‘Explosive diarrhoea’ outbreak grips US: how researchers are hunting its source. The CDC's parasite surveillance team dropped from 11 to 3 people last year, complicating efforts to track down what is now the largest Cyclospora outbreak in US history.

A microscopic parasite found in contaminated food products is ruining summer meals across the United States in the largest outbreak of its kind in the country’s history. And health officials have yet to identify the source, although the state of Michigan, which has reported the highest number of infections, has signalled that lettuce or salad greens might be the culprit.

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Since 1 May, 1,645 infections with the parasite Cyclospora cayetanensis have been confirmed, and more than 5,100 further cases are under investigation. These figures — which have accumulated in just a few weeks — have quickly surpassed the total number of cases typically seen in a year, says Joel Barratt, a molecular parasitologist at Emory School of Medicine in Atlanta, Georgia. More than 100 people with C. cayetanensis infections have been hospitalized, but no deaths have been reported.

One issue that is probably impeding the investigation is that “the number of staff working on these outbreaks is not what it used to be”, says Barratt, who worked at the US Centers for Disease Control and Prevention (CDC) in Atlanta for eight years and led the parasite surveillance team there. Barratt left the agency ten months ago, after the administration of US President Donald Trump fired employees or encouraged them to leave in a bid to downsize the government.

Nature spoke to Barratt and Jitender Dubey, a microbiologist at the US Department of Agriculture in Beltsville, Maryland, to learn more about the outbreak and how health officials are responding.

How does Cyclospora make people unwell?

C. cayetanensis is an intracellular parasite, meaning that it invades and then hijacks a host’s cells, especially those in the lining of the intestines. Once inside, it rapidly begins to multiply, which damages the cells. It can take a week or more for the parasite to complete its life cycle and inflict enough harm for symptoms to appear.

The hallmark symptom of watery diarrhoea occurs because the parasite inflames the intestinal lining. This disrupts the body’s ability to absorb water and nutrients, and causes the body to flush excess fluid through the gut. Although the infection usually runs its course when the immune system expels the parasite from the body — meaning that most people will not require treatment — the illness can last for weeks or even months.

The main treatment that physicians use to fight the parasite is the combined antibiotic trimethoprim–sulfamethoxazole (sold as Bactrim). Antibiotics are not typically used against parasites, but Bactrim starves Cyclospora of an essential vitamin it needs to replicate.

How does Cyclospora get into food?

A Cyclospora infection means that someone has consumed food or water that was contaminated by human faeces. C. cayetanensis reproduces through egg-like structures called oocysts that are shed in human stool. These oocysts are not immediately infectious: they must first sporulate, or mature, in a warm environment at temperatures between 22 °C and 32 °C for a week or two — which is one reason why outbreaks tend to occur during summer.

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Although C. cayetanensis is most common in tropical climates and past US outbreaks were frequently tied to foods imported from such regions, it is increasingly becoming a domestic issue. Recent US outbreaks in 2018 and 2020 were linked to produce likely grown in the United States, indicating that the parasite’s reach is expanding beyond regions where it has been historically endemic.

Contamination generally occurs through sanitation failures, Barratt says. For example, raw sewage run-off might contaminate agricultural soil. Or farm workers who lack immediate access to toilets might defecate in or near crop fields.

What worries Barratt, however, is the increase in outbreaks that scientists are seeing. “They seem to be getting larger year by year,” he says. Climate change, which is creating warmer environments for the parasite’s oocysts to mature, could partially be to blame, research has suggested¹.

More studies of how latrines are used on farms and regular surveillance of the stool of farm workers could help to explain how these outbreaks start, Dubey says, as well as investigations of whether people who are infected but asymptomatic can transmit the parasite.

How are researchers trying to pinpoint the outbreak’s origin?

Tracking the parasite requires coordination between US state and federal health officials. First, state laboratories collect faecal samples from people with disease symptoms and send them to federal facilities. If a sample tests positive for Cyclospora, health officials will ascertain when the person started showing symptoms and record what the person ate in the weeks beforehand.