Understanding the Cyclospora Outbreak Linked to Sewage Contamination

Scientific researchers and public health officials have indicated that the recent nationwide cyclospora outbreak likely results from sewage contamination in the food supply. Cyclospora, a single-celled protozoan found only in humans, replicates in the intestinal tract, causing severe diarrhea. It reproduces by releasing oocysts, which are excreted in feces. In the United States, these feces often end up in sewage systems. In many states, sewage water is treated, although not always effectively enough to remove or kill the oocysts. This water can then be released into waterways and used to irrigate crops. Oocysts mature in warm environments in about a week, and if a person consumes the irrigated food or water, they can get sick, releasing more oocysts into the sewage system.

Background on Cyclospora
The first U.S. cyclospora outbreak occurred in Florida in 1995, with 45 cases initially linked to California strawberries. Later, the source was traced to contaminated raspberries imported from Guatemala. Guatemala’s raspberries were implicated in larger outbreaks in 1996 and 1997. Despite the challenges posed by this pathogen, cyclospora remains problematic, as evidenced by the largest outbreak ever recorded in the U.S., affecting over 22,000 people and contributing to two deaths in Michigan.

Sewage Use in Agriculture
Using treated sewage to irrigate crops is common, especially in areas with limited groundwater and surface water. Researchers collect sewage to assess disease levels in communities and evaluate sewage treatment efficacy. For instance, measuring SARS-CoV-2 in untreated sewage helped map COVID-19 spread and variant emergence. Cyclospora oocysts, while difficult to detect even with modern lab methods, have been found in sewage worldwide, leading to uncertainty about their widespread impact.

Sewage Treatment and Pathogens
There is limited data on how effective sewage treatment processes are in reducing cyclospora oocysts. Studies of other protozoa, like Cryptosporidium and Giardia, reveal that while chlorine disinfection removes a significant percentage of these pathogens, some remain intact even after treatment. This suggests that cyclospora oocysts might also survive sewage treatment, remaining a potential disease source.

Reuse of Treated Sewage
In the U.S., 200 billion gallons of treated sewage wastewater are used annually for agricultural irrigation. Some of this water undergoes additional filtration and disinfection, but the extent is unclear. Few states regulate efforts to remove or monitor protozoa like cyclospora. Effective filtration can remove protozoa, and ultraviolet light can inactivate certain pathogens, offering methods for protozoan control.

Looking Ahead: Risk Management
Floods and droughts increase the risk of sewage overflow into water bodies, enhancing the spread of diseases like cyclospora, transmitted via fecal matter ingestion. Rising temperatures may accelerate the maturation of cyclospora oocysts, further exposing populations to infection. Improved technology and techniques can monitor water quality, detect pathogens, and mitigate risks. Expanding wastewater monitoring for protozoan diseases could prevent future outbreaks and slow the spread of disease from contaminated water.