16/lettuce-supplier-is-potential-source-cyclosporiasis-outbreak-investigators-say/).

*Cyclospora* is contracted by consuming food or water contaminated with human faeces; its egg-like oocysts must mature in warm conditions for one to two weeks — one reason outbreaks peak in summer. Outbreaks in 2018 and 2020 were linked to US-grown produce, suggesting the parasite's reach is expanding beyond its historic tropical range.

Researchers warn the trend is worsening. "They seem to be getting larger year by year," Barratt said, adding that climate change — by creating warmer environments for oocysts to mature — could be partially to blame. He also noted that US outbreak investigations are being hampered by staffing cuts at federal health agencies under the Trump administration, which fired or encouraged employees to leave.

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