A woman in the US reportedly had to rush to the bathroom around 30 times a day after becoming infected with Cyclospora, a microscopic parasite that causes explosive diarrhoea, dehydration and hospitalisation.
The latest cyclospora outbreak is one of the largest ever recorded in the US, with case numbers already exceeding the previous record set in 2019. Dr Cyriac Abby Philips, hepatologist, clinical scientist and public health researcher, explains what makes the parasite so dangerous, and how common are its cases in India.
Cyclospora infections are often mistaken for routine food poisoning or viral gastroenteritis. Why?
The single most useful clue is the duration. Bacterial food poisoning burns itself out in one to three days and viral gastroenteritis in two or three, whereas cyclosporiasis runs for weeks. If left untreated, the illness may last from a few days to a month or longer and may relapse.
That rhythm, in which the patient improves, believes it is over, and then crashes again, is close to a signature. The incubation too is wrong for food poisoning: it averages about a week and ranges from two days to a fortnight or more. If the patient is blaming last night’s biryani, it’s not cyclospora.
Point to note is that Cyclospora is a protozoan, a single-celled microscopic organism that multiplies inside you, while worms are large, multi-celled creatures that cannot directly multiply inside the human body (they lay eggs instead). The feature of the illness also differs. The diarrhoea is watery and sometimes explosive, but what dominates is loss of appetite, weight loss, abdominal cramping and bloating, increased flatulence, and a prolonged fatigue extremely out of proportion to the bowel symptoms.
Vomiting is not the main event. The illness is non-inflammatory, so there are no blood and no pus cells in stool.
What about treatment?
Empirical use of antibiotics such as metronidazole, azithromycin or a fluoroquinolone (ciprofloxacin, norfloxacin) will fail, and then co-trimoxazole will produce a dramatic response, which in hindsight is itself a diagnostic.
Recognised complications include malabsorption, gallbladder inflammation and reactive arthritis. Any watery diarrhoea persisting beyond two weeks without blood is Cyclospora until proven otherwise. The important effect is that a routine “stool for ova and parasite” will not find it. The clinician has to specifically request testing for this parasite, or it will simply not be looked for.
Is cyclospora an old parasite? Why is it long-lasting?
Until the 1990s, cyclospora was thought to infect rodents and reptiles. The first published report of human infection dates to 1979 in Papua New Guinea. The name Cyclospora cayetanensis came in 1994, the etymology taken from Cayetano Heredia University in Lima, where the principal studies on the parasite had been done.
In 2023, scientists at the US Centers for Disease Control (CDC) showed that at least three isolated lineages caused human cyclosporiasis. Nearly half a century after its discovery, we are still working out what this organism actually is.
Cyclospora cannot be grown in a laboratory culture dish like bacteria, so scientists have to rely heavily on advanced genomic tools to study it. As for why the illness lasts, cyclospora is a tough parasite that hides and multiplies inside the lining of the small intestine, attacking neighbouring cells in waves that cause the illness to return just when doctors think the patient is recovering.
It damages the gut’s ability to absorb nutrients, which is why extreme fatigue and weight loss can drag on for weeks even after the diarrhoea has stopped. Furthermore, our body does not develop permanent immunity, meaning a person can catch it again.
The parasite is especially dangerous because its “eggs” have a rugged shell that survive for weeks in soil and water and completely resist chlorine, allowing it to easily hitch a ride onto fresh produce.
What about its prevalence in India?
We do not know because almost nobody looks. The most useful record is a 2025 study from the ICMR-National Institute of Cholera and Enteric Diseases in Kolkata, which found C. cayetanensis in around 2% of diarrhoeal patients across eastern India, rising to 7% during July and August, with children aged five to 12 worst affected at 18%. Poor household sanitation and rural residence predict infection.
Why is cyclosporiasis frequently underdiagnosed?
Outbreak investigations face a critical four-to-eight-week reporting-related delay that leaves no fresh produce for testing and relies on flawed recall data.
Cyclospora infections are frequently underdiagnosed because the parasite is invisible on standard tests and requires specific staining or ultraviolet fluorescence techniques. While the necessary technology is readily available, improving detection requires updating medical protocols to automatically screen for the parasite in chronic diarrhoea cases and providing specialised training to laboratory personnel.
Unlike many foodborne illnesses, Cyclospora requires time outside the human body to become infectious. How does this influence the way outbreaks occur?
Cyclospora requires one to two weeks to mature in the environment, making direct person-to-person transmission unlikely. Identifying the parasite on food serves as a “fossil record,” indicating contamination happened well before the food was harvested, shipped, or eaten. Consequently, restaurant hygiene protocols are irrelevant; outbreaks stem from environmental contamination upstream – such as contaminated irrigation water – requiring control measures at the farm level rather than the kitchen.
Fresh produce is often implicated in outbreaks. Are there vulnerabilities in the farm-to-fork supply chain?
Cyclospora is exceptionally skilful at evading food safety systems because it favours raw produce like leafy greens and berries, lacks an effective chemical kill step, and clings tenaciously to complex plant surfaces. Water, including contaminated irrigation, wash, and pesticide water, serves as the primary vector for this human exclusive parasite, meaning every infection traces back to inadequate sanitation and a lack of toilets. The risk is magnified when contaminated lots are blended into thousands of bags and consumed before short shelf lives expire.
While thorough cooking at 70°C is the only absolute kill step, risk reduction depends on farm-level controls like drip irrigation, treated water, worker toilets and strict lot traceability. Consumers can buy whole produce instead of pre-washed salad kits.
What are the biggest challenges public health agencies face when investigating cyclosporiasis, particularly in tracing contaminated produce across international supply chains?
While molecular genotyping is advancing, poor supply-chain documentation often prevents tracing, making paperwork – not science – the primary bottleneck. Additionally, current PCR testing cannot differentiate between live and dead thick-walled spores of the parasite due to a lack of culture methods. Public health advocates have noted that the size and tracking delays of the 2026 outbreak coincided with a reduction in CDC surveillance.
In 2023, CDC scientists discovered that Cyclospora has at least three distinct populations. (Photo: CDC) 
Share
