Cyclospora (Cyclosporiasis): A Physician's Guide to Symptoms, Testing, and the One Antibiotic That Treats It
An ER physician explains cyclosporiasis: symptoms, why it is missed on routine stool tests, and why trimethoprim-sulfamethoxazole (Bactrim) is the only reliable treatment.
Cyclospora (Cyclosporiasis): A Physician's Guide to Symptoms, Testing, and the One Antibiotic That Treats It
Cyclosporiasis is an intestinal infection caused by the microscopic parasite Cyclospora cayetanensis, and its defining feature is watery diarrhea that lingers for weeks and relapses if it is not treated. It matters right now because the US is experiencing a record outbreak: the CDC's July 2026 Health Alert counts at least 1,645 domestically acquired cases across 34 states, with 9 percent of patients hospitalized. Here is the single most important clinical fact for anyone worried about it: Cyclospora is treated with trimethoprim-sulfamethoxazole (the combination antibiotic sold as Bactrim or Septra), and the antibiotics people usually take for traveler's diarrhea, like azithromycin and ciprofloxacin, do not reliably work against it. In my ER practice, the travelers who suffer longest with this are the ones who were never tested for it, because most labs do not look for Cyclospora unless a clinician specifically asks. This guide walks through how to recognize it, how it is diagnosed, and how it is actually cured.
What Cyclospora Is and How You Catch It
Cyclospora cayetanensis is a single-celled parasite that infects the small intestine. You catch it by swallowing the parasite in food or water that has been contaminated with infected human feces, most often fresh produce that was tainted somewhere along the supply chain before it reached your plate. It is not spread from person to person the way a stomach virus is, because the parasite needs days to weeks in the environment to become infectious after it is shed. That biology is why cyclosporiasis shows up as clusters tied to a common food, rather than spreading through a household.
Past US outbreaks have been linked to fresh, raw produce including raspberries, basil, cilantro, snow peas, and lettuce. Historically the illness was mostly seen in travelers returning from tropical and subtropical regions, but the large, recurring summer outbreaks inside the US over the past decade have made it a domestic concern as well. As a physician, I now consider Cyclospora on the differential for prolonged diarrhea even in patients who never left the country, and during an active outbreak that suspicion should be high.
Symptoms: The Pattern That Should Make You Think Parasite
The hallmark of cyclosporiasis is frequent, watery diarrhea. Symptoms usually begin about a week after infection, though the range runs from as little as two days to more than two weeks, which is longer than the hours-to-days onset of most bacterial food poisoning. Alongside the diarrhea, people commonly experience loss of appetite, weight loss, stomach cramps and bloating, nausea, fatigue, and a low-grade fever.
Two features distinguish Cyclospora from an ordinary stomach bug. The first is duration: untreated, cyclosporiasis can last from a few days to a month or longer. The second is its relapsing course, where symptoms improve or seem to resolve and then return one or more times. If you have watery diarrhea that will not quit, keeps bouncing back, and is wearing you down with fatigue and appetite loss, that combination is a strong hint that you are dealing with a parasite rather than a passing bacterial illness. That is the moment to get tested.
Why Routine Stool Tests Miss It
Here is the diagnostic trap that catches both patients and clinicians. Cyclospora is not detected by a standard stool culture, and it is not reliably found on a routine ova-and-parasite exam unless the lab uses specific techniques. Because Cyclospora is a coccidian parasite, infected people shed oocysts in their stool, and finding those oocysts requires special stains (such as modified acid-fast staining), ultraviolet fluorescence microscopy, or a molecular test. Many US laboratories do not test for Cyclospora unless a provider requests it by name.
In practice, this means a patient can hand in a stool sample, get a "normal" result, and still have an active Cyclospora infection. The workaround is to ask directly. If you have the prolonged, relapsing diarrhea pattern, ask your clinician to specifically test for Cyclospora, which today is often done through a gastrointestinal PCR panel that includes it, or through a stool exam ordered with Cyclospora testing explicitly requested. Getting the right test ordered is more than half the battle with this infection.
Treatment: Trimethoprim-Sulfamethoxazole Is the Answer
The recommended treatment for cyclosporiasis is trimethoprim-sulfamethoxazole, abbreviated TMP-SMX and widely known by the brand names Bactrim and Septra. For otherwise healthy adults, the CDC describes a typical regimen of one double-strength tablet (160 mg trimethoprim and 800 mg sulfamethoxazole) taken twice daily for seven to ten days. Most people improve substantially within a few days of starting it, and treatment also shortens the illness and prevents the frustrating relapses that untreated infections cause.
I want to be direct about what does not work, because it is the source of a lot of unnecessary suffering. The CDC notes that azithromycin appears to be ineffective for Cyclospora, and that ciprofloxacin is largely ineffective in immunocompetent people despite one small older study suggesting modest activity. Those two drugs are exactly what many travelers carry for traveler's diarrhea, so a person with cyclosporiasis can take their entire travel antibiotic supply and get nowhere. TMP-SMX is genuinely the treatment of choice, and no highly effective alternative has been established for people who cannot take it.
That last point deserves emphasis for anyone with a sulfa allergy. If you are allergic to or intolerant of sulfa drugs, do not simply substitute another antibiotic on your own, because the alternatives are limited and less proven. This is a situation to manage with a clinician who can weigh your specific history.
What This Means for the Common Travel Question
We have had travelers ask, reasonably, whether they should carry Bactrim "just in case" they get Cyclospora during the outbreak. I understand the instinct, and it comes from a correct piece of knowledge: Bactrim is the right drug for this parasite. But there are two catches worth understanding. First, Cyclospora should be confirmed with testing rather than assumed, because prolonged diarrhea has several causes and the treatments differ. Second, TMP-SMX is a prescription antibiotic with its own considerations, including sulfa allergy, drug interactions, and effects on people with certain conditions, so it is not a medication to start blindly.
The better plan for most travelers is to prepare for the common problem and stay alert for the uncommon one. Carry a standard traveler's diarrhea kit for the bacterial illness you are far more likely to get, and if you develop the prolonged, relapsing, watery-diarrhea pattern, get evaluated and tested promptly so the right treatment can be prescribed. Wandr's clinicians can help you build that kit and, when appropriate, call the relevant prescriptions in to your local pharmacy for pickup. See your medication options and we will help you decide what makes sense.
Prevention: Cooking Beats Washing
Because you cannot treat your way out of an exposure you could have avoided, prevention is worth a paragraph. Cyclospora is remarkably difficult to remove from produce, since it clings to microscopic surface crevices, and the CDC advises that routine washing does not reliably remove it and that standard chemical sanitizing is unlikely to kill it. Thorough cooking, on the other hand, destroys the parasite, which is why every documented US outbreak has traced to raw produce.
For travelers during an active outbreak, the sensible adjustments are to favor cooked vegetables over raw when eating out frequently, be cautious with fresh herb garnishes and salad bars, and continue the standard traveler habits of eating food served hot and drinking safe water. These are small changes that meaningfully lower your odds without turning every meal into a risk calculation.
Frequently Asked Questions
What are the main symptoms of cyclosporiasis? The primary symptom is frequent, watery diarrhea, often accompanied by loss of appetite, weight loss, cramping, bloating, nausea, fatigue, and a low-grade fever. Symptoms usually start about a week after infection and, if untreated, can last a month or longer with a relapsing course where they improve and then return.
What is the treatment for Cyclospora? The treatment of choice is trimethoprim-sulfamethoxazole (Bactrim or Septra). A common adult regimen is one double-strength tablet twice daily for seven to ten days. There is no well-established, highly effective alternative for people who are allergic to or intolerant of sulfa drugs, so those cases should be managed by a clinician.
Can I just take my traveler's diarrhea antibiotic for Cyclospora? No. The CDC reports that azithromycin appears ineffective against Cyclospora and that ciprofloxacin is largely ineffective in healthy adults. These are the usual traveler's diarrhea antibiotics, so they will not cure cyclosporiasis. The parasite requires trimethoprim-sulfamethoxazole after a confirmed diagnosis.
How is Cyclospora diagnosed? It requires specific testing that most labs do not run automatically. Detection uses special stool techniques such as modified acid-fast staining, UV fluorescence microscopy, or a molecular (PCR) panel that includes Cyclospora. Ask your clinician to test for Cyclospora by name, especially if you have prolonged or relapsing diarrhea, because a routine stool test can miss it.
How long does cyclosporiasis last? Without treatment, it can last from a few days to a month or more, and it often relapses. With the correct antibiotic, most people improve within a few days. This long, waxing-and-waning course is one of the features that distinguishes it from short-lived bacterial food poisoning.
Is Cyclospora contagious from person to person? No, not directly. The parasite is shed in feces but needs days to weeks in the environment to become infectious, so it does not spread person to person the way a stomach virus does. Infections come from consuming contaminated food or water, which is why outbreaks cluster around a common produce source.
Medical Disclaimer
This article is for general educational purposes and is not a substitute for individualized medical advice, diagnosis, or treatment. Do not start or stop any prescription medication without consulting a licensed clinician. Outbreak details and clinical guidance can change; check official sources such as the CDC for the latest information.
Sources
- CDC, Clinical Care of Cyclosporiasis (TMP-SMX regimen; azithromycin and ciprofloxacin ineffectiveness; alternatives for sulfa allergy)
- CDC, Clinical Overview of Cyclosporiasis (diagnosis, oocyst shedding, special stains)
- CDC, About Cyclosporiasis (symptoms, onset, relapsing course, duration)
- CDC Health Alert Network, HAN00531, "Domestically Acquired Cyclosporiasis Cases in Multiple U.S. States, 2026"
- CDC, Preventing Cyclosporiasis (washing versus cooking)
Alec Freling, MD is a board-certified emergency medicine physician and co-founder of Wandr Health with ER experience treating returning travelers.