Understanding the Cyclospora Outbreak and Sewage Contamination

The nationwide cyclospora outbreak likely stems from sewage contamination in the food supply. This is according to scientific researchers and public health officials. Cyclospora is a parasite found only in humans. It replicates in the intestinal tract and causes severe diarrhea. The organism reproduces by releasing oocysts, excreted in human feces.

In the United States, fecal matter typically ends up in sewage systems. In many states, sewage water undergoes treatment. However, this treatment doesn’t always effectively remove or kill the oocysts. Treated water is then released into waterways and can be used for crop irrigation. The oocysts mature in warm temperatures over about a week. When humans consume irrigated food or drink contaminated water, they risk infection. This creates a cycle by which more oocysts enter sewage systems.

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As a public health water microbiologist, I collect and test sewage water for pathogens such as cyclospora. The first cyclospora outbreak recorded in the U.S. occurred in Florida in 1995, involving 45 cases. Initial suspicions pointed to strawberries from California, but later findings suggested imported raspberries from Guatemala. This country was linked to larger outbreaks in 1996 and 1997.

Today, the largest recorded U.S. outbreak sickened over 22,000 people, with two deaths in Michigan. The use of treated sewage for crop irrigation is common, especially where water resources are limited.

Scope of Surveillance and Sewage Treatment

Researchers monitor sewage to assess disease prevalence in communities. During COVID-19, measuring the virus in sewage provided insights into its spread and variant emergence. Another aim is understanding the effectiveness of sewage treatment in reducing pathogens. Measuring cyclospora oocysts proves challenging due to the difficulty in detecting low levels, which still pose disease risks.

Watch: Cyclospora and salmonella outbreaks raise concerns about U.S. food safety

Cyclospora oocysts appear in sewage worldwide, with studies reporting detection in up to 25% of samples. However, concentration levels vary. An infected person might excrete 100 to 10,000 oocysts per gram of feces, potentially over 60 days. Research at Michigan State University aims to improve detection methods for lower levels of oocysts in sewage.

This surveillance could track outbreak patterns and occasionally assist sewage plant managers in monitoring discharges.

Effectiveness of Sewage Treatment Processes

Data on cyclospora oocyst reduction during sewage treatment is lacking, but information exists for similar protozoa like Cryptosporidium and Giardia, which also cause diarrhea in humans. Between 2001 and 2003, my laboratory studied these protozoa in six sewage treatment plants across states, such as Arizona and California. All plants were approved for nonpotable reuse, including crop irrigation.

These protozoa were present in all tested untreated sewage, indicating that some of the population is excreting oocysts. Treatment plants using chlorine disinfection removed a significant percentage of these protozoa, but not completely. Some cysts and oocysts withstood treatment, posing ongoing disease risks.

Although wastewater chlorination does not destroy these protozoa, ultraviolet light can inactivate cryptosporidium and related protozoa like eimeria, used as a surrogate for cyclospora studies.

Reuse and Health Risks

Annually, 200 billion gallons of treated sewage wastewater irrigate agricultural lands across the U.S. This water may undergo further filtration and disinfection. Detailed volume data is scarce. Some treated sewage, undergoing only standard secondary treatment, reenters rivers and reservoirs used for irrigation. Few states regulate protozoa monitoring in treated sewage. Proper filtration can remove protozoa, but chlorination is ineffective.

Need for Improved Risk Assessment and Management

Floods and droughts can lead to sewage overflows into water bodies or direct use on crops. This heightens the risk of diseases, including protozoa like cyclospora, as warm temperatures expedite oocyst maturation. Modern technology enables water quality monitoring, pathogen detection, and elimination. Expanding wastewater surveillance for protozoa can prevent outbreaks and curb disease spread from sewage-contaminated sources.

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