A microscopic parasite is making thousands of Americans sick this summer, and public health officials are still figuring out exactly what foods are spreading it.

As of July 14, the U.S. Centers for Disease Control and Prevention has counted 1,645 cases of the intestinal illness cyclosporiasis in people across 34 states, and more than 5,100 additional cases are still awaiting confirmation. The highest number of cases has been identified in Michigan, which has counted roughly 4,312 this summer as of July 16. 

Federal agencies announced yesterday that many cases have been traced to Taylor Farms lettuce supplied to Taco Bell restaurants throughout the Midwest. The $7 billion agricultural industry giant has been implicated in multiple prior outbreaks of foodborne illness, including Cyclospora and E. coli.

Cyclosporiasis symptoms can last for weeks, causing particularly severe problems in young children, older adults, and immunosuppressed people. People get sick around a week after eating contaminated food—most commonly fresh produce, especially raspberries, leafy herbs, and bagged salad mixes—but it can take as little as two days and as long as two weeks to develop symptoms.     

Cyclospora cayetanensis is a single-celled, spherical parasite, too small to see without a microscope, first described in humans in 1994. It causes watery, sometimes explosive diarrhea that can wax and wane for weeks, along with cramping, gas, nausea, fatigue, and loss of appetite. Most healthy people recover, and the illness responds to a generic antibiotic, trimethoprim-sulfamethoxazole. 

Many cases of cyclosporiasis are never caught because the parasite isn’t on every diagnostic panel, and clinicians often have to proactively think of the infection to order the right test. The growing use of multi-organism test panels that detect Cyclospora as a matter of routine has led to a steady increase in cases over the last few years.

Nationally, cyclosporiasis case totals swing widely from year to year, and this season is running high, even though the CDC’s figure lags behind individual state counts. The clearest signal is at the state level: Michigan, which typically identifies about 50 cases a year, has seen more than 50 times that number so far this summer, and many other states have reported clusters of their own. 

It’s not currently clear whether multiple items or one broadly distributed product is to blame. Lawyer and food safety advocate Bill Marler has represented victims in major foodborne illness outbreaks for 35 years, and he has his suspicions. “Given the size of what is happening and that the time frame is pretty tight,” he says, “my sense is that there’s probably one product that is explaining most of this.”

How does Cyclospora spread?

One feature sets Cyclospora apart from most gut bugs: it doesn’t spread person to person. Almost every case begins when a person ingests contaminated food, usually fresh produce. 

But Cyclospora doesn’t exit a sick human in an infectious state. The oocyst, the parasite’s egg-like stage, has to sit in a warm environment for days to weeks to mature into a form that can invade the cells of the human digestive tract, which is why the U.S. season rises with warm, wet weather. Given the scale of the outbreak this year, “a pretty efficient transmission process” has likely been taking place.

Cyclospora oocysts are hard to kill. They can live in soil or water for weeks to months, and are resistant to regular chlorine, the most common method farmers use to get rid of bacteria and viruses in agricultural water. To eradicate the bug, agricultural operations must use sophisticated and expensive methods like microfiltration, ultraviolet light irradiation, or treating the water with ozone.

Once tainted water deposits an oocyst on a plant, the organism clings tightly to the plant’s outer layer. It can linger there alive for weeks or months before making it through sorting facilities, shipping systems, and grocery stores onto your kitchen counter. 

Finding the source of a cyclospora outbreak usually involves asking affected people detailed questions about what they ate in the two weeks before symptoms started, comparing the genetic fingerprints of different people’s germs, and tracing the movement of potentially responsible foods through complex distribution networks. Michigan and other statehealth department officials have been administering patient questionnaires and performing genetic tests on their stool samples, and the U.S. Food and Drug Administration has initiated tracebacks related to several clusters of the infection. 

Even when working at full speed to investigate cyclospora outbreaks, it often takes public health authorities weeks to zero in on their cause. After eating a contaminated item, it can take two weeks for a person to get sick, and several more days before they see a clinician. Many clinicians don’t test for causes of diarrhea until a person has had symptoms for at least a week. Add on a few more days for a patient to collect and submit a stool sample, and a few more days for lab testing. The work of talking to patients and potentially implicated suppliers happens simultaneously but can also be delayed for a whole range of reasons. The timeliness of all of this can sometimes be really challenging.

The pathogen presents some home food safety challenges, too: A casual splash of water isn’t enough to dislodge it from the outer layer of a snow pea, the tiny hairs on a raspberry, or a leaf of lettuce, basil or cilantro. But longer rinsing does help. 

To reduce the risk that a salad will make you sick, opt for whole heads of lettuce over pre-washed, bagged lettuce or pre-mixed salad kits. Throw away the outer two or three layers of leaves, and thoroughly wash the inner leaves under running tap water. 

Careful produce handling isn’t the only thing people can do to help stop this outbreak. If you end up getting a Cyclospora infection, sharing as much as you can remember with public health authorities can ultimately help stop the outbreak from making other people sick. 

If the health department calls you to ask about what you ate, by providing information you can help them identify what the cause of the outbreak is.