Cyclosporiasis: The Parasite Behind the Produce

Imagine you eat fresh berries, leafy greens, or herbs for lunch. They taste fresh and delicious. You can't see anything wrong with them. You can't smell anything wrong with them. You can't taste anything wrong with them. Three days to two weeks later, you're suddenly hit with watery diarrhea so intense and frequent that you can't leave the bathroom. Your stomach cramps so badly you can barely stand. You lose your appetite. You feel exhausted. The diarrhea continues for weeks, sometimes coming and going with multiple relapses. You might never suspect that microscopic parasite you consumed along with your lunch. You might never connect the fresh produce you ate days ago to the illness that's incapacitating you now.

This is what happens when you contract cyclosporiasis, an intestinal parasitic infection caused by a microscopic organism called Cyclospora cayetanensis. The parasite is so small you can never see it, smell it, or taste it. It hides in contaminated water and food. Thousands of people contract it every year, and the number of outbreaks has been increasing dramatically since the 1990s.

This is the story of cyclosporiasis: what it is, how you get it, what it does to your body, how it's treated, and what's being done to prevent outbreaks.

What Is Cyclosporiasis? Understanding The Disease

Cyclosporiasis is an intestinal illness caused by the parasite Cyclospora cayetanensis, often just called Cyclospora. It's a unicellular organism that is so tiny it can only be seen under a microscope. The organism belongs to a group called coccidia, which are parasites that typically infect the intestines of animals. Cyclospora cayetanensis is unique because it specifically infects humans. You cannot get it from animals, and it doesn't spread directly from person to person the way cold or flu viruses do.

Instead, you get cyclosporiasis by consuming food or water contaminated with the parasite's oocysts, which are the dormant infectious forms of the organism. These oocysts are passed in the feces of infected people. If those contaminated feces enter water supplies or contaminate food during growing or handling, the parasite can infect the next person who eats or drinks that contaminated food or water.

The key point is that you cannot catch cyclosporiasis from someone who has it by hugging them, touching them, or even sharing food with them. The parasite needs time, typically days to weeks, to become infectious after being passed in feces. It needs specific environmental conditions including warmth and humidity. This is why the disease is most common in tropical and subtropical regions and why it spreads primarily through contaminated food and water rather than person-to-person contact.

Cyclosporiasis is not typically fatal, but it can be severe enough to hospitalize people, especially the very young, the elderly, and people with weakened immune systems. For most healthy people, cyclosporiasis is miserably unpleasant rather than life-threatening.

How Does The Parasite Work? What Happens Inside Your Body

When you swallow contaminated food or water containing Cyclospora oocysts, the parasite travels down your digestive tract to your small intestine. There, the oocysts rupture and release the organism inside. The parasite invades and replicates inside the cells lining your small intestine, called enterocytes. As the parasite multiplies, it damages these intestinal cells. The brush border of your intestine, which is responsible for absorbing nutrients, gets damaged. Digestive enzymes attached to the intestinal lining are lost. The tiny finger-like projections called villi that increase the surface area for nutrient absorption become blunted and atrophied.

As the intestinal lining is damaged, inflammatory cells including lymphocytes, plasma cells, and sometimes eosinophils flood in. This inflammation makes things worse, causing the diarrhea and cramping. The damaged intestinal barrier can't absorb water properly, so instead of normal stools, you get watery diarrhea. It can't absorb nutrients properly either, so you experience malabsorption and weight loss.

The parasite also damages the intestinal barrier in another way. Normally, your intestinal lining is a tight barrier that lets nutrients through while keeping harmful organisms out. Cyclospora damages this barrier, making it more permeable. This increased permeability can allow other bacteria to cross the barrier and cause additional problems.

As the parasite replicates, the organism also produces new oocysts that are shed in your feces. These oocysts are immediately not infectious. They need to spend days to weeks in the environment, exposed to warmth and humidity, before they become infectious to someone else. This sporulation process is one reason cyclosporiasis doesn't spread directly from person to person.

Symptoms: From Mild To Absolutely Miserable

Symptoms of cyclosporiasis typically begin about one week after infection, though they can appear as soon as two days or as late as two weeks or more. The timing varies depending on how much of the parasite you ingested, your immune system, and individual factors.

The main symptom is watery diarrhea. Not just normal diarrhea, but frequently described as explosive diarrhea. People report sudden, urgent, frequent bowel movements. Some people describe it as having multiple urgent bowel movements throughout the day, especially in the early morning. The diarrhea can be so severe and frequent that people can't go to work or school.

Along with the diarrhea come other gastrointestinal symptoms. Many people experience stomach cramps and pain that can be quite severe. Nausea is common. Some people vomit. Loss of appetite happens to many infected people, which combined with the diarrhea often leads to weight loss. Bloating and increased gas are typical. Many people feel generally nauseous and unwell.

Beyond the digestive symptoms, people often experience systemic symptoms like fatigue, low-grade fever, body aches, and headaches. Some people describe flu-like symptoms. The exhaustion can be profound, making it difficult to do normal daily activities.

What makes cyclosporiasis particularly frustrating is that the symptoms can persist for weeks or even months. Without treatment, the illness may last from a few days to over a month. Even worse, symptoms often don't disappear completely. Instead, they improve and then return. People experience remissions followed by relapses. Some people have symptoms for one to two months with multiple separate periods of severe diarrhea interspersed with periods of relative wellness.

The prolonged diarrhea can lead to dehydration, which is particularly dangerous in infants and young children. It can also lead to malnutrition as the damaged intestines can't absorb nutrients properly and the infected person often doesn't feel like eating.

In people with weakened immune systems, particularly those with HIV or AIDS, the infection can spread beyond the intestine. The parasite can infect the biliary tree and gallbladder, causing additional problems.

How Do People Get Infected? The Contamination Chain

Cyclosporiasis transmission requires contamination of food or water with oocysts from infected people's feces. Unlike many other parasitic infections, there is no animal reservoir. Cyclospora only infects humans. The contamination most commonly occurs in countries with warm climates and poor sanitation. When sewage systems are inadequate, contaminated feces can seep into groundwater, which may be used for irrigation. When crops are grown using contaminated water, the oocysts contaminate the produce. The parasite can also contaminate food during handling by infected workers with poor hygiene.

In developed countries like the United States, most cases are either imported from travel or from contaminated imported produce. Since the 1990s, the United States has seen increasing outbreaks linked to fresh produce imported from tropical and subtropical countries where the disease is endemic.

Past outbreaks in the United States have been traced to raspberries, basil, cilantro, snow peas, and mesclun lettuce. The produce comes primarily from Latin America, particularly countries like Mexico, Guatemala, Dominican Republic, and Peru where the parasite is common and sanitation conditions support its spread.

More recently, summertime outbreaks in the United States have also been linked to locally grown produce. In 2024, for example, outbreaks were linked to cilantro and other fresh herbs and produce grown in the United States. This suggests that contamination can occur in developed countries too if infected produce handlers or agricultural workers contaminate crops during growing, harvesting, or processing.

The parasite can survive on produce if the conditions are right. It can persist on leafy greens and herbs, especially in cool, moist conditions. Unlike some pathogens, Cyclospora is not typically killed by standard washing of produce. Simply rinsing lettuce or herbs under water will not reliably remove the parasite.

Treatment: Antibiotics That Work

Cyclosporiasis is effectively treated with the antibiotic trimethoprim/sulfamethoxazole, often abbreviated as TMP/SMX or known by brand names like Bactrim or Septra. This combination antibiotic is highly effective at killing the parasite and relieving symptoms. The medication works by interfering with the parasite's ability to produce certain nutrients essential for its survival. When the parasite can't produce these nutrients, it dies and is eliminated from the body. Symptoms typically improve within a few days of starting the medication.

For people who are allergic to sulfa drugs, an alternative antibiotic called nitazoxanide can be used. Nitazoxanide is also effective and is increasingly used, particularly in cases where patients have sulfa allergies or where resistance has been observed. Without treatment, cyclosporiasis will eventually resolve on its own in most healthy people, but this can take weeks to months. The prolonged illness and risk of complications like dehydration and malnutrition make treatment much preferable to waiting it out.

People with weakened immune systems, particularly those with HIV, may need more aggressive treatment and longer courses of antibiotics. In some cases, the parasite can be difficult to completely eliminate in severely immunocompromised individuals.

Supportive care is also important. Staying hydrated is crucial, especially with the severe diarrhea. Electrolyte replacement drinks help restore balance of salts and minerals lost through diarrhea. Rest and avoiding foods that are likely to aggravate symptoms can help people feel better while the medication takes effect.

Outbreaks: The Growing Problem

Before 1996, cyclosporiasis was extremely rare in the United States. Only three outbreaks had ever been reported. Then everything changed. In the summer of 1996, almost 1,000 laboratory-confirmed cases were reported to the CDC between May and mid-July. The outbreak was linked to contaminated raspberries imported from Guatemala. This was shocking to the public health community and marked the beginning of regular cyclosporiasis outbreaks in the United States.

Since 1996, outbreaks have become increasingly common. Every summer, the CDC now expects to see multiple outbreaks. Some recent significant outbreaks include cases linked to cilantro (2018-2020), basil (2022), and various other fresh produce items. The outbreaks have expanded geographically. Cyclosporiasis is now a nationally notifiable disease and is reportable in 43 states and the District of Columbia.

What's concerning is that the number of reported cases has increased dramatically since the 1990s. This increase is likely due to a combination of factors. Improved diagnostic testing has made it easier to detect the parasite, leading to more cases being identified. Increased trade in fresh produce from tropical countries has increased the opportunity for contamination. Climate change may be expanding the geographic range where the parasite can persist in the environment.

Additionally, the outbreak detection system has improved. In 1996, it took time for the public health system to realize there was a problem. Modern molecular testing and communication networks allow rapid detection of outbreaks now. So some of the increase in reported cases reflects better surveillance.

The outbreaks are particularly challenging because they're difficult to trace and prevent. The produce can be contaminated far away and shipped across the country before anyone realizes there's a problem. By the time cases are identified and the source is traced, the contaminated produce may have already been consumed or removed from stores.

Who's Most At Risk?

Cyclosporiasis affects people across all age groups and backgrounds, but certain groups face higher risk. People living in or traveling to tropical and subtropical regions where the parasite is endemic face the highest risk. This includes countries in Latin America, Asia, and Africa where sanitation conditions support transmission. Travelers to these regions account for a significant portion of cases in developed countries.

Food workers, agricultural workers, and farm laborers in endemic areas face occupational exposure. If they become infected and don't have access to treatment, they can continue shedding oocysts and contaminating food.

In developed countries, most cases are either travelers or people who consumed contaminated imported produce. Consumers cannot reliably determine whether produce is contaminated, so anyone eating fresh produce from endemic areas has some risk.

Certain groups experience more severe disease. Infants and young children can become severely ill with dehydration and malnutrition. The elderly may also experience more severe disease. People with weakened immune systems, particularly those with HIV or AIDS, can experience severe, prolonged illness that's difficult to treat.

Pregnant women who contract cyclosporiasis can pass antibodies to their babies, potentially providing some protection, but pregnant women experiencing severe symptoms need careful management.

Prevention: What You Can Do

The best way to prevent cyclosporiasis is to avoid contaminated food and water. The challenge is that you can't see, smell, or taste the parasite, so identifying contaminated food is impossible for consumers. The CDC recommends several prevention strategies. If you're traveling to areas where cyclosporiasis is common, you should drink bottled water and avoid tap water, ice, and untreated water. You should avoid raw and undercooked foods. You should eat fruits and vegetables that you've peeled yourself or that were prepared in hygienic conditions.

In developed countries where the disease is less common, the best prevention is proper food handling and sanitation. Washing hands thoroughly before eating and after using the bathroom is important. Food workers should follow strict hygiene protocols to avoid contaminating food. For produce specifically, standard washing may not eliminate Cyclospora oocysts. Some studies suggest that produce should be washed vigorously under running water, but this won't guarantee removal of the parasite. Some research suggests that the parasite can survive on produce even after washing. The most reliable prevention for high-risk individuals might be to avoid produce from countries where the parasite is common or to consume only cooked vegetables and fruits from endemic areas, since the parasite is killed by cooking.

However, for people in the United States, avoiding all fresh produce from Latin America is impractical. Much of the year-round supply of fresh berries, leafy greens, and herbs comes from these regions. Most people are willing to accept the small risk of contracting cyclosporiasis in exchange for access to affordable fresh produce year-round.

Diagnosis: Why It's Difficult

Diagnosing cyclosporiasis is more difficult than diagnosing many other foodborne illnesses. The parasite produces characteristic oocysts that appear in stool samples, and these can be identified under a microscope by someone trained to recognize them. However, the oocysts are small and don't stain easily with standard tests, making them easy to miss.

A healthcare provider who suspects cyclosporiasis based on symptoms (prolonged watery diarrhea in someone who traveled or consumed imported produce) can order a stool test. The laboratory must use special staining methods to visualize the oocysts. Even with proper testing, the parasite can be difficult to detect, especially in people with light infections or later in the illness when oocyst shedding is decreasing. More recent advances include molecular testing using polymerase chain reaction (PCR), which can detect the parasite's DNA in stool samples. These tests are more sensitive and specific than microscopy and are increasingly being used, particularly in outbreaks.

Diagnosis is important because the symptoms of cyclosporiasis are similar to many other gastrointestinal illnesses. Without proper diagnosis, people might go untreated for weeks, or they might be treated with medications that don't work against this particular parasite. In some cases, healthcare providers miss the diagnosis initially. People might be evaluated for irritable bowel syndrome or other chronic conditions before cyclosporiasis is considered. This is why the history of travel or consumption of imported produce is so important for diagnosis.

The Bottom Line

Cyclosporiasis is an intestinal parasitic illness caused by the microscopic organism Cyclospora cayetanensis. The parasite is transmitted through contaminated food or water and causes watery, sometimes explosive diarrhea along with stomach cramps, nausea, fatigue, and loss of appetite.

The parasite infects the small intestine, where it invades and damages the cells lining the intestine. This damages the intestinal barrier and interferes with nutrient absorption. Symptoms typically begin one week after infection but can appear as early as two days or as late as two weeks. Without treatment, symptoms can last from days to over a month, often with relapses.

Cyclosporiasis is effectively treated with the antibiotic trimethoprim/sulfamethoxazole, or with nitazoxanide for people who are sulfa-allergic. Symptoms improve within days of starting treatment.

The parasite is found worldwide but is most common in tropical and subtropical regions with poor sanitation. It is transmitted when infected feces contaminate food or water. The parasite does not spread directly from person to person because it requires days to weeks in the environment to become infectious.

In the United States, cyclosporiasis was extremely rare before 1996 but has become increasingly common since then, with regular summer outbreaks linked to contaminated fresh produce, particularly raspberries, cilantro, basil, and lettuce from Latin America. Recent outbreaks have also been linked to locally grown produce.

Prevention requires awareness that contaminated produce cannot be reliably identified before purchase. Proper food handling and sanitation are important. Travelers to endemic areas should be cautious about food and water. Most people in developed countries accept the small risk of infection in exchange for access to affordable fresh produce.

Diagnosis can be challenging and requires specific testing methods to identify the parasite. With proper diagnosis and treatment, cyclosporiasis is readily curable, though without treatment it can persist for weeks or months.

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