For decades, America’s food safety system has relied on a network of laboratories, state health departments, and federal investigators working together to quickly identify contaminated foods before outbreaks spread nationwide. In many cases involving pathogens such as Salmonella, E. coli, or Listeria, investigators are able to trace illnesses back to a single farm, processing plant, or food distributor within days.

This summer, however, health officials are confronting a very different challenge.

A growing outbreak of Cyclospora, a microscopic parasite that causes severe intestinal illness, has spread across 34 states, leaving public health officials searching for a common source that remains frustratingly elusive. While the Centers for Disease Control and Prevention (CDC) has confirmed more than 1,600 cases, several states are investigating thousands of additional probable infections, making this one of the largest Cyclospora outbreaks the United States has seen in years.

Unlike many foodborne illnesses, investigators still have not identified a single product, farm, or distributor responsible for the outbreak.

A Growing National Outbreak

According to the CDC’s latest surveillance data, 1,645 laboratory-confirmed cases have been reported nationwide. State health departments are simultaneously reviewing more than 5,000 additional suspected or probable cases, bringing the total number of illnesses under investigation close to 7,000.

Although 141 patients have required hospitalization, no deaths have been reported.

The largest clusters remain concentrated across the Midwest and Northeast.

Michigan has experienced the most significant surge, with health officials investigating more than 3,300 reported illnesses since early July.

New York has confirmed more than 500 cases, many centered in New York City, while Ohio, Illinois, Kentucky, West Virginia, and several neighboring states continue reporting new infections each week.

Because many investigations remain ongoing, officials caution that case counts are likely to continue rising.

Current Outbreak Snapshot

• 34 states reporting confirmed infections
• 1,645 CDC-confirmed cases
• More than 5,000 additional probable cases under review
• 141 hospitalizations
• No confirmed deaths

Why Investigators Still Don’t Know the Source

Finding the contaminated food has proven exceptionally difficult.

Cyclospora behaves differently than many bacteria responsible for foodborne outbreaks.

After someone consumes contaminated produce or water, symptoms often don’t appear for one to two weeks. By the time patients become seriously ill, the food they ate has usually been discarded, making interviews about recent meals far less reliable than they are during a typical food poisoning investigation.

That long delay makes it much harder for epidemiologists to connect patients to a single grocery store, restaurant, or supplier.

Adding to the challenge is the parasite itself.

Cyclospora organisms are microscopic and can be extremely difficult to detect on fresh produce.

Steven Manderach, executive director of the Association of Food and Drug Officials, compared the process to searching for something almost impossibly small.

Instead of looking for “a needle in a haystack,” he said, investigators are effectively searching for a microscopic fragment of a needle hidden somewhere inside an entire truckload of produce.

Because of that, laboratory testing often requires washing large quantities of fruits or vegetables before scientists can even begin searching for traces of the parasite.

Foods Frequently Linked to Cyclospora

Officials have not identified a common source in the current outbreak.

However, previous North American outbreaks have been linked to fresh produce, including:

• Bagged salad mixes
• Fresh cilantro
• Basil
• Green onions
• Imported raspberries
• Other uncooked fruits and vegetables

Cyclospora is not spread directly from one person to another in ordinary daily contact.

Instead, infection usually occurs after consuming food or water contaminated with microscopic parasite eggs, often through contaminated irrigation water or poor sanitation during harvesting and handling.

Funding Questions Add Another Layer

As investigators work to solve the mystery, some public health experts argue that recent reductions in federal surveillance programs have complicated the response.

Several epidemiologists interviewed by the BBC said cutbacks affecting the Foodborne Diseases Active Surveillance Network (FoodNet) have reduced the amount of routine data available to investigators.

FoodNet has historically served as one of the nation’s primary systems for tracking foodborne illnesses and identifying emerging outbreaks.

Jodie Guest, a professor of epidemiology at Emory University’s Rollins School of Public Health and a former FoodNet scientist, said investigators today have fewer baseline data points than they once did.

“When we see an outbreak or a cluster,” Guest told the BBC, “we don’t have the data we normally expect to go back to use to help us. You’re starting in the dark.”

Some state health departments have also acknowledged staffing pressures and reduced resources while interviewing thousands of patients during the current investigation.

Federal officials, however, dispute suggestions that surveillance reductions are responsible for the slow investigation.

The Department of Health and Human Services maintains that funding for foodborne disease monitoring remains available through multiple surveillance systems beyond FoodNet and says investigators continue working with state agencies to identify the outbreak’s source.

For now, the debate over surveillance funding continues while epidemiologists focus on stopping the spread.

Symptoms Can Be Severe

Cyclospora infections often begin gradually before worsening over several days.

Common symptoms include:

• Frequent watery diarrhea
• Severe stomach cramps
• Loss of appetite
• Fatigue
• Nausea
• Weight loss
• Bloating

Without treatment, symptoms can last several weeks and may return even after initially improving.

Doctors typically prescribe antibiotics for confirmed cases.

What Consumers Should Do

Because investigators have not yet identified a contaminated product, health officials are advising consumers to take extra precautions when preparing fresh foods.

The CDC recommends:

• Thoroughly washing fruits and vegetables under running water.
• Refrigerating perishable produce promptly.
• Avoiding food preparation when experiencing diarrhea.
• Seeking medical care if symptoms persist for more than a few days.

Some state health departments have also encouraged vulnerable individuals to cook leafy greens when possible until investigators determine the outbreak’s source.

Watch: Health experts on NBC’s TODAY explain what Cyclospora is, why this outbreak has proven so difficult to trace, and the simple precautions consumers can take while investigators search for the source.

Related: How Foodborne Illness Investigations Track Contaminated Produce Across State Lines

A Race Against Time

Every day that passes without identifying the contaminated food allows more potentially affected products to move through grocery stores, restaurants, and home kitchens.

Public health investigators continue interviewing patients, tracing food purchases, reviewing supply chains, and testing produce from multiple distributors in hopes of finding the common link.

Until they do, one of the country’s largest parasitic outbreaks in recent years remains an unsolved public health mystery—one that demonstrates just how difficult modern foodborne investigations can become when the pathogen leaves few clues behind.

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Sources

• Centers for Disease Control and Prevention (CDC) — Cyclospora surveillance updates and case counts.
• BBC News — Reporting by Madeline Halpert, Kayla Epstein, and Sareen Habeshian on the outbreak and expert interviews.
• Association of Food and Drug Officials — Comments from Executive Director Steven Manderach regarding laboratory detection challenges.
• Johns Hopkins Center for Health Security — Background information on Cyclospora transmission and foodborne disease investigations.