The widespread cyclospora outbreak in the U.S. is likely due to sewage contamination in the food supply, according to researchers and public health experts. This parasite, found solely in humans, thrives in the intestinal tract and causes severe diarrhea. It reproduces by releasing oocysts, which are then excreted. In the U.S., these excretions usually enter sewage systems.
Sewage water often undergoes treatment. However, not all methods effectively eliminate oocysts, which mature in warm environments within about a week. If a person consumes contaminated food or water, they become ill and contribute more oocysts to the sewage, continuing the cycle.
A self-sustaining cycle of disease can occur if not adequately managed.
I am a public health water microbiologist, specializing in testing sewage for harmful pathogens like cyclospora. My first study of cyclospora was during a 1995 outbreak in Florida, initially attributed to California strawberries but later linked to imported Guatemalan raspberries. Those imports also caused significant outbreaks in 1996 and 1997. The largest recorded U.S. outbreak has now affected over 22,000 people and led to deaths in Michigan, where I reside.
Using treated sewage for crop irrigation is common, particularly in areas with limited water supplies. Researchers study sewage for two main reasons: assessing community disease levels and evaluating treatment effectiveness in pathogen removal. Sewage has been critical in understanding diseases like COVID-19.
Detecting cyclospora oocysts, even with modern methods, poses challenges due to their low concentration levels in sewage, contaminated water, or food. Studies demonstrate cyclospora presence in up to 25% of sewage samples globally, though concentrations vary. Those infected can excrete 100 to 10,000 oocysts per gram of feces for up to 60 days. Our lab at Michigan State University is developing techniques to enhance detection accuracy for this parasite in wastewater.
This type of surveillance can help pinpoint when outbreaks start declining and identify where they persist. It may also aid sewage plant managers in monitoring discharge quality.
Effectiveness of Sewage Treatment
Current data is inconclusive on how well standard treatment processes reduce cyclospora oocyst numbers. Studies on similar protozoa, namely Cryptosporidium and Giardia, which cause severe diarrhea, provide insights. From 2001 to 2003, we studied these protozoa at six facilities in Arizona, California, and Florida, where water is reused for nonpotable purposes, including landscape and crop irrigation.
All untreated sewage tested contained these protozoa, signaling infected individuals were excreting them. Of the facilities using chlorination, a significant portion of the protozoa was removed, yet some survived, posing health risks. Chlorination alone does not eliminate these parasites.
It is plausible that some cyclospora oocysts also endure sewage treatment, surviving months in the environment after discharge.
Impact of Reuse Practices
Annually, 200 billion gallons of treated sewage are used for agricultural irrigation in the U.S. Additional filtration and disinfection often occur before distribution on crops, though precise volumes remain undisclosed. Many states lack regulatory measures for removing or monitoring protozoa in treated sewage. While filtration can remove protozoa if properly managed, chlorination is ineffective. Ultraviolet light can inactivate related protozoa like Cryptosporidium.
Improving Risk Management
The regular occurrence of floods and droughts increases the risk of sewage overflow or direct application on crops, raising the potential for diseases like cyclospora to spread. This risk is heightened as rising temperatures accelerate oocyst maturation, exposing more individuals to infection.
Advancements in technology allow for effective water quality monitoring and pathogen removal. Enhancing wastewater surveillance can help mitigate future outbreaks and slow disease spread due to sewage contamination.