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

The nationwide cyclospora outbreak is likely due to sewage contamination in the food supply, according to researchers and public health officials. Cyclospora, a parasite found only in humans, replicates in the intestinal tract and causes severe diarrhea. It reproduces by releasing oocysts, which are excreted in human feces. In the U.S., these oocysts typically end up in sewage systems, which are then released into waterways and used to irrigate crops.

The oocysts mature in warm temperatures within about a week. If a human consumes the irrigated food or drinks the water, they can become sick and release more oocysts, perpetuating the cycle. The first documented U.S. outbreak occurred in 1995 in Florida, with the cause initially suspected to be strawberries from California but later determined to be imported contaminated raspberries from Guatemala.

In 1996 and 1997, Guatemala's raspberries were linked to larger outbreaks. More recently, a major outbreak in 2021 sickened over 22,000 people and caused two deaths in Michigan. Researchers collect sewage to monitor disease levels and assess if sewage treatment effectively removes or inactivates pathogens. Detecting cyclospora oocysts in sewage is challenging due to their low levels, but laboratory methods are continuously being improved.

Studies have shown that about 25% of sewage samples can contain cyclospora oocysts, with concentrations ranging from 1 to 100 oocysts per liter. While standard sewage treatment processes may not eliminate all oocysts, some plants can reduce their numbers. However, there is uncertainty about the survival rate of cyclospora oocysts after treatment, and their presence in treated wastewater can still pose a risk of infection.