In a summer already characterized by extreme heat and a surplus of ticks, we didn’t need an outbreak of a nightmare stomach bug. But here we are, with the highest ever count of cyclosporiasis cases in a season. It’s an illness caused by consuming the eggs of the microscopic Cyclospora parasite, usually as a result of contaminated produce or water.
This surge in cases has gotten a lot of attention, in part because infection with the parasite causes terrible bouts of diarrhea. Google searches for “cyclospora” have spiked more than 5,000 percent over the past month. Fellow parents keep asking me if they should stop feeding their children raspberries or lettuce.
That’s understandable, because no one wants these symptoms. But a look at the numbers suggests that you can probably chill out a bit. Our collective freakout about the parasite tells us something important about how we process information and evaluate risk about disease.
The number of documented cases — even with this highest number ever — is really quite small. The Centers for Disease Control and Prevention has reported 1,645 confirmed cases since May 1 and is aware of an additional 5,100 suspected cases that need more analysis. By contrast, the agency this year has reported nearly 35,000 cases of Campylobacter infection, which is another serious diarrheal illness. Both of these pale in comparison to the likely number of norovirus cases. While the C.D.C. doesn’t track norovirus cases individually (because they are so numerous), it estimates that up to 21 million Americans a year get sick from the virus, more than 100,000 are hospitalized, and nearly 1,000 die.
Cyclospora can last a lot longer than norovirus or Campylobacter — sometimes a month or longer. But it also usually resolves on its own and is treatable with antibiotics. Deaths are very rare, and it’s not contagious from person to person.
Cases are much higher than usual in a handful of states, with Michigan and Ohio appearing to be the biggest outliers. But in much of the rest of the country, cases appear to be flat or only modestly elevated relative to a typical year. Summer always brings some isolated cases of Cyclospora. In California, the Department of Health has reported 41 cases, versus 80 during the same period last year. In Connecticut, cases are higher than usual, but that means 35 cases as of earlier this week, versus 19 last year at this time. That’s roughly one case for every 100,000 people.
To be clear: The case counts could go up, and it’s appropriate to pay attention. The data is woefully incomplete, which is the first big obstacle to understanding the scope of the problem. Already, Michigan is reporting far more cases within the state than the C.D.C. has confirmed for the whole country — a result of the lag time it takes for the federal agency to collect state numbers.
Every year, this illness is likely undercounted. Watery diarrhea is a very common problem. It can be caused by all kinds of things, and people mostly suffer at home. In a typical year, many of the people with Cyclospora infection will never get tested or learn they had it. But when many people are talking about the parasite, more people will seek medical help for these symptoms, especially since there is a treatment option. That means more cases get reported. It’s hard to know how much of the higher case count to attribute to more parasite infections (which I’m convinced is happening in some states) versus how much is being driven by greater awareness and reporting.
We do not have the kind of broad public surveillance we’d need to answer that question (wastewater testing would be an option here, but it’s not clear it’s being done) — a problem that came up during Covid and hasn’t gotten much better. Without that, we’ll never really know how many cases there are, or how that number compares with those from the past.
Another problem is public health communication. There is a lot of uncertainty around this outbreak. We do not yet know what its source is, or even if there is a single source. Michigan’s health department has suggested that the common thread may be lettuce or salad greens, but that’s unconfirmed. This is a tough problem because, among other factors, there can be a long incubation period between ingesting the parasite and becoming ill, and few people remember what they ate two weeks ago.
Better funded state public health departments would certainly speed up answers. But, for now, this uncertainty is unavoidable. The public would benefit from clear messaging focused on what we know, what we don’t know, how we are planning to learn more and when the next update will arrive. Unfortunately, public health messaging often fails to provide this level of consistent detail.
And then there’s us. Humans have evolved to react to the risks that we see in front of us. When we see a tiger, we need to react by running, not by thinking about the statistical likelihood of being killed by a tiger. If our social media feeds are full of posts about Cyclospora, that’s the tiger — and we want to react. That makes it harder to react rationally.
The rational response would be to think about your risk of getting cyclosporiasis relative to the other risks you’re already living with — including the base-line summer risk of this parasite, which you probably never thought about before. That’s a lot less scary. But it’s hard to put your mind into that frame.
In the face of all this, what should you actually do? I’d recommend finding a single source you trust for information on this, and tracking that. By following one source, you’re likely to pick up more signal and less noise. Also, consider your location before deciding what to do. There is a real elevated risk this year in Michigan and Ohio — much less so outside of those places.
And please do not stop eating fruits and vegetables. If you are really worried about this, just cook your vegetables and fruits, which kills the parasite.
Emily Oster is the founder and chief executive of ParentData, an information platform that helps parents think about risk.
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