Skip to Content



Richard D. Pearson

, MD, University of Virginia School of Medicine

Last full review/revision May 2020| Content last modified May 2020

Cyclosporiasis is an infection of the small intestine caused by the parasite Cyclospora cayetanensis. The main symptoms are watery diarrhea with abdominal cramping and nausea.

  • People can become infected with Cyclospora by consuming imported food or water contaminated with the parasite.
  • Cyclosporiasis symptoms may be more severe in people with a weakened immune system, such as people with AIDS.
  • Symptoms vary but include watery diarrhea, abdominal cramping, fever, and weight loss.
  • Doctors diagnose the infection by identifying Cyclospora in a sample of stool.
  • Trimethoprim/sulfamethoxazole (TMP/SMX) is used to treat cyclosporiasis.

(See also Overview of Parasitic Infections.)

Cyclosporiasis is most common in tropical and subtropical climates where sanitation is poor. Residents and travelers to endemic areas are at risk. In the United States, outbreaks of this infection have been attributed to imported fresh produce, such as raspberries, basil, snow peas, mesclun lettuce, and cilantro.

Symptoms of Cyclosporiasis

The primary symptom of cyclosporiasis is sudden, nonbloody, watery diarrhea, and nausea. Other symptoms include fever, abdominal cramps, vomiting, fatigue, and weight loss. Symptoms in people with a normal immune system last from a few days to a month or longer. Relapses may occur.

In people with a weakened immune system, including people with AIDS, cyclosporiasis may cause severe diarrhea that may persist for a long time.

Diagnosis of Cyclosporiasis

  • Stool tests

To diagnose cyclosporiasis, a stool sample is examined under a microscope for Cyclospora eggs. Specialized techniques can be used to increase the chances of identifying the eggs. Molecular techniques are available in some reference laboratories to identify parasite DNA.

When stool examination does not reveal a cause of persistent diarrhea, doctors may use a flexible viewing tube (endoscope) to examine the upper part of digestive tract and obtain a sample of tissue (a biopsy) to be examined under a microscope and analyzed for parasite DNA.

Prevention of Cyclosporiasis

When outbreaks are reported, people should avoid eating potentially contaminated fruits or vegetables from the area. When traveling to tropical and subtropical areas where the infection is common, people should avoid eating uncooked foods, including salads and vegetables, and should avoid consuming potentially contaminated water and ice. Hand washing with soap and water is important. Drinking water that has been boiled is safe. Filtering water through a 0.1 or 0.4 micron filter can remove cysts of Cyclospora and other parasites, as well as bacteria that cause diseases.

Treatment of Cyclosporiasis

  • Trimethoprim/sulfamethoxazole (TMP/SMX)

Infected people who have symptoms can be treated with trimethoprim/sulfamethoxazole (TMP/SMX) taken by mouth.

In people with AIDS, it is very important that the HIV infection is treated as effectively as possible with antiretroviral drugs. Such treatment can strengthen the weakened immune system, which usually helps control the diarrhea and other symptoms. People with AIDS may benefit from a higher dose of TMP/SMX and a longer course of treatment.

Drugs Mentioned In This Article

Generic Name Select Brand Names
Trimethoprim No US brand name

Copyright © 2022 Merck & Co., Inc., known as MSD outside of the US, Kenilworth, New Jersey, USA. All rights reserved. Merck Manual Disclaimer