Diarrhea: coming to a town near you this week?

I’ve been watching the outbreak of cyclospora in the U.S., for a few weeks now. It’s probably time for a primary care update about this, even though I’d rather be focusing on the World Cup semifinals! Here’s my take.

The United States is in the middle of a rapidly expanding, multi-cluster outbreak of cyclosporiasis. This is an intestinal illness caused by the microscopic parasite Cyclospora cayetanensis. Based on recent reporting, it’s coming from several simultaneous clusters, not a single confirmed nationwide common-source outbreak.

The numbers are moving quickly, and whatever is being detected is likely just the tip of the diagnostic iceberg (more on why later).

• As of July 10, the CDC had confirmed 843 domestically acquired cases across 31 states and was evaluating more than 1,500 additional reports to determine whether they qualify.

• I like how Your Local Epidemiologist Katelyn Jetelina derived her numbers better: “This outbreak, the largest in U.S. history, has topped 3,000 cases, and it’s not slowing down. That number comes from adding up state-level case counts, which are running well ahead of the counts on CDC’s website. For context, the U.S. typically sees around 3,000-4,000 cyclosporiasis cases in an entire year…”

• I agree that many more people are not seeking care or getting diagnosed yet. The logistics for testing involve people seeing a doctor/urgent care, getting the right order for the right stool testing (cyclospora is not detected in the usual “ova and parasites” screen that most doctors order), and often multiple stool samples need to be checked.

• The Midwest is bearing the brunt: Michigan alone had surpassed 1,500 cases by July 10, against a typical annual total of roughly 50, and Ohio and New York appear bad, too.

• At least 86 people have been hospitalized so far, no deaths have been reported, and officials expect counts to keep climbing.

The CDC has launched a traceback investigation but has not yet named a source. Previous U.S. outbreaks have implicated fresh produce like berries, leafy greens, and herbs… but no specific item, farm, retailer, or restaurant chain has been linked to this event yet, and there is no food recall as of this writing.

After an average incubation of about a week, cyclosporiasis typically causes:

• prolonged, watery, sometimes explosive diarrhea

• abdominal cramps

• bloating

• nausea

• fatigue

• loss of appetite

• weight loss

• sometimes a low-grade fever

• The signature feature is its course: symptoms often wax and wane, can persist for weeks, and may relapse if the infection isn’t treated. That drawn-out, stop-and-start pattern is another big reason source attribution is so hard. People struggle to recall what they ate a week or two earlier. Can you? Me neither! Plus, I ate more than one thing a week ago.

Several summer diarrheal illnesses look similar at first. The main parasitic mimics are Cryptosporidium, Giardia, and Isospora. The most common viral cause of diarrhea is usually norovirus.

Two clues point toward Cyclospora however:

• diarrhea that lasts longer than a week and waxes and wanes

• illness that doesn’t spread the way a typical stomach bug does

Because Cyclospora is shed in a non-infectious form and needs one to two weeks in the environment to mature, it is not transmitted person-to-person.

Contrast that with the dreaded norovirus, which spreads readily within households (and even pools), and only needs a tiny viral dose to cause infection.

Here is some family doc practical advice, should you find yourself with a suspicious case of diarrhea this summer. The single most important point: you have to ask for cyclospora testing specifically, AND YOUR DOC HAS TO ORDER IT BY NAME.

*A routine stool ova-and-parasite exam (what most of us docs order, and what comes up on our electronic medical record’s menu of test options) does not include Cyclospora. I checked my EMR just now. I could not even order “cyclospora” on the system we use. That system, called EPIC, is used by about half of the nation’s doctors, and that is a real problem, but I digress. So I will have to break out the paper prescription pad and write out: “Cyclospora stool testing.”

That will prompt the lab (usually Quest or Labcorp) to run either:

• A GI multiplex PCR panel that includes Cyclospora. Looks like Labcorp uses this one. Apparently it is the most sensitive option, at roughly 95% sensitivity and specificity.

• If the lab uses microscopy, they will run a modified acid-fast staining; the oocysts are 8–10 microns and can also be seen by fluorescence. From my quick research it looks like Quest runs this one, and Labcorp will, too.

• Because shedding is intermittent, providers may/should collect multiple stool specimens 2–3 days apart. Have you ever done one? Getting people to sign up for multiple stool samples is not easy… and I don’t blame them. But in the context of the current outbreak, this should be strongly considered, especially if the first tests do not find a different explanation for the diarrhea.

• You can imagine how all of this reduces the reported case numbers, right?!

The first-line treatment is an antibiotic many of us have heard before: Bactrim. Scientific/generic name is trimethoprim-sulfamethoxazole (TMP-SMX).

Until a source is identified, the sensible approach is to reduce risk without giving up produce entirely. I admit that I’m going to keep eating strawberries and the like, but just be aware that if I develop symptoms consistent with cyclospora, I need to be on top of that.

• Skip bagged and boxed salads for now. Whole heads of lettuce you wash and cut yourself are a safer bet.

• Favor produce you can peel, wash thoroughly, or cook. Bumpy, folded items like leafy greens and berries give the parasite more places to hide.

• Know washing’s limits. Rinsing removes some oocysts but not all, and the parasite resists standard chlorine disinfection, so a rinse won’t sterilize produce. Cooking reliably kills it. Anyone have a good recipe for boiled Caesar salad?

How about two little reassurances?

• No household isolation or bleach protocols are needed, since Cyclospora doesn’t spread through shared bathrooms or casual contact.

• U.S. public pools are not a meaningful risk — unlike another similar-sounding critter called Cryptosporidium, Cyclospora must sporulate in the environment before it can infect anyone. It’s not going to do that in a pool very well, and certainly not quickly.

• No need to boil tap water or stock up on plastic water bottles.

Many online and public health analysts in general have argued that the sprawling, slow-to-be-traced nature of this summer diarrhea outbreak reflects bad choices about how the U.S. funds and coordinates disease surveillance. As in using a chainsaw.

From my reading, the most relevant weaknesses seem to be the lack of centralized federal coordination, “siloed” health systems that don’t have easy coordination anymore, chronically underfunded state and local health departments doing this work while also chasing a historic measles outbreak and other constant threats. Every day of delayed source identification means more and more illnesses are being seeded in these multiple locations. Reasonable people weigh these factors differently, but the common thread is that outbreak detection depends on a capacity that has to be paid for, sustained, and respected.

Addendum 7/14/2026, via Heather Cox Richardson:

…on July 1, 2025, the Centers for Disease Control and Prevention (CDC), overseen by Health and Human Services Secretary Robert F. Kennedy Jr., would no longer track infections caused by cyclospora and five other common causes of foodborne illnesses.

Cyclospora and other diseases appreciate the evolutionary and survival advantages of an ideological system that tears down public health experts and science in general. Why wouldn’t they? Hard to blame the cysts.

The outlook is generally good (i.e. gets diagnosed and treated, and the outbreak is contained).

Most websites will say: “cases are mild and self-limiting in otherwise healthy people, and treatment works well.” But I want to examine that a bit more, as “self-limiting” makes this seem like no big deal. From OpenEvidence:

Cyclosporiasis is described as self-limiting in most immunocompetent patients, though it may present as severe, protracted, or chronic diarrhea in some cases. The disease can be symptomatic (mostly acute and self-limiting) or asymptomatic.

Duration of untreated illness: Without treatment, symptoms can persist for days to weeks to a month or longer, with a waxing and waning pattern. In the largest early study of immunocompetent patients, untreated diarrhea lasted a mean of 43 ± 24 days.

A placebo-controlled trial in Nepal found that after 7 days, 88% of placebo recipients still had Cyclospora detected in stool, indicating that spontaneous clearance within a week is uncommon. In the treated group in the Nepal trial, only 6% had detectable Cyclospora leftover after treatment with Bactrim.

A Canadian outbreak in immunocompetent adults showed diarrhea lasting 4–18 days and fatigue lasting 11–42 days, with a small proportion experiencing delayed recovery.

Endemic vs. non-endemic populations: In endemic areas, children develop partial immunity with age — in a Peruvian study, only 32% of infected children were symptomatic, suggesting that many infections in endemic populations are subclinical or asymptomatic. Conversely, travelers from non-endemic countries tend to be more severely affected.

In summary, while the infection does eventually self-resolve in immunocompetent individuals, the illness can be prolonged and debilitating without treatment, which is why TMP-SMX remains recommended to shorten symptom duration and oocyst shedding.

The main goals then might be patience and vigilance. Symptoms can relapse over weeks (one case report ran 107 days apparently), which is itself a hint to test for Cyclospora rather than assume a passing stomach bug.

Good luck out there, and hopefully this will all be figured out soon. Just stay aware for now, perhaps make a few different choices at the grocery store, and remember this if you get a nasty bout of diarrhea this summer.