Cyclospora Outbreak and Sewage Contamination

The nationwide cyclospora outbreak likely originates from sewage contamination in the food supply, as confirmed by scientific researchers and public health officials. Cyclospora, a parasite found solely in humans, replicates in the intestinal tract causing severe diarrhea. It reproduces through oocysts excreted in human feces.

In the U.S., feces enter sewage systems. Sewage water is treated in many states, though these processes do not always remove or kill cyclospora oocysts. This treated water is released into waterways and used to irrigate crops. Oocysts mature in warm environments in about a week. Humans then consume the irrigated food or water, become infected, and continue the cycle by releasing more oocysts into sewage systems.

Historical Context and Personal Experience

As a public health water microbiologist, I collect and test sewage water for harmful pathogens like cyclospora. My first study on cyclospora involved the first U.S. outbreak in Florida in 1995, where the cause was initially thought to be contaminated strawberries from California. It was later linked to imported raspberries from Guatemala.

Currently, the largest U.S. outbreak has affected over 22,000 people, resulting in two fatalities in Michigan. Treated sewage is frequently used to irrigate crops, especially where water resources are limited.

The Role of Wastewater Surveillance

Researchers collect and monitor sewage for two main reasons. First, to assess community disease levels. Tracking viruses like SARS-CoV-2 in sewage helped map the spread of COVID-19. Second, to evaluate the sewage treatment process in terms of pathogen elimination.

Detecting cyclospora oocysts in sewage, contaminated water, or food poses challenges due to their low but disease-causing levels. Globally, studies find cyclospora oocysts in up to 25% of sewage samples, though concentrations are rarely detailed. Infected individuals excrete about 100 to 10,000 oocysts per gram of feces over two months. Estimations suggest between 1 to 100 oocysts per liter in sewage.

Our team at Michigan State University is developing methods to detect lower levels of parasites in sewage reliably. This surveillance may pinpoint when an outbreak wanes and affected areas. It also aids sewage treatment facilities in monitoring discharges.

Sewage Treatment and Protozoa

Data on cyclospora oocyst reduction through standard sewage treatment are unclear. However, research on similar protozoa like Cryptosporidium and Giardia shows treatment plants using chlorine can reduce but not eliminate protozoa. Our lab studied these protozoa in treatment plants across the U.S., showing their presence in untreated sewage.

Despite attempts at disinfection, some protozoa remain viable post-treatment, and wastewater chlorination is ineffective against them. It’s reasonable to assume some cyclospora oocysts survive these processes and re-enter the environment.

Treated Sewage Reuse Concerns

Over 200 billion gallons of treated sewage are reused annually for agricultural irrigation in the U.S. Some undergo further filtration and disinfection before being applied to crops or landscapes. Specific volumes remain unknown. Other treated sewage, only undergoing secondary treatment, gets discharged into potential irrigation sources.

Few states regulate removing or monitoring cyclospora or similar protozoa in treated sewage. Filtration can remove protozoa, but only if properly designed and operated. Chlorination fails against these organisms, whereas ultraviolet light can inactivate similar protozoa.

Improving Risk Management

Rising temperatures and climate events like floods or droughts increase the chance of sewage contaminating water or crops, escalating the risk of diseases spread through fecal matter. Cyclospora oocysts mature faster with heat, posing further threats as climates warm.

Modern technologies offer capabilities for pathogen detection and removal in water quality monitoring. Expanding wastewater surveillance for protozoan diseases can prevent outbreaks and slow the spread from sewage-contaminated water.