The nationwide cyclospora outbreak is very likely a result of sewage contamination in the food supply, according to scientific researchers and public health officials. Cyclospora is found only in humans, where it replicates in the intestinal tract, causing severe diarrhea. The organism reproduces by releasing oocysts, which are excreted in those human feces.
In the U.S., feces usually end up in sewage systems. In many states, sewage water is treated ― though not always in ways that remove or kill the oocysts ― and then released into waterways and can be used to irrigate crops. The oocysts mature in the environment with warm temperatures in about a week. A human comes along and eats the irrigated food or drinks the water and gets sick, releasing
more oocysts into sewage systems.
The largest outbreak ever recorded in the U.S. has sickened more than 22,000 people. Using treated sewage to irrigate crops is common in many places, especially where groundwater and surface water are in short supply.
There are two reasons researchers like me collect sewage and monitor what’s in it. The first is to evaluate levels of disease in a community. Measuring viruses, such as SARS-CoV-2 in untreated sewage, helped paint a picture of the spread of COVID-19 and the emergence of variants as the pandemic continued. The second reason is to determine whether and how sewage treatment removes, inactivates or kills pathogens.
It is not easy to measure cyclospora oocysts in sewage, contaminated water or food. Even modern laboratory methods have trouble reliably detecting low levels of oocysts, which can still cause disease.
Our laboratory at Michigan State University is developing a method to more accurately detect this parasite even at lower levels in sewage. This type of wastewater surveillance may help determine when an outbreak is beginning to subside and where larger numbers of people are still affected. In addition, this information could allow sewage treatment plant managers to regularly monitor their discharges.
States aren't keeping up with treating tainted water
There isn’t clear data on how well standard sewage treatment processes reduce the numbers of cyclospora oocysts. But there is information on two other similar protozoa that also cause significant diarrhea in humans: Cryptosporidium produces an oocyst about half the size of cyclospora, and giardia produces a cyst that is similar in size to cyclospora.
From 2001 to 2003, my laboratory studied the occurrence of those protozoa at six sewage treatment plants ― one each in Arizona and California and four in Florida. All had been approved by their respective state regulators for nonpotable reuse, including irrigation of landscapes and, in some cases, crops.
In that study, we found these protozoa in all the untreated sewage we tested, indicating there is some portion of the population that is infected and excreting their cysts or oocysts. The five treatment plants whose processes included disinfection with chlorine were able to get rid of a high percentage – but not all – of the protozoa in their discharged wastewater. Some cysts and oocysts remained intact, with the potential to cause disease, even in water that had gone through the entire treatment process, as it is known that wastewater chlorination does not kill these protozoa.
From that data, it seems reasonable to assume that at least some small proportion of cyclospora oocysts also survive sewage treatment processes and are released back into the environment, where they can survive for months.
Across the U.S., 200 billion gallons of treated sewage wastewater are used for irrigation of agricultural lands each year. Some of that undergoes additional filtration and disinfection for reuse before being spread directly on landscapes or crops. The volumes, however, are not readily known. And other treated sewage, which undergoes only standard secondary treatment, is discharged into rivers, streams and reservoirs that could provide irrigation water.
But 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, but ultraviolet light does inactivate cryptosporidium and eimeria, a chicken protozoan highly related to and used as a surrogate in studies testing methods of killing cyclospora by the food safety industry.
As floods and droughts occur routinely across the U.S. and around the world, sewage may overflow into bodies of water or be used directly on crops, and because cyclospora oocysts mature in outdoor heat, it is possible that increasing temperatures could speed up their maturation, exposing more people to the infectious form of the parasite.
Modern technology and techniques are capable of monitoring water quality, detecting harmful pathogens and eliminating them. Expanding monitoring of wastewater for protozoan diseases and other dangers can help prevent outbreaks and slow the spread of disease from sewage-contaminated water in the future.
Joan Rose is professor of Water Research and director of Water Alliance at Michigan State University. She wrote this piece for The Conversation, an independent news organization dedicated to unlocking the knowledge of experts for the public good.
This article originally appeared on Palm Beach Post: Forget produce. Check the sewage for cyclospora | Opinion
