Cyclospora and other parasites can survive sewage treatment in small quantities, and crops are sometimes irrigated with treated water.
That’s why scientific researchers and public health officials are pointing to sewage contamination in the food supply as one of the most plausible theories for the current U.S. cyclospora outbreak. The government says it is still investigating the cause.
The parasite’s spread can become a self-perpetuating cycle. Cyclospora is found only in humans, where it replicates in the intestinal tract, causing severe diarrhea. The organism reproduces by releasing oocysts, which are excreted in those human feces. In the United States, feces usually end up in sewage systems.
In many states, sewage water is treated — though not always in ways that remove or kill the oocysts — and then released into waterways that can be used to irrigate crops. The oocysts mature in the environment with warm temperatures in about a week. Then a human comes along and eats the irrigated food or drinks the water and gets sick, releasing more oocysts into sewage systems.
I am a public health water microbiologist who collects sewage water and tests it for evidence of harmful pathogens, such as cyclospora. The first time I studied cyclospora was in the first outbreak ever documented in the U.S., a small, 45-case outbreak in Florida in 1995.
The cause was initially suspected to be strawberries grown in California, but later it was determined it was more likely caused by imported contaminated raspberries from Guatemala. That country’s raspberries were also associated with much larger outbreaks in 1996 and 1997. At the time, little was known about this single-celled protozoan pathogen.
More than 30 years later, the largest outbreak ever recorded in the U.S. has sickened more than 22,000 people and contributed to the deaths of two in Michigan, where I live and work.
What’s in wastewater
Using treated sewage to irrigate crops is common in many places, especially where groundwater and surface water are in short supply.
There are two reasons researchers like me collect sewage and monitor what is in it.
The first is to evaluate levels of disease in a community. Measuring viruses such as SARS-CoV-2 in untreated sewage helped paint a picture of 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.
It is not easy to measure cyclospora oocysts 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 shows that they can be detected in up to 25 percent 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 100 to 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 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. In addition, this information could allow sewage treatment plant managers to regularly monitor their discharges.
What happens during sewage treatment?
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 protozoa that also cause significant diarrhea in humans: Cryptosporidium produces an oocyst about half the size of cyclospora, and giardia produces a cyst that is similar in size to cyclospora.
From 2001 to 2003, my laboratory studied the occurrence of those protozoa at six sewage treatment plants — one each in Arizona and California and four in Florida. All had been approved by their respective state regulators for non-potable reuse, including irrigation of landscapes and, in some cases, crops.
In that study, we found these protozoa in all the untreated sewage we tested, indicating there is some portion of the population that is infected and excreting their cysts or oocysts.
The five treatment plants whose processes included disinfection with chlorine were able to get rid of a high percentage — but not all — of the protozoa in their discharged wastewater. Some cysts and oocysts remained intact, with the potential to cause disease, even in water that had gone through the entire treatment process, as it is known that wastewater chlorination does not kill these protozoa.
From that data, it seems reasonable to assume that at least some small proportion of cyclospora oocysts also survive sewage treatment processes and are released back into the environment, where they can survive for months.
What about 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 protozoa such as cyclospora in treated sewage. Filtration can remove protozoa but must be designed and operated correctly. Chlorination is not effective, but ultraviolet light does inactivate cryptosporidium and eimeria, a chicken protozoan highly related to cyclospora and used as a surrogate in studies testing methods of killing cyclospora by the food safety industry.
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 protozoa such as cyclospora, which are transmitted through ingestion of fecal matter, may spread through food and water supplies.
And because cyclospora oocysts mature in outdoor heat, it is possible that increasing temperatures, such as heat domes, could speed up their maturation, exposing more people to the infectious form of the parasite.
Joan Rose is laboratory director/principal investigator in water research at Michigan State University.
This article was produced in collaboration with The Conversation, a nonprofit news organization.
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