Cyclosporiasis in Solid Organ and Hematopoietic Cell Transplant Recipients: Diagnosis, Treatment, and Prevention

ABSTRACT Background Cyclospora cayetanensis is a foodborne coccidian protozoan that causes prolonged, often relapsing watery diarrhea. Once considered largely a travel‐associated infection, it has become a recurring cause of large, domestically acquired, produce‐associated outbreaks in temperate countries. The 2026 US season was the largest on record and prompted a CDC Health Advisory, placing substantial numbers of solid organ transplant (SOT) and hematopoietic cell transplant (HCT) recipients at risk of exposure. Aim To provide a practical, transplant‐specific approach to the diagnosis, treatment, and prevention of cyclosporiasis, using the 2026 outbreak as a worked example of a problem that will recur in future seasons. Observations As of August 17 2026, CDC had reported 15 716 laboratory‐confirmed domestically acquired cases since May 1, including 828 hospitalizations and two deaths, with at least 11 841 further cases under investigation; 1180 cases were reported during the whole of the 2025 season. A large multistate outbreak was traced to shredded iceberg lettuce sourced from central Mexico. Published data describing cyclosporiasis in transplant recipients are limited to small case series and seven individual case reports that provide detailed clinical descriptions. Among these seven reported cases, all occurred in SOT recipients more than one year after transplantation, none were receiving trimethoprim‐sulfamethoxazole (TMP‐SMX) prophylaxis at the time of diagnosis, and chronic watery diarrhea with weight loss predominated. Stool polymerase chain reaction (PCR), usually within a multiplex gastrointestinal panel, is the preferred diagnostic test, as routine ova‐and‐parasite examination does not reliably detect the parasite; unstained oocysts are easily mistaken for yeast or debris unless modified acid‐fast or safranin staining or ultraviolet autofluorescence is specifically applied. TMP‐SMX remains the treatment of choice; longer treatment courses may be required in immunocompromised patients, and secondary prophylaxis may be considered in select patients. Because person‐to‐person transmission is unlikely, Contact Precautions are only recommended for diapered or incontinent patients, and environmental control depends on mechanical removal because the bilayered oocyst wall resists chemical disinfection. Conclusion Cyclospora should be considered in any transplant recipient with persistent or relapsing diarrhea during the May–August season, irrespective of travel history. Clinicians should specifically request Cyclospora testing, obtain stool PCR early, treat with TMP‐SMX, and reinforce food‐ and water‐safety counseling. The 2026 outbreak demonstrates how rapidly population exposure can escalate, and the same approach will apply to future seasons. Published transplant‐specific experience remains strikingly limited.