Ask the expert: How does cyclospora make its way into our food? 

A growing nationwide outbreak of cyclospora continues to raise alarm among scientists and public health officials who point to sewage contamination in the food supply as the likely cause.

The outbreak has sickened more than 22,000 people in the U.S. and contributed to the deaths of two in Michigan. This is not the first nationwide outbreak of the parasite, as 30 years ago imported raspberries from Guatemala were associated with outbreaks in 1996 and 1997.

Joan Rose is the Homer Nowlin Chair in Water Research at Michigan State University’s College of Agriculture and Natural Resources where she also directs the MSU Water Alliance. Her research explores advanced ways to detect and monitor pathogens in water, track their sources and improve water treatment processes.

Here, Rose provides insights into cyclospora, including how it can contaminate fresh produce, how it can enter our food supply and how challenging it can be to mitigate.

Why is it important to monitor wastewater? How is cyclospora monitored?

There are two reasons researchers collect wastewater and monitor what’s in it. The first is to evaluate levels of disease in a community. Measuring viruses such as SARS-CoV-2 in untreated sewage helped evaluate the spread of COVID-19 and the emergence of variants as the pandemic continued.

The second reason is to determine whether and how sewage treatment removes, inactivates or kills pathogens in the wastewater.

It is not easy to measure cyclospora oocysts — the infectious stage of the parasite — in sewage, contaminated water or food. Even modern laboratory methods have trouble reliably detecting low levels of oocysts, which can still cause disease.

Cyclospora oocysts have been found in sewage around the world. A range of studies across the world show that they can be detected in up to 25% of sewage samples — but not all studies report how high or low the concentrations of the oocysts were. So, it can be hard to say exactly how widespread it is.

A person who is infected with cyclospora excretes somewhere between 100 and 10,000 oocysts per gram of feces for as long as 60 days. Based on what is known about other fecal pathogen excretions from patients related to the concentrations of those pathogens in sewage, I estimate that there could be anywhere from 1 to 100 oocysts per liter in sewage.

Our laboratory at Michigan State University is developing a method to more accurately detect this parasite even at lower levels in sewage. This type of wastewater surveillance may help determine when an outbreak is beginning to subside and where larger numbers of people are still affected.

How does sewage treatment reduce parasite outbreaks?

There isn’t clear data on how well standard sewage treatment processes reduce the numbers of cyclospora oocysts. But there is information on two other similar parasites that also cause significant diarrhea in humans: cryptosporidium and giardia.

From 2001 to 2003, my laboratory studied the occurrence of cryptosporidium and giardia at six sewage treatment plants. All had been approved by their respective state regulators for reuse for irrigation of landscapes and, in some cases, crops.

The five treatment plants whose processes included disinfection with chlorine were able to get rid of a high percentage — but not all — of the parasites in their discharged wastewater. Some remained with the potential to cause disease, even in water that had gone through the entire treatment process.

From that data, it seems reasonable to assume that at least some small proportion of cyclospora also survive sewage treatment processes and are released back into the environment, where they can survive for months.

What steps are taken to make wastewater safe for reuse?

Across the U.S., 200 billion gallons of treated sewage wastewater are used for irrigation of agricultural lands each year. Some of that wastewater undergoes additional filtration and disinfection for reuse before being spread directly on landscapes or crops. The volumes, however, are not readily known. And other treated sewage, which undergoes only standard secondary treatment, is discharged into rivers, streams and reservoirs that could provide irrigation water.

But few states regulate efforts to remove or monitor parasites such as cyclospora in treated sewage. Filtration can remove parasites but must be designed and operated correctly. Chlorine is not effective at killing them. However, ultraviolet, or UV, light can inactivate cryptosporidium and Eimeria, a chicken parasite closely related to cyclospora that food safety researchers use to test ways to kill cyclospora.

What is needed for better risk assessment and management?

As floods and droughts occur routinely across the U.S. and around the world, sewage may overflow into bodies of water or be used directly on crops. That increases the risk that diseases, including cyclospora, may spread through food and water supplies.

Cyclospora oocysts mature in warm outdoor temperatures, so it is possible rising temperatures could make them mature faster and increase the spread of infection.

Modern technology and techniques can monitor water quality, detect harmful pathogens and eliminate them. Expanding monitoring of wastewater for protozoan diseases and other dangers can help prevent outbreaks and slow disease spread from sewage-contaminated water in the future.

Responses and excerpts from an article originally published in The Conversation.