The cyclospora outbreak tied to bagged iceberg lettuce has spread to four more states, the CDC and FDA said on 24 July. The parasite, which causes explosive diarrhoea, is behind what officials call the largest reported US cyclosporiasis outbreak, now spanning nine states.

The expanding outbreak

Illinois, Kansas, Oklahoma and Pennsylvania joined Michigan, Ohio, Indiana, Kentucky and West Virginia as the states with confirmed cases linked to the Taylor Farms lettuce. At least 1,947 people nationally have been sickened and 98 hospitalised in this cluster. The total number of cyclospora infections across the US since mid-May now exceeds 11,000 across 41 states, though many are from unrelated outbreaks.

“That count was probably an undercount.”

Michigan remains the hardest-hit state. On Thursday state health officials reported 7,664 cases, a figure that rose to more than 8,100 by Friday. Some Michigan labs were running out of reagents needed to test for the parasite, the state's executive medical director Natasha Bagdasarian told The New York Times.

Recall and false positive

Taylor Farms, a major US food supplier, voluntarily recalled all its iceberg lettuce sourced from central Mexico on 17 July. The recall covered bagged lettuce sold in 27 states, including the nine outbreak states, and shipped to restaurants, retailers and food-service distributors such as Taco Bell and Walmart. The company said it acted "out of an abundance of caution." The FDA initially reported that a Taylor Farms lettuce sample had tested positive for cyclospora but later retracted the statement, saying it was a false positive.

Slow-moving investigation

Health experts told the BBC that tracing cyclospora is notoriously difficult because symptoms can take weeks to appear. That delay makes it hard for investigators to reconstruct what someone ate and find a common link. Wade Syers, a food safety and security expert at Michigan State University, said the inquiry into the multistate outbreak is likely to be prolonged.

“For Cyclospora, it's not unusual for the picture to become clearer a couple of months after the first illnesses occur. Because these investigations are unfortunately more likely to be marathon investigations rather than a quick sprint.”

Other unrelated outbreaks

The FDA disclosed a new cyclospora outbreak with 72 cases on 24 July, adding to three other active national investigations. Officials stressed that these are not linked to the Taylor Farms lettuce recall. Nationally, the CDC is investigating or has confirmed more than 11,500 cyclosporiasis cases, caused by a parasite that typically spreads through food or water contaminated with human faeces.