Wastewater testing explored to track Michigan’s cyclospora outbreak

As Michigan experiences the largest cyclosporiasis outbreak in its history, Michigan State University researchers say wastewater surveillance, the same public health tool widely used during the COVID-19 pandemic, could help identify hotspots, monitor the outbreak’s progression and guide public health response.

While public health officials continue investigating the outbreak’s source, researchers in MSU’s Water Quality and Environmental Microbiology Lab, also known as the Rose lab, are developing methods to detect Cyclospora cayetanensis in wastewater. The approach could eventually provide a community-wide picture of where infections are increasing or declining without relying solely on clinical testing. The lab regularly monitors wastewater for SARS-CoV-2 (the virus that causes COVID-19), influenza, RSV, Candida auris and norovirus. Researchers are now adapting those methods to detect cyclospora.

Michigan is experiencing one of the largest outbreaks of cyclosporiasis in the state's history, with 10,773 reported cases as of July 31. This represents an increase of more than 200 times the state's typical annual average of about 50 cases and is the highest case count reported by any state in the nation this year as public health officials continue working to identify the source.

The highest number of cases have been reported in Wayne County (1,104), followed by Washtenaw (813), Oakland (760), Ingham (732) and Genesee (385). And as of July 31, 193 people in Michigan have been hospitalized, and no deaths have been reported.

Nationally, the Centers for Disease Control and Prevention has focused on contaminated fresh produce as the potential cause of the current outbreak, while locally, the Michigan Department of Health and Human Services specifically flagged lettuce and salad greens as the possible culprit on July 13.

On July 16, the Food and Drug Administration initially announced that iceberg lettuce supplied to Taco Bell restaurants by Taylor Farms was the source of the outbreak; however, the agency quickly retracted that finding, stating the laboratory result was a false positive.

Because of its long incubation period, identifying the source for cyclospora is always incredibly difficult, but that may be only one piece of a larger puzzle.

As thousands of infected people shed the parasite, public health experts say millions of cyclospora oocysts (the parasite’s hardy, egg-like stage) could enter Michigan’s wastewater systems each day. Practices used at wastewater treatment facilities to kill common bacteria such as E. coli and Salmonella are not effective against cyclospora.

According to Joan Rose, a water microbiologist at MSU and director of the MSU Water Alliance, “wastewater treatment removes only a small percentage of the oocysts, and cyclospora is not inactivated by wastewater chlorination.”

Rose also explained that while cases of waterborne disease are rare — meaning it’s very unlikely to get sick from cyclospora while swimming or engaging in other recreational water activities — there is still some risk.

“Massive sewage contamination from sewage spills or CSOs along with very warm water could lead to recreational exposure via natural ponds and shallower lake areas,” Rose said.

Could this lead to smaller, localized spikes in infections down the road? And if so, how can we track it?

How cyclosporiasis spreads

Cyclosporiasis is an intestinal illness caused by the parasite Cyclospora cayetanensis. It’s only found in humans and spreads through fecal-oral contact — in other words, by consuming food or water contaminated with human feces. It’s an obligate pathogen, meaning it does not grow or replicate outside of the human body and requires a human host to live out its life cycle.

After someone ingests a mature oocyst, it travels through the stomach into their small intestine where digestive fluids and bile break open its protective shell. Once released, the active parasites burrow into the cells lining your gut and begin rapidly multiplying.

This activity causes severe inflammation, leading to painful stomach cramps and explosive, watery diarrhea as your body attempts to flush out the parasite. These symptoms can last anywhere from a few days to over a month, sometimes fading before returning. While rarely fatal, this cycle can cause dehydration in extreme cases and prolonged illness in individuals with compromised immune systems.

In Michigan, 160 people have been hospitalized as of July 23 but there have been no reported deaths regionally or otherwise.

Eventually, new, immature oocysts are excreted in stool. These are noninfectious and must mature in the environment at temperatures between 73.4 and 89.6 degrees Fahrenheit (23 and 32 degrees Celsius) to become infectious — around 40% of oocysts sporulate and become infectious within 14 days of being excreted. The process of replication begins again only after the now mature oocyst enters another human body.

Tracking cyclospora through wastewater monitoring

Wastewater monitoring could be used to examine the outbreak by identifying regions of concern and, perhaps more importantly, monitoring progress toward its end. Likewise, tracking areas where clinical testing should be prioritized is critical to ensure resources are available for high-risk individuals.

Nishita D’Souza, an assistant professor at MSU and water microbiologist in the Rose lab, explained that, “While state health authorities and wastewater facilities in Michigan do not routinely track cyclosporiasis, there is potential to optimize existing methods used for wastewater monitoring of viruses and fungi for protozoa like cyclospora.”

D’Souza and the Rose lab are developing and testing a method to detect cyclospora in wastewater. The process concentrates pathogens from sewage, extracts their DNA, and uses a highly sensitive laboratory technique — droplet digital PCR, or ddPCR — to detect even trace amounts of the parasite.

Once validated, D’Souza believes wastewater monitoring could help identify impacted areas in the state and provide key indications of areas where testing or interventions should be prioritized. Wastewater could also help monitor how interventions being suggested and implemented by health professionals are making a difference in keeping our communities protected from disease spread.

“Outbreaks like these underscore the importance of sustaining environmental and public health monitoring capacity, which can pivot to agile monitoring of new pathogens when warranted,” D’Souza said.

Limitations of traditional traceback

Wade Syers, a food safety specialist with MSU Extension is intrigued by the potential of wastewater monitoring for the parasite.

“I’d be very interested in what wastewater surveillance might reveal,” he said. “I wonder if signals in wastewater could help identify areas where contamination may be occurring. That could help with traceback efforts.”

Though it hasn’t been confirmed, it’s very likely the current outbreak is linked to widespread sewage-contaminated produce.

One of the first and largest outbreaks of cyclosporiasis in the U.S. occurred in 1996 because of sewage-irrigated raspberries imported from Guatemala. Other outbreaks in the U.S. have been linked to cilantro, bagged salads, snow and snap peas and vegetable trays.

Syers explained the difficulty in tracing outbreaks to their original source, stating, “One of the challenges with cyclospora is that fresh produce is distributed so broadly. By the time illnesses are identified, contaminated product may have been consumed in multiple states and mixed with products from numerous farms and distributors.”

Instead of waiting for patients to seek care and fill out food and exposure questionnaires, wastewater offers a more reliant, community-wide view. It could reveal not only where the outbreak is surging but also where it’s winding down.

But choosing the right method to guarantee high specificity is important.

“Direct protozoa monitoring of irrigation water is difficult,” Rose said. “However, microbial source tracking to detect human sewage can be used for irrigation waters and in the field to develop a safer system for produce in the future.”