Zebra of the week: Cyclosporiasis

Cyclosporiasis is an intestinal infection caused by the coccidian protozoan Cyclospora cayetanensis. Although uncommon in routine clinical practice, it should be considered in patients with prolonged or relapsing watery diarrhea, particularly during the spring and summer months or following consumption of fresh imported produce. In the US, most cases have historically been associated with foodborne outbreaks linked to contaminated fresh herbs, leafy greens, berries, and other produce, although sporadic travel-associated infections also occur.

Transmission occurs through ingestion of sporulated oocysts in contaminated food or water. Importantly, freshly excreted oocysts are not immediately infectious; they require days to weeks in the environment to sporulate. As a result, direct person-to-person transmission is considered unlikely. This unique life cycle distinguishes cyclosporiasis from many other infectious causes of gastroenteritis.

Symptoms typically develop 2 to 14 days after exposure (most often about 1 week) and include profuse watery diarrhea, anorexia, abdominal cramping, bloating, nausea, fatigue, low-grade fever, myalgias, and weight loss. Illness can be prolonged, lasting weeks to months if untreated, and is often characterized by relapsing symptoms. Although infection is usually self-limited in immunocompetent individuals, severe or persistent disease is more likely in older adults, immunocompromised patients, and those with delayed diagnosis. Dehydration and weight loss are the most common complications.

Diagnosis requires a high index of suspicion. Routine stool ova-and-parasite examinations frequently do not detect Cyclospora unless specific testing is requested. Clinicians should order stool PCR gastrointestinal pathogen panels that include Cyclospora or request modified acid-fast staining or ultraviolet fluorescence microscopy when PCR is unavailable. Because oocyst shedding may be intermittent, repeat stool testing may improve diagnostic yield.

Trimethoprim-sulfamethoxazole (TMP-SMX) remains the treatment of choice. For immunocompetent adults, a 7- to 10-day course is generally effective, while immunocompromised patients may require longer treatment or secondary prophylaxis because of relapse. There is no highly effective alternative for patients with sulfonamide allergy; management in these cases is primarily supportive, although several less effective agents have been used anecdotally. Oral rehydration and electrolyte replacement are important adjunctive therapies.

Prevention focuses on safe food and water practices. Because Cyclospora oocysts are resistant to many routine disinfectants and may adhere to fresh produce, washing alone may not completely eliminate contamination. Clinicians should also report confirmed cases to local public health authorities to facilitate outbreak detection and traceback investigations.

Clinical update: 2026 U.S. outbreak

In July 2026, the CDC issued a Health Alert Network (HAN) advisory after identifying 1,645 laboratory-confirmed domestically acquired cases across 34 states since May 1, with more than 5,100 additional reports under investigation. Most illnesses are suspected to be linked to contaminated fresh produce, although a definitive source has not yet been identified. CDC advises clinicians to consider Cyclospora in any patient with prolonged or relapsing watery diarrhea during the May–August season—even in the absence of international travel—and to specifically request Cyclospora testing, as routine stool O&P examinations may miss the diagnosis.

Sources:

NIH GARD. Cyclosporiasis

Centers for Disease Control and Prevention. (2026 July 14) CDC Health Alert Network. Domestically Acquired Cyclosporiasis Cases in Multiple U.S. States, 2026 (HAN No. 00531)