The ongoing outbreak of cyclosporiasis in the US from an as yet unidentified source has expanded to at least 34 states, with a confirmed case count of 1645 that is likely largely underestimated, said Gwen Biggerstaff, MD, deputy director of the CDC’s Division of Foodborne, Waterborne, and Environmental Diseases, in a July 14 press briefing. The CDC is aware of at least 5100 additional potential cases undergoing additional analysis, she said.

By comparison, 249 confirmed cyclosporiasis cases were reported nationally to the CDC between May 1 and July 16, 2025.

Although past outbreaks of cyclosporiasis in the US have been linked to overseas travel, more cases in recent years have been traced to both domestic and imported produce, said Biggerstaff.

The peak season for cyclosporiasis, an intestinal illness caused by the Cyclospora parasite, is May 1 through August 31, when more people are consuming the fresh produce that serves as a primary carrier for the parasite, according to the CDC.

Cyclosporiasis remains an epidemiologic challenge in part because of the delay of approximately 2 days to 2 weeks between exposure to a parasite and the emergence of symptoms, Biggerstaff noted.

Current Status

The latest data from the CDC indicate that the confirmed cyclosporiasis patients became ill after eating food in the US, with none reporting travel in the 2 weeks before their symptoms appeared. The patients range in age from 2 to 95 years, with a median age of 44 years, and approximately half are women. Although 141 of the confirmed patients have been hospitalized, no deaths have been reported to date.

More details about the current outbreak, the status of the investigation, and guidance for clinicians are available in the Health Advisory posted by the CDC on July 14. Other actions taken by the CDC include creation of a new outbreak investigation notice webpage with details on the potentially linked multistate outbreak involving Michigan, West Virgina, Kentucky, and Ohio. The CDC intends to update the surveillance and investigation webpages on a weekly basis at minimum, rather than the monthly updates previously posted, Biggerstaff said in the briefing.

The investigation into a source of the current outbreak remains ongoing. Donald Prater, DVM, the FDA’s acting deputy commissioner for food, acknowledged reports from Michigan of lettuce as a possible source, but did not comment further during the briefing.

What to Look For

Cyclosporiasis has occurred sporadically every year since its discovery in the late twentieth century, said Thomas Moore, MD, clinical professor of medicine at The University of Kansas School of Medicine, Wichita, in a press briefing sponsored by the Infectious Diseases Society of America (IDSA).

Cyclosporiasis, characterized mainly by watery diarrhea, sometimes accompanied by loss of appetite or nausea, and less frequently by low-grade fever and vomiting, is usually self-limiting and therefore vastly underreported, Moore said.

“The main thing to keep in mind is that diarrheal diseases of all kinds are more common in the summer,” Moore emphasized. One day of loose stools could be cyclospora but could be almost anything else, he noted. “A clinician’s suspicion should be raised for a patient with diarrhea that has persisted over about 3 days, given the current outbreak,” Moore said. In non-outbreak scenarios, clinicians could wait a few more days before suspecting cyclosporiasis, he said.

For most patients with cyclosporiasis symptoms, hydration and simple foods remains the standard guidance, and testing may not be needed, said Paul G. Auwaerter, MD, MBA, FIDSA, professor of medicine an infectious diseases specialist at Johns Hopkins School of Medicine in Baltimore, in the briefing.

However, for patients in groups at higher risk for complications, such as the frail elderly, or immunocompromised persons of any age, maintain a lower threshold for testing to see whether they might benefit from antimicrobial therapy, Auwaerter said.

Notably, many testing panels are expensive, and may not be covered by patients’ insurance. If someone is at increased risk for complications, they may warrant antimicrobial treatment in the absence of a confirmed diagnosis, but this decision depends on the patient and whether they are at risk or seem to be improving, said Moore.

The delay in detecting a source for the current outbreak may be attributable in part to CDC’s cuts in budget and personnel, Moore said in the briefing. In addition, many states have cut back on public health infrastructure, said Moore. “When you are trying to fight an outbreak with one hand tied behind your back, it makes it very difficult,” he added. However, many state public health offices, notably Michigan, where the outbreak has hit especially hard, have stepped up to provide tracking information and clinical resources at the local level, said Jeanne Marrazzo, MD, MPH, FIDSA, CEO of the IDSA, in the briefing.

For patients who do need antimicrobial therapy, the first-line treatment for otherwise healthy individuals aged 2 months or older is a 7- to 10-day course of trimethoprim-sulfamethoxazole; immunocompromised patients may need a longer duration, according to the CDC.

The experts had no relevant financial conflicts to disclose.