Water microbiologist ties cyclospora outbreak to sewage-contaminated food

Largest U.S. cyclospora outbreak has sickened more than 22,000 people

Joan Rose, a professor of water research and director of the Water Alliance at Michigan State University, said the nationwide cyclospora outbreak is very likely a result of sewage contamination in the food supply. Rose, a public health water microbiologist who collects sewage and tests it for harmful pathogens, published the analysis on Aug. 10.

Rose described a self-perpetuating cycle that carries the parasite from human waste through sewage systems onto crop fields. Cyclospora is found only in humans, where it replicates in the intestinal tract and causes severe diarrhea, she wrote. The organism reproduces by releasing oocysts, which are excreted in feces. In the United States, feces usually end up in sewage systems, and sewage water is treated — though not always in ways that remove or kill the oocysts — before being released into waterways or used to irrigate crops. The oocysts mature in warm temperatures in about a week, after which a person who eats the irrigated food or drinks the water can fall sick and shed more oocysts into sewage, continuing the cycle.

The outbreak has sickened more than 22,000 people and contributed to the deaths of two in Michigan, where Rose lives and works. She identified it as the largest cyclospora outbreak ever recorded in the United States.

Rose first studied the parasite during the first cyclospora outbreak ever documented in the country, a 45-case outbreak in Florida in 1995. The cause was initially suspected to be strawberries grown in California, she wrote, but investigators later determined it was more likely imported contaminated raspberries from Guatemala, which were also associated with larger outbreaks in 1996 and 1997.

An infected person excretes between 100 and 10,000 oocysts per gram of feces for as long as 60 days, Rose wrote. Based on what is known about other fecal pathogens, she estimated that sewage contains anywhere from 1 to 100 oocysts per liter. Studies around the world have detected cyclospora oocysts in up to 25 percent of sewage samples, though not all report how high or low the concentrations were.

Rose said there is no clear data on how well standard sewage treatment reduces cyclospora oocysts, but she pointed to findings from an earlier study of two similar protozoa. From 2001 to 2003, her laboratory studied cryptosporidium — an oocyst about half the size of cyclospora — and giardia — a cyst similar in size — at six sewage treatment plants, one each in Arizona and California and four in Florida. All had been approved by their state regulators for nonpotable reuse, including irrigation. The five plants whose processes included chlorine disinfection removed a high percentage, but not all, of the protozoa in their discharges; some cysts and oocysts remained intact with the potential to cause disease. Rose noted that wastewater chlorination does not kill these protozoa, and wrote that it is reasonable to assume at least some cyclospora oocysts also survive treatment and are released into the environment, where they can survive for months.

Across the United States, about 200 billion gallons of treated sewage wastewater are used to irrigate agricultural land each year, Rose wrote. Some of that water undergoes additional filtration and disinfection for reuse before being spread on landscapes or crops, but the volumes are not readily known. She noted that few states regulate efforts to remove or monitor protozoa such as cyclospora in treated sewage. Filtration can remove protozoa but must be designed and operated correctly; chlorination is not effective, while ultraviolet light does inactivate cryptosporidium and eimeria, a chicken protozoan used as a surrogate in studies testing methods of killing cyclospora.

Rose’s laboratory at Michigan State University is developing a method to detect the parasite at lower levels in sewage. Such wastewater surveillance, she wrote, could help determine when an outbreak is beginning to subside and where larger numbers of people are still affected, and could allow sewage treatment plant managers to regularly monitor their discharges.

Rose wrote that as floods and droughts occur routinely across the United States and around the world, sewage may overflow into bodies of water or be used directly on crops, increasing the risk that diseases transmitted through ingestion of fecal matter spread through food and water. Because cyclospora oocysts mature in outdoor heat, she added, rising temperatures such as heat domes could speed up their maturation and expose more people to the infectious form of the parasite. She argued that expanding monitoring of wastewater for protozoan diseases and other dangers could help prevent outbreaks and slow the spread of disease from sewage-contaminated water in the future.