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Cyclosporiasis Outbreak Spotlights Risk of Uncommon Pathogen

By Amesh A. Adalja, MD, FACP, August 9, 2013

A multistate outbreak of more than 400 cases of Cyclospora infection has thrust a relatively rare—and seldom tested for—protozoan pathogen into the headlines.* The estimated incidence of Cyclospora is 0.03 per 100,000 population, which is orders of magnitude lower than the incidence for Salmonella or Campylobacter. Nonetheless, this outbreak, in which 27 individuals have been hospitalized, reinforces the need for clinicians to be aware of the presentation, diagnosis, and treatment of infection for this pathogen.1

Pre-packaged Lettuce Implicated

As of this writing, investigators believe that the food vehicle that harbored Cyclospora was pre-packaged lettuce that originated at a facility in Mexico (owned by a California company). The lettuce was sold to restaurants in Iowa and Nebraska, after which case clusters occurred in June and July. At this point, it is not clear whether all 438 cases are part of the same outbreak or the result of heightened testing.1

Humans are the Only Host

Cyclospora cayetanensis is a protozoan first identified as a cause of human disease in 1977. Infection results from the ingestion of a sporulated oocyst. Once ingested, excystation occurs in the small intestine and is followed by reproduction. Oocysts are subsequently secreted in the stool. Outside the (exclusive) human host, sporulation occurs over 7 days, resulting in infective oocysts. Because of the sporulation requirement, person-to-person transmission is not possible.2

Cyclosporiasis occurs worldwide, and in the US the highest incidence is during the months of May through July.2 The incubation period is approximately 7 days and is followed by watery diarrhea that may last for several weeks, especially in people who are immunocompromised. Abdominal cramps and myalgias are common; fever may or may not be present. Asymptomatic cases are known to occur, even in people with HIV infection.

Diagnosis and Treatment

Diagnosis usually relies on recognition of the characteristic oocyst in a stool sample, and special staining may be required to ensure proper identification. Because Cyclospora is not a common pathogen, it is essential that the microbiology laboratory in receipt of samples be notified that cyclosporiasis is suspected based on an ongoing outbreak or a patient’s history. There are no rapid tests commercially available.2

The treatment of choice for cyclosporiasis is a 7-day course of trimethoprim/sulfamethoxazole (TMP/SMX). Alternative therapies are ciprofloxacin and nitazoxanide. In HIV-infected patients, chronic suppression may be necessary due to high relapse rates.2

More Recognition of Cyclosporiasis

Awareness about the current outbreak may serve to lower the threshold for testing, which may result in detection of cases unrelated to the lettuce-born outbreak and, in turn, help elucidate the true burden of disease in the US. As this outbreak illustrates, recognition and definitive diagnosis of unusual pathogens still relies on the combined skills of astute clinicians and microbiologists.2

* Before the outbreak this summer, the most well-known appearance of this pathogen occurred during an outbreak in the mid-1990s that was traced to raspberries imported from Guatemala.2


  1. Cyclospora cases rise to 425 as feds add investigative help. CIDRAP News. August 5, 2013. Accessed August 6, 2013.
  2. Suh KN, Kozarsky P, Keystone JS. Cyclospora cayetanesis, Isospora belli, Sarcocystis Species, Balantidium coli, and Blastocystis hominis. In: Mandell GL, Bennett JE, Dolin R, eds. Mandell, Douglas, and Bennetts Principles and Practice of Infectious Diseases. 7th ed. Philadelphia, PA: Churchill Livingstone; 2010.