Cyclospora spreads through human waste, so how did it get on food?

The current nationwide cyclospora outbreak is almost certainly due to sewage contaminating our food supply, according to medical professionals and government officials. This parasite, which only exists in humans, replicates within the human intestinal tract, causing intense diarrhea. It spreads through oocysts, which are released in human waste, and can survive in the environment for up to a week when exposed to warm temperatures.

These oocysts often end up in sewage systems, which are then released into waterways or used to irrigate crops. The cycle continues as humans consume the contaminated food or water, getting sick and releasing more oocysts back into the sewage system.

As a public health water microbiologist who tests sewage for harmful pathogens, I've seen this outbreak before. The first U.S. outbreak was in 1995, caused by contaminated raspberries imported from Guatemala. More recently, the largest outbreak ever recorded in the U.S. has sickened over 22,000 people and resulted in two deaths.

Treated sewage is commonly used for irrigation, particularly in regions with scarce water resources. The reasons for collecting sewage and monitoring its contents are two-fold: to assess disease levels in a community and to determine the effectiveness of sewage treatment in removing, inactivating, or killing pathogens.

Detecting Cyclospora oocysts in sewage is challenging due to their low levels and the difficulty of modern laboratory methods. They've been found in up to 25% of sewage samples, though concentrations can vary. A single infected person can release between 100 and 10,000 oocysts per gram of feces for up to 60 days. Based on this, there could be anywhere from 1 to 100 oocysts per liter in sewage.

Our lab at Michigan State University is working on a more accurate detection method for the parasite even at lower levels in sewage. This could help determine when an outbreak is subsiding and where more people are still affected, as well as allow sewage treatment plant managers to regularly monitor their discharges.

While there are no clear data on how well standard sewage treatment processes reduce Cyclospora oocysts, studies have shown that some treatments can get rid of a high percentage of protozoa, including Cryptosporidium and Giardia, but not all. Cyclospora oocysts can survive in treated sewage and may remain infectious for months, potentially causing disease in those who consume the contaminated food or water.

Approximately 200 billion gallons of treated sewage wastewater are used for irrigation in the U.S. annually, some of which undergo additional filtration and disinfection before being spread directly on crops. However, few states regulate efforts to remove or monitor protozoa such as Cyclospora in treated sewage. Filtration can remove protozoa, but it must be done correctly.

Chlorination is not effective, but ultraviolet light can inactivate Cyclospora. As floods and droughts occur regularly across the U.S. and around the world, it's crucial to understand how sewage treatment affects the spread of this parasite.